Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page i 2.11.2007 3:10pm Compositor Name: BMani Amino Acids and Proteins for the Athlete The Anabolic Edge Second Edition Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page ii 2.11.2007 3:10pm Compositor Name: BMani Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page iii 2.11.2007 3:10pm Compositor Name: BMani Amino Acids and Proteins for the Athlete The Anabolic Edge Second Edition Mauro G. Di Pasquale CEO, MetabolicDiet.com Ontario, Canada Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page iv 2.11.2007 3:10pm Compositor Name: BMani CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2008 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-4380-8 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. 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Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page v 2.11.2007 3:10pm Compositor Name: BMani Contents Preface to the Second Edition..........................................................................................................xv Preface to the First Edition ........................................................................................................... xvii Author .............................................................................................................................................xix Part I The Theory Chapter 1 Proteins and Amino Acids............................................................................................3 Introduction ........................................................................................................................................3 Amino Acids ......................................................................................................................................4 Peptide Linkage .................................................................................................................................4 Free Amino Acid Pool.......................................................................................................................5 Protein Synthesis................................................................................................................................6 Regulation of Protein Synthesis.....................................................................................................7 Factors Affecting Protein Synthesis and Catabolism ....................................................................9 Cellular Hydration and Protein Synthesis....................................................................................11 Cellular Hydration and Amino Acids..........................................................................................12 Nutrients and Protein Synthesis...................................................................................................13 Protein Catabolism...........................................................................................................................14 Adaptive Response to Exercise .......................................................................................................15 Exercise-Induced Amino Acid Flux ................................................................................................17 References ........................................................................................................................................17 Chapter 2 Exercise and Protein Metabolism ...............................................................................23 Introduction ......................................................................................................................................23 Effects of Exercise on Protein Synthesis and Degradation .............................................................24 Effects of Protein and Amino Acids on Exercise Performance ......................................................24 Hormonal Response to Exercise ......................................................................................................24 Hormones .........................................................................................................................................26 Growth Hormone .........................................................................................................................27 Growth Hormone and Athletic Performance ...........................................................................28 Problems with Exogenous Growth Hormone ..........................................................................30 Effects of Growth Hormone on Body Composition and Athletic Performance .....................30 GH, IGF-1, Insulin, and Amino Acids Synergism ..................................................................31 Nutrient and Hormone Delivery to Muscle .............................................................................32 Growth Hormone and Exercise ...............................................................................................32 Growth Hormone and Endurance Athletes..............................................................................33 Effects of GH on Core Temperature and Performance ...........................................................33 Effects of GH on Intramyocellular Triacylglycerol=Triglyceride Content..............................33 Regenerative and Cognitive Effects of GH and IGF-1 ...........................................................34 Effects of GH on Aging...........................................................................................................34 Growth Hormone Synthesis and Secretion..............................................................................34 v Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page vi 2.11.2007 3:10pm Compositor Name: BMani vi Myostatin .................................................................................................................................35 GH and Myostatin....................................................................................................................36 Insulin-Like Growth Factor 1 ......................................................................................................37 GH and IGF-1 Synergism........................................................................................................40 Mechano Growth Factor ..........................................................................................................42 Insulin...........................................................................................................................................43 Insulin’s Controversial Effects on Protein Synthesis and Degradation ...................................45 Insulin and Nutrient Delivery ..................................................................................................46 Insulin, GH, and IGF-1 Synergism..........................................................................................47 Thyroid Hormones .......................................................................................................................48 Thyroid Hormones and Protein Metabolism ...........................................................................50 Thyroid Hormone and Other Hormones..................................................................................51 Testosterone .................................................................................................................................52 Stress and Testosterone Levels ................................................................................................53 Exercise and Testosterone Levels............................................................................................53 Catabolic Hormones.....................................................................................................................55 Effect of the Catabolic Hormones on Skeletal Muscle Catabolism ........................................55 Catecholamines ............................................................................................................................56 Effects of Catecholamines on Protein Metabolism .................................................................57 Glucagon ......................................................................................................................................58 Glucagon and Insulin ...............................................................................................................59 Cortisol.........................................................................................................................................60 Cortisol, Amino Acids, and Other Hormones .........................................................................61 Testosterone to Cortisol Ratio .................................................................................................61 Testosterone to Cortisol Ratio and Exercise............................................................................62 Effect of Dietary Nutrients on Testosterone and the Testosterone to Cortisol Ratio.......................................................................................................................63 Cytokines and Muscle Protein Synthesis.........................................................................................64 Eicosanoids and Muscle Protein Synthesis......................................................................................66 Hormonal Effects of Amino Acids ..................................................................................................69 References ........................................................................................................................................70 Chapter 3 Energy Metabolism...................................................................................................107 Introduction ....................................................................................................................................107 Energy Sensing ..............................................................................................................................108 Metabolic Pathways Producing ATP.............................................................................................108 Anaerobic Energy Production........................................................................................................108 Cytoplasmic Anaerobic Substrate-Level Phosphorylation ........................................................108 Anaerobic Mitochondrial Phosphorylation ................................................................................109 Aerobic Energy Production ...........................................................................................................109 TCA Cycle .................................................................................................................................110 Role of Protein and Amino Acids in Energy Metabolism ............................................................111 Fate of Dietary Protein...................................................................................................................112 Protein Contribution to Energy Metabolism..................................................................................113 Amino Acid Catabolism ................................................................................................................113 Oxidation of Amino Acids ............................................................................................................113 Gluconeogenesis ............................................................................................................................115 Amino Acids and Gluconeogenesis...........................................................................................116 Hormonal Control of Gluconeogenesis .....................................................................................117 Effects of Amino Acids on Hepatic Glucose Metabolism ............................................................118 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page vii 2.11.2007 3:10pm Compositor Name: BMani vii Use of Amino Acids for Energy—Catabolic Effects of Exercise .................................................118 Amino Acid Metabolism in Muscle ..........................................................................................119 Pathways of Amino Acid Metabolism in Muscle .....................................................................119 Skeletal Muscle Catabolism.......................................................................................................119 Oxidation of Amino Acids ........................................................................................................120 Glucogenic and Ketogenic Amino Acids ......................................................................................122 Glucogenic Amino Acids...........................................................................................................123 Metabolized to AKG, Pyruvate, Oxaloacetate, Fumarate, or Succinyl-CoA ........................123 Ketogenic Amino Acids.............................................................................................................124 Metabolized to Acetyl-CoA or Acetoacetate.........................................................................124 Alanine and Glutamine ..................................................................................................................124 Interorgan Exchange of Amino Acids ...........................................................................................125 Protein Metabolism and Ammonia ................................................................................................126 Metabolism of Ammonia ...........................................................................................................126 Urea Formation by the Liver .....................................................................................................128 High Levels of Protein Intake and Ammonia............................................................................129 Low-Carbohydrate, High-Protein Diets and Energy Metabolism .................................................130 Metabolic Advantage of a High-Protein, Low-Carbohydrate Diet ...........................................130 Dietary Calories from Macronutrients .......................................................................................130 Low-Carbohydrate Controversy.................................................................................................131 Conclusions and Recommendations ..........................................................................................132 References ......................................................................................................................................133 Chapter 4 Dietary Protein and Amino Acids ............................................................................139 Effects of Protein on Dietary Intake and Appetite ........................................................................139 Classification of Proteins ...............................................................................................................139 Simple Proteins ..........................................................................................................................139 Conjugated Proteins ...................................................................................................................140 Functions of Proteins .....................................................................................................................141 Growth .......................................................................................................................................141 Maintenance ...............................................................................................................................141 Regulatory..................................................................................................................................141 Energy ........................................................................................................................................142 Coagulation and Denaturation .......................................................................................................142 Protein Digestion, Absorption, and Metabolism ...........................................................................142 Requirement for Dietary Protein—Amino Acid Needs.................................................................144 Quality of Proteins .........................................................................................................................144 Slow and Fast Dietary Proteins .....................................................................................................145 Effects of Dietary Protein on Protein Metabolism ........................................................................147 Protein Quality—Amino Acid Requirements................................................................................147 Vegetarian Diets.............................................................................................................................149 Types of Vegetarians .................................................................................................................150 Nutritional Quality of Vegetarian Diets.....................................................................................150 Vegetarian Food Guide—Basic Four Food Groups ..................................................................151 Supplying Required Nutrients ...................................................................................................152 Nutritional Considerations of Vegetarians.................................................................................152 Special Nutrient Needs of Vegetarians..................................................................................152 Vegetarian Athletes....................................................................................................................153 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page viii 2.11.2007 3:10pm Compositor Name: BMani viii Common Vitamin and Mineral Deficiencies .............................................................................153 Specific Nutritional Needs of Vegetarians.................................................................................154 Protein ........................................................................................................................................154 Ensuring Adequate and High-Quality Protein...........................................................................154 Complementary Proteins............................................................................................................154 Nutritional Responses to Combining Two Dietary Proteins .....................................................156 Nutritional Supplements and the Vegetarian Athlete ................................................................157 Creatine ......................................................................................................................................158 Food Processing .............................................................................................................................158 Measuring Protein Quality.............................................................................................................158 Protein Efficiency Ratio.............................................................................................................158 Biological Value ........................................................................................................................159 Protein Digestibility Corrected Amino Acid Score ...................................................................160 Dietary Protein Requirements........................................................................................................161 How Was the RDA Established?...................................................................................................163 Recommended Daily Intakes for Athletes .....................................................................................164 Historical Overview ...................................................................................................................164 Effects of Exercise on Dietary Protein Requirements ...................................................................165 Protein (Nitrogen) Balance ........................................................................................................168 Effect of Dietary Protein on Protein Metabolism......................................................................170 Amino Acid Metabolism ...........................................................................................................171 The Dietary Protein Paradox—The Probable Need for Protein and Amino Acid Supplements Even in Diets High in Dietary Protein....................................................172 Protein Needs in Calorie-Restricted Diets .................................................................................174 References ......................................................................................................................................174 Chapter 5 Protein Foods versus Protein and Amino Acid Supplements ..................................185 Hydrolysates—Comparison to Whole Protein and Intact Whole-Protein Supplements ...............186 Whey Protein Hydrolysates and the Athlete .................................................................................187 BV of Whey Hydrolysates.............................................................................................................188 Whey Protein and the Immune System .........................................................................................188 Effects of Whey Protein on Glutathione Levels............................................................................188 Whey and Casein and Antigenicity—Advantages of Hydrolysates..............................................189 Whey Protein—Conclusions..........................................................................................................190 Free-Form Amino Acids versus Di- and Tripeptides ....................................................................190 Practical Guide to Commercial Amino Acid Preparations............................................................191 Free-Form Amino Acid Mixtures ..............................................................................................192 Peptide-Bonded Aminos ............................................................................................................192 Factors Affecting Amino Acid Bioavailability..............................................................................192 Role of Amino Acid Supplementation in Muscle Hypertrophy and Strength ..............................193 References ......................................................................................................................................194 Chapter 6 Physiological and Pharmacological Actions of Amino Acids .................................197 Introduction ....................................................................................................................................197 Central Nervous System Effects of Amino Acids.........................................................................199 Effects of Amino Acids on Growth Hormone Release .................................................................200 Hepatoprotectant and Cytoprotective Effects of Amino Acids .....................................................201 References ......................................................................................................................................201 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page ix 2.11.2007 3:10pm Compositor Name: BMani ix Chapter 7 Essential Amino Acids .............................................................................................207 Use of Essential Amino Acids after Training Increases 24 h Protein Balance.............................207 Branched-Chain Amino Acids: Isoleucine, Leucine, and Valine..................................................208 BCAAs and Muscle Hypertrophy..............................................................................................212 BCAAs and Hypoxia .................................................................................................................213 Regulation of Muscle Protein Synthesis by Leucine.................................................................213 Leucine and Body Composition ................................................................................................214 Branched-Chain Keto Acids ......................................................................................................215 a-Ketoisocaproic Acid...............................................................................................................215 b-Hydroxy-b-Methylbutyrate ....................................................................................................216 Lysine.............................................................................................................................................218 L-Carnitine..................................................................................................................................218 L-Carnitine Metabolism..........................................................................................................219 L-Carnitine Functions .............................................................................................................220 LCAR Effects on Body Composition and Exercise Performance.........................................221 L-Carnitine and Choline .........................................................................................................224 Acetyl-L-Carnitine ......................................................................................................................224 Cognitive and Antiaging Effects of ALCAR ........................................................................226 Methionine .....................................................................................................................................227 S-Adenosyl-L-Methionine ..........................................................................................................229 Creatine ..........................................................................................................................................229 Creatine Supplementation ..........................................................................................................230 Creatine Supplementation Combined with Other Ingredients...................................................232 Creatine and Caffeine ................................................................................................................232 Phenylalanine .................................................................................................................................233 Threonine .......................................................................................................................................233 Tryptophan .....................................................................................................................................233 References ......................................................................................................................................235 Chapter 8 L-Arginine Conditionally Essential Amino Acids ......................................................................253 ......................................................................................................................................253 and Nitric Oxide ......................................................................................................255 L-Arginine and the Athlete.........................................................................................................256 Thiamin, Arginine, Caffeine, and Citric Acid Combination .....................................................258 Recovery and Athletic Injuries ..................................................................................................258 Possible Ergolytic Effects of Nitric Oxide ................................................................................258 L-Arginine and Caffeine.............................................................................................................258 L-Citrulline .....................................................................................................................................259 Citrulline Malate ........................................................................................................................260 L-Cysteine.......................................................................................................................................260 N-Acetylcysteine ............................................................................................................................261 L-Glutamine....................................................................................................................................262 Glutamine Metabolism...............................................................................................................263 Anabolic and Anticatabolic Effects of Glutamine.....................................................................263 Glutamine and Cortisol ..............................................................................................................264 Anabolic Effects of Glutamine ..................................................................................................266 Glutamine and Athletic Performance.........................................................................................267 Glutamine and Overtraining ......................................................................................................269 Oral Glutamine Supplementation...............................................................................................270 L-Arginine Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page x 2.11.2007 3:10pm Compositor Name: BMani x Glutamine in Supplements.........................................................................................................271 Summary ....................................................................................................................................271 a-Ketoglutarate ..........................................................................................................................272 Histidine .........................................................................................................................................272 L-Carnosine ....................................................................................................................................273 Buffering Effects of Carnosine ..................................................................................................274 Tyrosine .........................................................................................................................................275 Proline and Hydroxyproline...........................................................................................................276 References ......................................................................................................................................277 Chapter 9 Nonessential or Dispensable Amino Acids ..............................................................297 Alanine ...........................................................................................................................................297 Glucose–Alanine Cycle..............................................................................................................297 Alanine Needs and Exercise ......................................................................................................300 b-Alanine ...................................................................................................................................301 Asparagine and Aspartic Acid .......................................................................................................302 Malate–Aspartate Shuttle ...........................................................................................................304 D-Aspartic Acid ..........................................................................................................................305 Citrulline ........................................................................................................................................305 Citrulline Malate ........................................................................................................................306 Glutamic Acid................................................................................................................................306 Ornithine ........................................................................................................................................306 Ornithine a-Ketoglutarate ..........................................................................................................307 Serine .............................................................................................................................................308 Phosphatidylserine .....................................................................................................................309 Glycine ...........................................................................................................................................310 Taurine ...........................................................................................................................................311 Levodopa (L-Dopa) ........................................................................................................................313 Analogues and Derivatives of Amino Acids .................................................................................314 Protein and Amino Acids and Muscle Hypertrophy .....................................................................314 References ......................................................................................................................................315 Chapter 10 Summary and Conclusions .....................................................................................327 Part II Naturally Anabolic Chapter 11 Artificial Enhancement ...........................................................................................331 Introduction ....................................................................................................................................331 Drugs in Sports ..............................................................................................................................331 Nutritional Supplements ............................................................................................................331 Gene Therapy and Sports ..............................................................................................................331 The Easy, but Dangerous Way Out ...............................................................................................333 References ......................................................................................................................................334 Chapter 12 Getting It Together .................................................................................................335 Hormonal Manipulation.................................................................................................................335 Maximizing Lean Body Mass and Athletic Performance without Drugs .....................................336 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xi 2.11.2007 3:10pm Compositor Name: BMani xi Genetics..........................................................................................................................................336 Usual Suspects—Lifestyle, Training, Diet, and Nutritional Supplements ....................................337 Performance and Body Composition Enhancement Pipeline....................................................337 Factors That Maximize the Pipeline ..............................................................................................338 Step Number One—Lifestyle ....................................................................................................338 Step Number Two—Training without Overtraining .................................................................338 Recovery Phase—Protein Synthesis after Exercise...............................................................340 Hormonal Changes with Exercise .........................................................................................340 Maximizing the Anabolic and Minimizing the Catabolic Effects of Exercise......................342 Step Number Three—Maximizing Diet and Nutrition..............................................................343 Weight Loss and Body Composition.............................................................................................345 Effective Fat Loss ......................................................................................................................346 Phase Shift Diets........................................................................................................................346 Evolution of My Phase Shift Diets............................................................................................346 Man’s Survival: The History of Food........................................................................................347 Recipe for Disaster.....................................................................................................................348 Control of Weight and Body Composition ...............................................................................348 Unstructured Phase Shift Diets ..................................................................................................350 Structured Phase Shift Diets ......................................................................................................351 Metabolic Diet ...........................................................................................................................351 Cultural Carbohydrates ..............................................................................................................353 Carbohydrate Dilemma ..............................................................................................................353 Higher-Carbohydrate Phase .......................................................................................................353 Low-Carbohydrate Phase ...........................................................................................................353 Scientific Validation...................................................................................................................354 Glucose Metabolism at the Start of the Low-Carbohydrate Phase of the Metabolic Diet .................................................................................................................355 Postexercise Carbohydrates are Counterproductive ......................................................................356 Low-Carbohydrate, High-Protein Diets, and Energy Metabolism ............................................358 Metabolic Advantage of a High-Protein, Low-Carbohydrate Diet ...........................................358 Dietary Calories from Macronutrients .......................................................................................358 Low-Carbohydrate Controversy.................................................................................................359 Step Number Four—Nutritional Supplements...............................................................................360 Why, When, and How to Take Them and How Much to Take................................................360 Why Use Nutritional Supplements? ..........................................................................................361 Sports Performance ....................................................................................................................362 Cycling of Supplements.............................................................................................................362 Stage One: Detraining or Rest Phase.....................................................................................362 Stage Two: Beginning Training Phase ..................................................................................363 Stage Three: Precompetition and Competition Phase ...........................................................363 Current Nutritional Supplement Use .........................................................................................363 Benefits of Nutritional Supplements..........................................................................................366 Vitamins and Minerals...............................................................................................................366 Antioxidants ...............................................................................................................................368 Vitamin C...................................................................................................................................368 Coenzyme Q10 (Ubiquinone-10)...............................................................................................369 Zinc ............................................................................................................................................369 Magnesium.................................................................................................................................369 Calcium ......................................................................................................................................369 Chromium ..................................................................................................................................370 Chromium and Conjugated Linoleic Acid (CLA).....................................................................370 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xii 2.11.2007 3:10pm Compositor Name: BMani xii Potassium ...................................................................................................................................370 Alpha Lipoic Acid .....................................................................................................................371 Glucosamine Sulfate ..................................................................................................................371 Green Tea Extract ......................................................................................................................372 Hydroxycitrate............................................................................................................................372 Citrus aurantium........................................................................................................................373 Yohimbe.....................................................................................................................................374 Dhea ...........................................................................................................................................374 Essential Fatty Acids .................................................................................................................375 Omega-3 Fatty Acids .................................................................................................................375 EFAs and Body Composition and Exercise Performance.........................................................376 Gamma Linolenic Acid..............................................................................................................377 Conjugated Linolenic Acid........................................................................................................377 Glycerol......................................................................................................................................378 Pyruvate .....................................................................................................................................378 Caffeine and Ephedrine .............................................................................................................378 Anticortisol Supplements ...........................................................................................................379 Amino Acid and Protein Supplementation ................................................................................379 Timing of Nutrient and Protein Intake in Relation to Exercise ....................................................380 Postexercise Carbohydrates May Be Counterproductive ..........................................................381 References ......................................................................................................................................382 Chapter 13 Examples of Useful Nutritional Supplement Formulations ...................................397 Example of a Weight and Fat Loss, and Body Composition Supplement....................................397 Thiamin, Arginine, Caffeine, and Citric Acid Combination .....................................................397 Yohimbe.....................................................................................................................................397 Citrus aurantium........................................................................................................................398 Evodiamine ................................................................................................................................398 Cayenne Pepper .........................................................................................................................399 Green Tea Extract ......................................................................................................................399 Phenylalanine .............................................................................................................................399 Tyrosine .....................................................................................................................................399 Tyrosine, Capsaicin, Catechins, Calcium, and Caffeine Combo ..............................................399 Conjugated Linoleic Acid ..........................................................................................................400 Guarana ......................................................................................................................................400 Conjugated Linoleic Acid and Guarana Combo .......................................................................400 Chromium ..................................................................................................................................400 Chromium and CLA ..................................................................................................................401 Yerba Maté.................................................................................................................................401 Niacin .........................................................................................................................................401 Dandelion ...................................................................................................................................401 Amino Acids ..............................................................................................................................402 Histidine .....................................................................................................................................402 Taurine .......................................................................................................................................402 Calcium ......................................................................................................................................402 Other Ingredients........................................................................................................................403 Boosting Endogenous Growth Hormone.......................................................................................403 Postexercise Amino Acid Formulation..........................................................................................406 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xiii 2.11.2007 3:10pm Compositor Name: BMani xiii Joint Support Formula ...................................................................................................................406 Glucosamine Sulfate ..................................................................................................................406 Chondroitin Sulfate ....................................................................................................................407 Combined Use of Glucosamine and Chondroitin Sulfate .........................................................407 Hyaluronic Acid.........................................................................................................................408 Methylsulfonylmethane..............................................................................................................408 Alpha Lipoic Acid .....................................................................................................................408 Amino Acids ..............................................................................................................................409 Antioxidants ...............................................................................................................................409 Boswellia serrata Extract...........................................................................................................410 Bromelain, Papain, Trypsin, and Rutin .....................................................................................410 Cayenne Pepper .........................................................................................................................410 Carnosine ...................................................................................................................................410 Coenzyme Q10 ..........................................................................................................................411 Devil’s Claw Root (Uncaria tomentosa) ...................................................................................411 Ginger.........................................................................................................................................411 Ginkgo biloba ............................................................................................................................411 Green Tea Extract ......................................................................................................................411 Melatonin ...................................................................................................................................412 Omega-3 and Omega-6 Oils—GLA, DHA, and EPA ..............................................................412 Pau D’arco..................................................................................................................................412 Rutin and Quercetin ...................................................................................................................413 S-Adenosyl-L-Methionine—Increasing Endogenous Production ..............................................413 Shark Cartilage...........................................................................................................................413 Silicon ........................................................................................................................................413 Stinging Nettle Extract...............................................................................................................414 Turmeric.....................................................................................................................................414 Vitamins and Minerals...............................................................................................................414 White Willow Bark....................................................................................................................415 Yucca Leaf Extract ....................................................................................................................415 Other Ingredients........................................................................................................................416 Summary ........................................................................................................................................416 References ......................................................................................................................................416 Index..............................................................................................................................................427 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xiv 2.11.2007 3:10pm Compositor Name: BMani Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xv 2.11.2007 3:10pm Compositor Name: BMani Preface to the Second Edition In the last decade research on protein and amino acids has increased almost exponentially. New information has increased our knowledge on the use and function of proteins and amino acids. Concepts have been clarified and new concepts have arisen on the requirements and functions of the amino acids, as the building blocks of protein, and their roles in energy metabolism, and in metabolic signaling. Although the new information greatly expands our knowledge it has become even more obvious that discussing any one aspect of metabolism soon brings into view other elements that in turn bring up still other elements of metabolism. The end result is that any attempt to discuss any one topic in detail soon involves others and makes the whole project unmanageable. In fact, today you will need a separate book to fully discuss many of the topics that I attempt to cover in this one. As a result, in an attempt to cover such a broad topic as the effects of protein and amino acids on athletic performance and body composition, some obvious concessions have to be made. These concessions include giving an umbrella type of coverage to most topics of interest and a somewhat detailed coverage to the points I consider most important for the readers. As such, this new edition is an attempt to bring the information on proteins and amino acids up to date, to speculate on what is in store down the line, and to give some solid advice on nutritional supplementation. Significant changes have been made to all chapters, with some being totally rewritten. In addition, the practical section of this book has been expanded to provide an overall look at what is needed to improve body composition and exercise performance, and a more detailed look at the role played by protein, amino acids, and other nutritional supplements. Mauro G. Di Pasquale xv Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xvi 2.11.2007 3:10pm Compositor Name: BMani Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xvii 2.11.2007 3:10pm Compositor Name: BMani Preface to the First Edition We need water and food to survive. Without water we can only survive 3 to 4 days. Without food, we can last a while longer. However, once our internal energy stores are exhausted, we perish. Our food contains, or at least should contain, the nutrients we need to live and grow. While the carbohydrates, fats, and proteins that make up our food are all important macronutrients, of the three, protein is the most important and versatile. Not only do proteins make up three-quarters of body solids (including structural and contractile proteins, enzymes, nucleoproteins, and proteins that transport oxygen) but protein and amino acids have potent biological effects on the body that involve all tissues in the body and extend to almost all metabolic processes. A large amount of research has been conducted to determine the protein requirements of athletes and the effects of increasing both the amount and quality of dietary protein; the effects protein and amino acid supplements have on both athletic performance, primarily on muscle size and strength and energy metabolism; and the effects of specific amino acid supplements on metabolic and physiological responses to strength and endurance exercise. There has recently been an increased interest in determining the ergogenic effect of specific amino acids. As we shall see, numerous studies have indicated that various amino acids are involved in the metabolic and physiological responses to both acute and chronic exercise. Because of a perceived need for protein, athletes have increased both their dietary protein consumption and the use of commercial protein and amino acid supplements. In this book we will examine the available scientific and medical information in order to determine the physiological and pharmacological effects of protein and amino acids on lean body mass, body fat, strength, and endurance. Some of the information presented below will serve as a brief review of energy and protein metabolism in order to help the reader understand some of the more important information on just how proteins and amino acids can be used to increase lean body mass and strength, lose body fat, and have a positive impact on athletic performance, health, disease, and longevity. xvii Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xviii 2.11.2007 3:10pm Compositor Name: BMani Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xix 2.11.2007 3:10pm Compositor Name: BMani Author Mauro G. Di Pasquale, M.D., MRO, MFS, is a licensed physician in Ontario, Canada, concentrating on sports medicine, nutrition, and nutritional supplements as they apply to body composition and exercise performance. He was an assistant professor at the University of Toronto for 10 years (1988–1998) lecturing and researching on athletic performance, nutrition, nutritional supplements, and drug use in sports. Dr. Di Pasquale holds an honors degree in biological science, majoring in molecular biochemistry (1968), and a medical degree (1971), both from the University of Toronto. He was certified for more than a decade as a medical review officer (MRO) by the Medical Review Officer Certification Council (MROCC) and is certified as a master of fitness sciences (MFS) by the International Sports Sciences Association (ISSA). Dr. Di Pasquale was the drug program advisor to the World Wrestling Federation (WWF) and past medical director and drug program advisor to the now defunct World Bodybuilding Federation (WBF). He was also the acting MRO for the National Association for Stock Car Auto Racing (NASCAR). He has been involved in international sports and drug testing for the last 40 years as an athlete, an administrator, and a physician. He was a world-class athlete for over 20 years, winning the world championships in powerlifting in 1976 and in the sport of powerlifting at the World Games in 1981. He was Canadian champion eight times, Pan American champion twice, and North American champion twice. He was the chairman of the International Powerlifting Federation’s Medical Committee for 8 years (1979–1987) and was on the committee for a total of 12 years. As chairman of the IPF Medical Committee, the Canadian Powerlifting Union Medical Committee, and the Canadian Amateur Federation of Bodybuilding Medical Committee and as a consultant to various sporting federations he has had extensive exposure to athletic injuries and disabilities and drug use by athletes. He was also IPF Vice President for North America, and President of the Pan American and United World Powerlifting Federations. In the early 1980s, he initiated and developed the IPF drugtesting protocols and procedures. Dr. Di Pasquale has written several books dealing with the use of drugs and nutritional supplements by athletes, including Drug Use and Detection in Amateur Sports, Beyond Anabolic Steroids; Anabolic Steroid Side Effects, Fact Fiction and Treatment; and The Bodybuilding Supplement Review. In the last 35 years, he has been on several editorial boards for various fitness and strength magazines and has written extensively in the field of exercise and nutrition. His writing includes hundreds of articles on supplements, nutrition, drugs, and exercise for magazines and association journals, as well as numerous contributed chapters on anabolic steroids and drug testing to several fitness, weight, and sports medicine books. He has written over a dozen books on ergogenic aids, drug use and detection in sports, nutrition, and nutritional supplements, and hundreds of articles displayed on various Internet sites. He continues to write a regular column in Powerlifting USA and regularly contributes to a half dozen Internet sites. He was the editor-in-chief of the quarterly international newsletter Drugs in Sports, published in English, Spanish, and Italian. He is currently editor-in-chief of the bimonthly newsletter The Anabolic Research Review. xix Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C000 Final Proof page xx 2.11.2007 3:10pm Compositor Name: BMani xx Dr. Di Pasquale acts as an international consultant and expert witness for athletes, amateur and professional sports bodies, and government agencies on legal matters relating to the use and abuse and drug testing of anabolic steroids, growth hormone, and other ergogenic drugs and supplements. Over the past few decades he had concentrated on providing nutritional alternatives to ergogenic drugs for both competitive and recreational athletes. In 1999, Dr. Di Pasquale formed his own nutritional company and formulated a complete nutritional supplement line, which now includes two dozen cutting-edge products designed to work with his phase shift diets to maximize body composition, athletic performance, and the beneficial effects of exercise. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Part I The Theory 43803_C001 Final Proof page 1 29.10.2007 7:46pm Compositor Name: JGanesan Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 2 29.10.2007 7:46pm Compositor Name: JGanesan Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 3 29.10.2007 7:46pm Compositor Name: JGanesan 1 Proteins and Amino Acids INTRODUCTION The word protein comes from the Greek ‘‘proteios,’’ which means ‘‘of the first rank or importance.’’ Protein is indeed important for life and is involved in every biological process within the body. The average human body comprises approximately 18% protein. Proteins are essential components of muscle, skin, cell membranes, blood, hormones, antibodies, enzymes, and genetic material and almost all other body tissues and components. They serve as structural components, biocatalysts (enzymes), antibodies, lubricants, messengers (hormones), and carriers. The contribution made by proteins to the energy value of most well-balanced diets is usually between 10% and 15% of the total and seldom exceeds 20%. In some athletes in power sports and bodybuilders who may be on very high protein diets, the contribution can be as high as 50% and in selective cases even more. In addition to proteins, some of the individual amino acids have important functions in regulating protein synthesis and various other aspects of body metabolism. However, the energy contribution that proteins make in diets and their involvement in many metabolic pathways, although important, is usually considered secondary. Their real importance generally lies in the fact that every cell in the body is partly composed of proteins which are subject to continuous wear and replacement. Carbohydrates and fats contain no nitrogen or sulfur, the two essential elements in proteins. Although the fat in the body can be derived from dietary carbohydrates and the carbohydrates from proteins, the proteins of the body are inevitably dependent for their formation and maintenance on the proteins in food, which are digested and the resultant amino acids and peptides are absorbed and used to synthesize body proteins. Proteins consist of large molecules with molecular weights ranging from 1,000 to over 1,000,000 Da. In their native state some are soluble and some insoluble in water. Although there are a great variety of proteins that can be subdivided into various categories, they are all made up of the same building blocks called amino acids. Every species of animal has its characteristic proteins. The proteins of beef muscle, for instance, differ from those of pork muscle. It is the proteins that give each species its specific immunological characteristics and uniqueness. Plants can synthesize all the amino acids they need from simple inorganic chemical compounds, but animals are unable to do this because they cannot synthesize the amino (NH2) group. In order to obtain the amino acids necessary for building protein they must eat plants, or other animals which live on plants. The human body has a certain limited capability of converting one amino acid into another. This is achieved in the liver partly by the process of transamination, whereby an amino group is shifted from one molecule across to another under the influence of aminotransferase, the coenzyme of which is pyridoxal phosphate. However, the ability of the body to convert one amino acid into another is restricted. There are several amino acids which the body cannot make for itself and so must be obtained from the diet. These are termed essential amino acids. Under normal circumstances, the adult human body can maintain nitrogenous equilibrium on a mixture of eight pure amino acids as its sole source of nitrogen. They are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Several amino acids, including 3 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 4 43803_C001 Final Proof page 4 29.10.2007 7:46pm Compositor Name: JGanesan Amino Acids and Proteins for the Athlete: The Anabolic Edge arginine, histidine, and glutamine are felt by some to be conditionally essential. That is, under certain conditions such as growth, these amino acids cannot be synthesized in adequate amounts and thus need to be supplied in the diet. Synthesis of the conditionally essential and nonessential amino acids depends mainly on the formation of appropriate a-keto acids, the precursors of the respective amino acids. For instance, pyruvic acid, which is formed in large quantities during the glycolytic breakdown of glucose, is the keto acid precursor of the amino acid alanine. Then, by the process of transamination, an amino radical is transferred from certain amino acids to the a-keto acid while the keto oxygen is transferred to the donor of the amino radical. In the formation of alanine, for example, the amino radical is transferred to the pyruvic acid from one of several possible amino acid donors including asparagine, glutamine, glutamic acid, and aspartic acid. Transamination is promoted by several enzymes among which are the aminotransferases, which are derivatives of pyridoxine (B6), one of the B vitamins. Without this vitamin, the nonessential amino acids are synthesized only poorly and, therefore, protein formation cannot proceed normally. The formation of protein can also be affected by other vitamins, minerals, and nutrients. All proteins are made up of varying numbers of amino acids attached together in a specific sequence and having a specific architecture. The sequence of the amino acids differentiates one protein from another, and gives the protein special physiological and biological properties. AMINO ACIDS Amino acids are all characterized by the presence of an amino (NH2) group with basic properties (hence the term amino) and a carboxyl (COOH) group with acidic properties (hence the term acid), attached to the same carbon atom. The rest of the molecule varies with the particular amino acid. Since all amino acids contain both an acid and a base, they, unlike other biological material, are capable of both acid and base reactions in the body. The structure of an amino acid may be represented by the formula: NH2 R-CH-COOH where NH2 is the amino group, COOH is the carboxyl group, and R represents the remainder of the molecule. PEPTIDE LINKAGE In the formation of a protein, amino acids are linked together by the peptide linkage in which the basic (amino) group of one amino acid is linked to the carboxyl group of another, with the elimination of a molecule of water. Since all amino acids contain both NH2 and COOH groups, long chains of amino acids may be formed. | | | | NH2 H H CH3 | | | CH C N CH COOH || O | | | H CH3 | | H N CH COOH | | | | NH2 H | CH C OH || O H2O The resultant chain of amino acids therefore has an amino group at one end (the N-terminus) and a carboxyl group at the other end (the C-terminus). Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 5 29.10.2007 7:46pm Compositor Name: JGanesan 5 Proteins and Amino Acids | | | | | | H R H R | | | | C N CH C N CH COOH || || O O | | | | | | | | | | | NH2 R H R H R H R | | | | | | | CH C N CH C N CH C N CH || || || O O O Amino acids with side chains possessing carboxyl groups are known as acidic amino acids; those with side chains possessing amino groups are known as basic amino acids; the remaining amino acids are termed neutral amino acids. These amino acid chains are called peptides or proteins. A chain of up to 100 amino acids is called a polypeptide. Two joined amino acids form a dipeptide, three form a tripeptide, and so on. A string of amino acids form a protein when more than 100 amino acids are joined together. The great variation in properties observed among proteins, whether it be the insulin secreted by the pancreas, the collagen found in connective tissues, or the hemoglobin of the blood, is largely due to the enormous number of ways in which these amino acids can be arranged. However, there is more to a protein than the number of amino acids that makes it up. A protein is a polypeptide chain (or a number of such chains) that has attained a unique, stable, three-dimensional shape (referred to as its native conformation), and is biologically active as a result. The shape of the protein is a result of interactions between various portions of the amino acids. If a protein is denatured by way of enzymes, heating, or by chemicals, changes occur in the shape of the protein. These changes, usually an uncoiling or unfolding of the protein structure, make the protein more vulnerable to degradation or digestion, destroying its biological effectiveness. Although the number of amino acids in various proteins can vary considerably (some proteins may contain hundreds of thousands of amino acids) there are only about 23 different amino acids that are used as building blocks for biological matter. About 20 of these amino acids are commonly found in animal protein and are the amino acids upon which we will concentrate in this book. The nature of the protein is determined by the types of amino acids in the protein, and also by the order in which they are joined. All amino acids, except glycine, are asymmetrical and can assume different configurations in space giving them the characteristic of chirality. Amino acids can, therefore, exist in two forms, designated D and L, both of which are found in nature. While both forms are found in nature and have biological effects, only the L form is found in proteins. In this book, the L form amino acids are either explicitly stated or are simply written as the amino acid itself. For example, leucine means the L form of leucine or L-leucine, so L-leucine and leucine are one and the same. Protein and amino acids, like lipids and carbohydrates, are classified as macronutrients since they make up a large part of our diets. While there are recommended dietary protein intakes, there are no recommended dietary allowances set for individual amino acids. Amino acids are involved in a number of important metabolic functions besides protein synthesis and the regulation of protein synthesis and catabolism. They can be precursors to neurotransmitters and also function as neurotransmitters. They are involved in energy production, ammonia and urea production, hormone synthesis, hormone activation and release, and in preventing oxidative damage through their antioxidant effects. FREE AMINO ACID POOL Of the body’s vast content of amino acids, only a small amount is not bound up in protein structures of one sort or another. Only 0.5%–1% of the amino acids in the body is present as free amino acids and make up the body’s free amino acid pool. The free amino acid pool represents the most bioactive part of the body’s proteins and amino acid content. This pool is made up of free amino acids in plasma and in the intracellular and Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 6 43803_C001 Final Proof page 6 29.10.2007 7:46pm Compositor Name: JGanesan Amino Acids and Proteins for the Athlete: The Anabolic Edge extracellular space, with the concentration of amino acids being higher in the intracellular pool. The relatively small amounts of free amino acids in the free amino acid pool, in equilibrium with the body’s much larger amounts of proteins and amino acids, are responsible for all the metabolic and substrate protein and amino acid interactions that take place in the body, including muscle protein balance.1 Various amino acids are represented in this pool with the nonessential or conditionally essential amino acids glutamine, glycine, and alanine making up the highest concentrations. Of the essential amino acids, lysine, threonine, and the branched-chain amino acids (BCAAs)—valine, leucine, and isoleucine—are present in the highest concentrations. The amino acids present in the plasma portion and extracellular portions of the pool are in a sort of equilibrium with the amino acids in the intracellular portion. Since this equilibrium between the intracellular and extracellular portions of the amino acid pool is in most cases due to active transport of the amino acids, it is not a true equilibrium but in fact a sort of steady state that is based on both concentrations of amino acids and the existing metabolic state. As such, the levels of free and protein-bound amino acids vary considerably. Measuring one portion of the pool without the other can give a false impression of the movement or flux of amino acids through the free pool. The small free amino acid pool, while highly regulated and active, is relatively stable in size and content. However, there are some small but important changes that take place in amino acid concentrations in response to various stimuli such as exercise, food intake, and various diseases. The delivery of amino acids from the extracellular to the intracellular pool is important in both protein synthesis and breakdown since higher intracellular levels will increase synthesis and decrease breakdown and thus allow for muscle gains to occur.2 Increasing dietary protein has a positive effect on the amino acid pool and that in turn has a positive effect on protein metabolism. For example, it has been postulated that in endurance athletes the provision of exogenous amino acids from dietary protein may augment this free amino acid pool, thereby facilitating protein synthesis and decreasing the need for a concurrent increase in breakdown following exercise.3 PROTEIN SYNTHESIS In order to duplicate exact amino acid sequences, the body uses complex processes that allow copying of genetic information and translation of that information into the formation of specific proteins. Nucleic acids are polymers of nucleotides. Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are responsible for the storage, expression, and transmission of genetic information. The DNA of each individual creature is unique in the sequence of its nucleotide bases. It is this sequence which provides a set of instructions that ultimately directs each cell to synthesize a characteristic set of proteins. Thus when a certain protein is needed, the cellular DNA provides a template of that protein, which is carried to the ribosomes, the site of protein synthesis in the cell. Ribosomal RNA (rRNA) is an important constituent of the ribosomes and has a major functional role in many, and possibly all, ribosomal processes. The amino acid sequence of a polypeptide is specified by messenger RNA (mRNA). The information is carried to the ribosome by a molecule of mRNA. The mRNA transcribes, or copies, this genetic information from the DNA in a process appropriately called transcription. In the process called translation, amino acids are assembled on ribosomes according to the sequence specified by the mRNA. As the ribosome travels along the mRNA chain and translates its code, the required amino acids (floating free in the cytoplasm) are brought to the ribosome by a specific transfer RNA (tRNA) molecule, of which there is at least one for each different amino acid. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 7 29.10.2007 7:46pm Compositor Name: JGanesan Proteins and Amino Acids 7 One by one, amino acids are linked together by peptide bonds until the sequence has been completed. A recognition process between the tRNA that is linked to the amino acid and the mRNA that is bound to the ribosome on which polypeptide synthesis takes place ensures that the correct amino acid is chosen for each successive amino acid addition. In addition to information (specific order of amino acids), peptide bond formation in the cell requires energy to activate the incoming amino acid and link it to the tRNA. Following resistance training, the ribosomal machinery of skeletal muscle increases in activity and begins to decode mRNA molecules into protein more quickly.4 This is likely in preparation for the replacement and hypertrophic adaptive response to exercise. REGULATION OF PROTEIN SYNTHESIS The mechanisms underlying changes in protein metabolism are ongoing but a number of working hypotheses have been formulated. Although a discussion on these hypotheses and the evolving detailed mechanics of protein synthesis is beyond the scope of this book, I will give an overview of the processes involved with details. However, anyone interested in pursuing the intricacies involved can have a look at some of the excellent reviews5–11 on this topic. Protein synthesis in the skeletal muscle is initiated by an increased availability of amino acids and exercise. Translational control is considered to be an important area of control of gene expression and thus regulator of protein synthesis. Protein synthesis (mRNA translation) is conventionally divided into three stages: initiation, elongation, and termination. Initiation brings the mRNA to the ribosome and uses a large number of initiation factors to assemble the ribosome and begin translation. Elongation then continues to assemble amino acids to form the protein until the process is terminated. Many mechanisms and factors impact one or more of these stages. For example, both initiation and elongation can be controlled by insulin. Initiation is the stage where the mRNA and the initiator methionine tRNA (tRNAmMet) are recruited to the ribosome, initially to the small 40S subunit. The start codon is then located by a process known as ‘‘scanning’’ during which it is recognized by the anticodon of tRNAmMet. This culminates in assembly of the complete 80S ribosome at the start codon, ready to proceed into elongation where the new polypeptide is assembled. Translation initiation can be functionally divided into three stages5: 1. Binding of initiator methionyl-tRNA (met-tRNAi) to the 40S ribosomal subunit to form the 43S preinitiation complex 2. Binding of mRNA to the 43S preinitiation complex to form the 48S preinitiation complex 3. Binding of the 60S ribosomal subunit to the 48S preinitiation complex to form the active 80S initiation complex The first stage is mediated by eukaryotic initiation factor-2 (eIF2) and regulated by the activity of eIF2B, which is not under the control of the mammalian target of rapamycin (mTOR) but which may be more involved in the activation of protein synthesis by amino acid than eIF4E.12 The second regulatory stage is catalyzed by a multi-subunit complex of eukaryotic factors referred to as eIF4F.13 In contrast to formation of 43S preinitiation complex, acute provision of nutrients enhances assembly of active eIF4F complex.14,15 The step in translation initiation catalyzed by eIF4F appears to control protein synthesis and is considered the essential factor in promoting mRNAs for translation. The eukaryotic initiation factors have multiple functions in translation initiation and their various roles as we know them now are illustrated in Table 1.1. Without going into too much detail it is suffice to say that eIF4F is composed of eIF4A, eIF4E, and eIF4G. Binding of eIF4G with eIF4E appears important in accelerating mRNA translation initiation. Phosphorylation of eIF4G is associated with enhanced formation of active eIF4GeIF4E Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 8 43803_C001 Final Proof page 8 29.10.2007 7:46pm Compositor Name: JGanesan Amino Acids and Proteins for the Athlete: The Anabolic Edge TABLE 1.1 Eukaryotic Initiation Factors and Their Functions Initiation Factor eIF-1 eIF-2 eIF-2A eIF-2B (also called GEF) (guanine nucleotide exchange factor) eIF-3 composed of ~10 subunits Initiation factor complex often referred to as eIF-4F composed of three primary subunits: eIF-4E, eIF4A, eIF-4G and at least two additional factors: PABP, Mnk1 (or Mnk2) PABP: polyA-binding protein Mnk1 and Mnk2 eIF-4E kinases eIF-4A eIF-4E 4E-BP (also called PHAS), three known forms eIF-4G eIF-4B eIF-5 eIF-6 Activity Repositioning of met-tRNA to facilitate mRNA binding Ternary complex formation AUG-dependent met-tRNAimet binding to 40S ribosome GTP=GDP exchange during eIF-2 recycling Ribosome subunit antiassociation, binding to 40S subunit mRNA binding to 40S subunit, ATPase-dependent RNA helicase activity, interaction between polyA tail and cap structure Binds to the polyA tail of mRNAs and provides a link to eIF-4G Phosphorylate eIF-4E increasing association with cap structure ATPase-dependent RNA helicase 50 cap recognition When dephosphorylated 4E-BP binds eIF-4E and represses its activity, phosphorylation of 4E-BP occurs in response to many growth stimuli leading to release of eIF-4E and increased translational initiation Acts as a scaffold for the assembly of eIF-4E and -4A in the eIF-4F complex, interaction with PABP allows 50 - and 30 -end of mRNAs to interact Stimulates helicase, binds simultaneously with eIF-4F Release of eIF-2 and eIF-3, ribosome-dependent GTPase Ribosome subunit antiassociation Source: From http:==isu.indstate-edu=mwking complex leading to an increase in the rate of protein synthesis. Moreover, elevated phosphorylation of eIF4G correlates with mRNA translation in skeletal muscle during perfusion of hind limb with buffer containing leucine16 or following oral administration of a single bolus of leucine.17 eIF4E availability is dependent in part on the translation repressor protein eIF4E-binding protein-1 (4E-BP1). 4E-BP1 is a small protein that, in the hypophosphorylated state, tightly binds eIF4E and blocks the ability of eIF4E to bind to eIF4G, thereby limiting protein synthesis. Hence, 4E-BP1 may regulate eIF4GeIF4E complex assembly and protein synthesis. The mTOR is thought to be the upstream kinase that phosphorylates 4E-BP1 and thus leads to an increase in protein synthesis. Since translation initiation is regulated in part by mTOR and growthpromoting hormones such as insulin or insulin-like growth factor 1 (IGF-1) are involved in regulating it, the mTOR, which exists in signaling, controls several components of the translational machinery. mTOR exists in two complexes: mTOR complex1, which is rapamycin sensitive and phosphorylates S6K1 and initiation factor 4E binding proteins (4E-BPs), and mTOR complex2, which is rapamycin insensitive and phosphorylates protein kinase B (PKB, also known as Akt). Both mTOR complexes are stimulated by mitogens, but only mTOR complex1 is under the control of nutrient and energy inputs.18 Although the stimulation of mTOR activity and increased phosphorylation of 4E-BP1 are important in translation initiation and thus for increasing protein synthesis, other signaling Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 9 29.10.2007 7:46pm Compositor Name: JGanesan 9 Proteins and Amino Acids pathways, often interrelated and involving hormones and nutrients, also appear to modulate translation initiation,19 including S6K1,18 phosphoinositide-30 -kinase (PI-3-kinase),20 class I PI3Ks,21 protein kinase C (PKC),22 heat-shock-sensitive signaling,23 mitogen-activated protein kinases (MAPK),24 and PKB.25 For example, S6K1, a crucial effector of mTOR signaling, when phosphorylated, phosphorylates ribosomal protein S6, preferentially enhancing the translation of mRNAs including translation components that make up the protein-synthetic apparatus.26 In addition, activation of any of the above signaling pathways invariably has other ramifications. For example, activation of the PI3K=Akt pathway results in the phosphorylation of the FOXO proteins, a subgroup of the Forkhead family of transcription factors,27 which may play a critical role in the development of muscle atrophy.28 Post-exercise regulation of protein metabolism also involves various anabolic and catabolic pathways. Intense exercise results in an inflammatory response in skeletal muscles.29 Early in the recovery phase a number of events occur including the release of growth factors and proinflammatory cytokines by the damaged muscle fibers, including IL-6,30 which begins a cascade of signaling events that include the signal transducers and activators of transcription (STAT) family.31,32 Growth factors, cytokines, and other molecules are central mediators of repair and adaptation in skeletal muscle.33 Although some of the relevant pathways have been discovered, the exact signaling pathways activated have yet to be fully elucidated. For example, a recent study found that while the significance of the STAT3 signaling pathway has yet to be determined, it may contribute to the remodeling and adaptation of skeletal muscle following resistance exercise.34 FACTORS AFFECTING PROTEIN SYNTHESIS AND CATABOLISM Amino acids, while important as substrates for various metabolic pathways, are also regulators of protein synthesis and other metabolic processes. For example, like exercise, amino acids increase protein synthesis with the signaling pathways influencing mTOR, which in turn stimulates protein synthesis.35,36 Accelerated protein breakdown and a net protein loss occurs secondary to exhaustive exercise, injury, and various diseases. The negative nitrogen balance observed in such cases represents the net result of breakdown and synthesis, with breakdown increased and synthesis either increased or diminished. Under certain conditions protein catabolism can be decreased. Protein synthesis also can be increased or decreased under certain conditions. The net result depends on the conditions present and the effects on both synthesis and catabolism. For example, in order to have a net increase in protein synthesis so that there is an increase in the concentration of a protein in a cell, its rate of synthesis would have to increase or its breakdown decrease or both. There are at least four ways in which the concentration of protein in a cell could be changed: 1. Rate of synthesis of the mRNA that codes for the particular protein(s) could be increased (known as transcriptional control). 2. Rate of synthesis of the polypeptide chain by the ribosomal-mRNA complex could be increased (known as translational control). 3. Rate of degradation of the mRNA could be decreased (also translational control). 4. Rate of degradation of the protein could be decreased. Table 1.2 outlines some of the conditions or factors affecting protein synthesis of which many are interrelated. Keep in mind that protein synthesis is increased as the result of a net positive change secondary to changes in both (or less commonly one of) synthesis and catabolism. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 10 29.10.2007 7:46pm Compositor Name: JGanesan 10 Amino Acids and Proteins for the Athlete: The Anabolic Edge TABLE 1.2 Some Conditions and Factors Affecting Protein Synthesis Conditions or Factors Decreased protein intake Increased protein intake Decreased energy intake Increased cellular hydrationa Decreased cellular hydrationa Increased intake of leucine in presence of sufficiency of other amino acids Increased intake of glutamine in presence of sufficiency of other amino acids Lack of nervous stimulation Muscle stretch or exercise Overtraining Testosterone (and anabolic steroids) Growth hormone Insulin-like growth factor 1 (IGF-1) Normal thyroid hormoneb Excess thyroid hormone Catecholamines (including synthetic b-adrenergic agonist such as clenbuterol) Glucocorticoids Physical trauma, infection a b Effect on Rate of Protein Synthesis Decreased Increased Decreased Increased Decreased Increased Increased Decreased Increased Decreased Increased Increased Increased Increased Decreased Increased Decreased Decreased Cellular hydration refers to an intracellular state and as such is different from extracellular hydration. Thyroid hormones (thyroxine and triiodothyronine) stimulate both protein synthesis and degradation depending on their levels in the body. Not enough or too much can be detrimental to protein synthesis. Higher than normal levels of thyroid hormones lead to the catabolism of protein. The amount of protein synthesis that takes place during and after exercise is dependent on several factors, including a complete complement of precursor amino acids, specific acetylating enzymes, tRNA, and adequate ATP levels. While the relation of protein synthesis to the ambient concentrations of amino acids in the intracellular and extracellular pools has not been fully defined, it is possible to identify sets of intracellular amino acids that predict the level of protein synthesis, and to delineate combinations of plasma amino acids whose levels account for a significant portion of the variance in the intracellular predictor amino acids in normal human infants and adults. In one study, the intracellular concentrations of most amino acids were found to be higher than their concentrations in plasma, except for valine and citrulline, which were lower.37 The aminograms in the two pools also were very different: 44% of the variance in protein synthesis was accounted for by the intracellular concentrations of leucine, glycine, alanine, and taurine in neonates, and 45% by a combination of threonine, valine, methionine, and histidine in adults. The intracellular concentrations of each of these predictor amino acids in adults were, in turn, related to different combinations of the plasma concentrations of threonine, phenylalanine, tryptophan, isoleucine, histidine, citrulline, ornithine, arginine, and glycine. The increases in intracellular amino acid concentrations seen with exercise may reflect decreased protein synthesis, accelerated protein catabolism, an increase in amino acid transport into the cell, or combinations of these conditions. For example, an increase in protein synthesis would be expected to cause a decrease in amino acids but this may be offset by an increase in intracellular availability due to increased transport. Moreover, altered intracellular amino acid levels may directly regulate exchange diffusion of intracellular for extracellular amino acids. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 11 29.10.2007 7:46pm Compositor Name: JGanesan 11 Proteins and Amino Acids CELLULAR HYDRATION AND PROTEIN SYNTHESIS There are many factors that determine overall nitrogen balance as taking place in either a positive (anabolic) or negative (catabolic) direction including stress, trauma, hormones, and nutritional substrate availability. One way in which these factors can directly trigger changes in protein turnover is by their effect on cellular hydration.38,39 The state of intracellular hydration, or the amount of water inside cells, which is related to overall cell volume, is now thought to be one of the most important mechanisms in determining cellular protein synthesis and catabolism. It now appears that when a muscle cell’s water content is high, anabolic or protein synthesis reactions are stimulated to occur. The converse is also true: when cells are dehydrated, protein synthesis decreases and catabolism increases. Thus cellular swelling leads to anabolism whereas cellular shrinkage promotes catabolism. It is important to understand that cellular hydration refers to an intracellular state and as such is different from extracellular hydration that is manifested either as water retention or volume depletion (extracellular dehydration) as measured by the degree of peripheral edema, overall blood volume, blood pressure, and serum concentrations of electrolytes. Although distinct, both intracellular and extracellular hydration have areas of overlap. Substances that promote extracellular water retention can likewise positively influence intracellular water retention. But this must not be misconstrued to mean that general, systemic water retention is in any way anabolic. In fact, one study showed that an increase in plasma volume induced by hypertonic fluids may come entirely at the expense of cell volume, not interstitial volume.40 In this study using isolated perfused cat calf muscle, the rapid increases in plasma osmolality caused by using NaCl or sucrose resulted in cellular dehydration. The cell’s intracellular hydration is largely determined by two factors that are interdependent on each other: 1. Concentration and activity of various intracellular ions (sodium [Naþ], potassium [Kþ], and chloride [Cl]) 2. Concentration of certain substrates (particularly amino acids) within cells Each cell in the body has an electrical potential or gradient, based upon the intracellular and extracellular concentrations of the various ions mentioned above. These cells have a specific ion pump which keeps a relatively constant, but different ion gradient outside and inside the cells by regulation of passage through the plasma membrane of the cell. These pump mechanisms require energy to maintain their operations, and this energy is derived from cellular metabolism of nutrients. Many amino acids, which are taken up in the cell for various bodily functions, are actively transported inside by means of one of these pumps: the sodium ion-dependent pumps or transport systems are most common. These systems convert the energy of the electrochemical sodium gradient across the plasma membrane into osmotically active amino acid gradients with intracellular= extracellular concentration ratios of up to 30. Such gradients cause water to move into the cell and lead to cell swelling. For example, it has been shown that liver cells distend as much as 12% within 2 min under the influence of physiological concentrations of glutamine and the increased cellular hydration is maintained as long as the amino acids are present.41 As the cell becomes more concentrated with substrate, free water diffuses through the cell’s plasma membrane (osmosis) causing the cellular volume to enlarge. This signals the cell to take in more nutrients, thus stimulating protein synthesis, and inhibiting further proteolysis. Also, as the cell expands, there is more available space within it for the three-dimensional interaction of intracellular receptors and various substrates, hormones, and messenger molecules that carry signals for transcription of RNA, and later, DNA protein synthesis. It is the extent of the change in hydration (change in volume), rather than the mechanism underlying the change, that determines the extent of the proteolytic or protein-synthetic response of the cell. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 12 29.10.2007 7:46pm Compositor Name: JGanesan 12 Amino Acids and Proteins for the Athlete: The Anabolic Edge Many factors, such as hormones, adenosine, cytokines, serotonin, oxygen radicals, nutrient availability, amino acids, and metabolic flux, can affect cellular hydration.42,43 These factors either effect the efficiency of the machinery function (cellular pump mechanisms), the specific intracellular= extracellular ion gradients (through permeability changes in the cellular membrane), or the availability of substrate. Cell shrinkage is caused by the loss of Naþ or Kþ concentration or the accumulation of Cl. For example, hormones in both physiological and pharmacological concentrations modulate the activity of ion transport systems in the plasma membrane. Insulin, b-adrenergic agonists, androgenic=anabolic steroids, and growth hormone (GH) increase cellular hydration by promoting the cellular accumulation of several ions or minerals (including potassium, sodium, and chloride secondary to the activation of the Naþ=Hþ-antiporter, Naþ, Kþ, 2Cl cotransport, and the Naþ=Kþ ATPase).44 Glucagon, glucocorticoids, certain cytokines, and oxidative stress, on the other hand, cause cell shrinkage by opening Ba2þ-sensitive Kþ channels, which allows loss of Kþ from the cell. Intracellular potassium plays a pivotal role in cellular hydration (and dehydration) by exerting a limiting effect on the other cell ions. In cellular hydration it controls the extent of cellular swelling, allowing just enough to promote the anabolic signal induced by swelling, but not enough to cause overhydration. Studies have shown that the nutritional state of an animal modifies the swelling potency of amino acids and hormones in liver and by this means affects proteolysis.45 In livers from rats deprived of food for 24 h, the swelling responses to glycine, glutamine, and alanine were enhanced, whereas the insulin- and IGF-1-induced increases of cell volume were diminished. A stronger inhibition of proteolysis was observed in livers from food-deprived rats upon addition of the amino acids, whereas the insulin- and IGF-1-mediated inhibition of proteolysis was attenuated. This study also concluded that independent of the nutritional state, a close relationship between the cellular hydration state and the corresponding inhibition of proteolysis was observed, regardless of whether cell volume was modified by amino acids, hormones, hypo-osmotic exposure, or bile acids. It appears that any condition that results in stress or inflammation may lead to cell dehydration and consequent catabolism. Sepsis, or infections, including viral replication and synthesis, that cause an increase in a substance called tumor necrosis factor also favor water loss from affected cells.46,47 This explains the catabolic effects typically seen in many viral diseases, including AIDS. Other factors leading to cell swelling or hydration include lactate, glycine oxidation, ketoisocaproate (KIC, a leucine amino acid derivative), and urea synthesis from amino acids.48 Experimentally induced oxidative stress decreases cellular hydration by opening potassium (Kþ) channels in the plasma membrane, possibly by oxidation of important thiol groups or by raising intracellular calcium (Ca2þ) concentration.49 Thus it is possible that the oxidative stress that typically occurs during exercise may also promote cellular dehydration through the loss of cellular potassium. CELLULAR HYDRATION AND AMINO ACIDS Several amino acids have cell volumizing effects and can stimulate protein synthesis in this way, as well as stimulating insulin production by affecting pancreatic b-cells.50 As such, these amino acids can have dual anabolic effects secondary to the cell swelling and the increased insulin secretion. The concentration uptake of some amino acids (i.e., those transported by sodium (Naþ)-dependent mechanisms into muscle and liver cells) can increase cellular hydration, thereby triggering a proteinanabolic signal. Such a case has been postulated for both glutamine and alanine.51 In one study it was concluded that ‘‘it seems plausible to conclude that stimulation of Kþ inflow plays a major role in the mechanism of alanine-induced stimulation of protein synthesis through changes in cell volume.’’52 Cellular hydration also affects, and is affected by, cellular glycogen and amino acid content. Cell shrinkage stimulates glycogenolysis, proteolysis, and formation of organic osmolytes such as Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 13 29.10.2007 7:46pm Compositor Name: JGanesan Proteins and Amino Acids 13 amino acids, methylamines, and polyols. Cell swelling stimulates formation of glycogen and proteins and cellular release of organic osmolytes. The effects of some hormones on protein synthesis may be partially mediated by their effects on glycogen. For example, while insulin promotes glycogen synthesis and increases cellular volume, glucagon works in an opposite direction through promoting liver glycogen breakdown and inducing cell shrinkage. Several amino acids are known to stimulate glycogen synthesis via activation of glycogen synthase. The authors of one study also concluded that stimulation of glycogen synthesis by amino acids is due, at least in part, to an increase in hepatocyte volume resulting from amino acid uptake, and that hepatocyte swelling per se stimulates glycogen synthesis.53 The effects of both glutamine and alanine on cellular hydration and protein synthesis are discussed in more detail below. NUTRIENTS AND PROTEIN SYNTHESIS Nutrients affect the various anabolic and catabolic signaling pathways leading to an increase in protein synthesis, not only via enhanced translation initiation but also via signaling promoting translation elongation or a decrease in protein catabolism. For example, a recent study found that the ingestion of a leucine-enriched essential amino acid– carbohydrate mixture resulted in significant increase in muscle protein synthesis, reduced AMPK phosphorylation, increased Akt=PKB and mTOR phosphorylation, increased mTOR signaling to its downstream effectors increasing both S6K1 and 4E-BP1 phosphorylation, and reduced eEF2 phosphorylation.19 Nutrient intake results in an increase in protein synthesis and accretion in skeletal muscle, indirectly through rises in insulin and directly due to the effects of the nutrients themselves. However, nutrient intake without a supply of amino acids protein synthesis does not increase as has been shown in studies in which ingestion of a mixed meal stimulates skeletal muscle protein synthesis in food-deprived animals, whereas consumption of a protein-deficient meal does not.14,54 Leucine appears to mediate most of the effects of the intake of protein=amino acid intake on protein metabolism. The mechanisms by which leucine increases protein synthesis, while not fully understood, have been partially uncovered in the past decade, including the potential for weight and fat loss, and the retention of muscle mass. It is likely that the mTOR pathway is involved in leucine signaling, it also appears that leucine may stimulate protein synthesis through insulin-independent mechanisms even though basal levels of insulin are important for a maximal effect of leucine on protein synthesis. Although protein and amino acids in meals provide substrate for increasing protein synthesis the increase is not necessarily in proportion to the increase in amino acid concentrations. The effects of meal feeding on potential signal transduction pathways leading to protein accretion in skeletal muscle are complex and not yet fully worked out.16,55–60 A recent study suggested the following scenario for stimulation of an assembly of eIF4GeIF4E complex after meal feeding.61 . . . . . Meal feeding causes a rise in both insulin and certain amino acids, including leucine. Initial rise in insulin causes stimulation of PI-3-kinase=PKB-signal pathway leads to enhanced mTOR phosphorylation and subsequent stimulation of phosphorylation of S6K1 and 4E-BP1. BCAAs are the most robust of the plasma amino acids in their ability to cause increased phosphorylation of mTOR. Meal feeding-induced activation of mTOR phosphorylation is maintained as long as the food is present. With removal of food, the assembly of eIF4GeIF4E returns to levels observed before feeding. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 14 29.10.2007 7:46pm Compositor Name: JGanesan 14 Amino Acids and Proteins for the Athlete: The Anabolic Edge . . Their findings suggest that acute leucine-induced stimulation of protein synthesis and the phosphorylation states of 4E-BP1 and S6K1 are facilitated by the transient increases in serum insulin concentrations. Meal feeding stimulates mRNA translation initiation through both insulin-dependent and insulin-independent mechanisms mediated by increased assembly of active eIF4F complex and phosphorylation of 4E-BP1, eIF4G, and S6K1. PROTEIN CATABOLISM Protein balance is a function of intake relative to output (utilization and loss). Body proteins are in a constant state of flux with both protein degradation and synthesis constantly going on. Normally, these two processes are equal with no net loss or net gain of protein taking place. Protein intake usually equals protein lost. However, increases of both protein synthesis and protein breakdown (as many do) might have a positive or negative anabolic effect. The overall effect is anabolic if the increase in protein synthesis is greater than the increase in protein breakdown, and catabolic if protein breakdown exceeds protein synthesis. It would also be useful to quantify the anabolic effect so we can tell which substances and procedures would be more effective than others for increasing muscle mass and strength. On the other hand, a substance or activity may decrease both protein breakdown and synthesis. Again the anabolic effect would depend on the degree it does on either one. If we examine clenbuterol we see that some studies show that while clenbuterol has potent anticatabolic properties, it also, to a lesser extent, suppresses protein synthesis.62 The overall effect, however, is anabolic since the net result of the decreased protein breakdown and protein synthesis is an increase in muscle protein content. There are many variables to be considered when trying to determine the most anabolic diet and exercise regimen for athletes. Even for those well-versed in the complex hormonal interactions involved in the anabolic catabolic processes of the body, there are many areas that are confusing because of contradictions in the literature and the lack of specific studies. Muscle, being a highly plastic tissue, is subject to gain or loss of mass depending on fluctuating demands for energy and a variety of signals that respond and relay the physiological state of the whole body.63–66 As such, skeletal muscle represents a potential reserve that can be mobilized in time of need to provide energy and nutrients to nonmuscle tissues. As mentioned above, there are a number of factors that either increase or decrease protein synthesis, and thus affect skeletal muscle mass. The other side of the protein metabolism coin is protein catabolism, a side that is often overlooked in determining anabolic and catabolic effects on the skeletal muscle. One of the reasons for the lack of concern is that many of the factors that increase protein synthesis also decrease protein catabolism. Nevertheless protein synthesis and protein catabolism are both regulated by nutritional, hormonal, and neural factors and must be considered together when trying to determine the overall effect on protein metabolism since the degree of each dictates whether there is a negative or positive protein=nitrogen balance. Normally, these two processes are equal with no net loss or net gain of protein taking place so that protein intake usually equals protein lost. There are three main pathways of protein catabolism in skeletal muscle: a lysosomal pathway, a calcium-dependent pathway, and an ATP-dependent ubiquitin-proteasome pathway. Of these three, the ubiquitin-proteasome pathway is mainly responsible for muscle catabolism, including the contractile proteins and damaged proteins secondary to oxidative stress.67,68 However, although usually treated separately, the various protein synthesis and protein catabolic pathways often interact and are influenced by some of the same factors.69–71 There are many nutritional, hormonal, and neural factors that culminate in metabolic adjustments to the most diverse situations. Recently, much progress has been made on the control of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 15 29.10.2007 7:46pm Compositor Name: JGanesan Proteins and Amino Acids 15 muscle protein breakdown and the role of the different proteolytic systems in several physiological and pathological states. As a result there has been some differentiation of the antiproteolytic effects of amino acids, insulin, and leucine. Studies have shown that both amino acids and insulin act on protein synthesis and hepatic autophagy through the mTOR signaling pathway, and that leucine acts as an inhibitor.72–82 For example, inhibition of the mTOR pathway by rapamycin abolishes the effect of leucine media starvation on autophagic proteolysis in C2C12 myotubes.83 However, the inhibitory effects on protein degradation by increasing amino acid concentration by the mTOR pathway, is different from the inhibitory effects of either insulin or leucine. Studies have shown that leucine suppresses proteolysis in skeletal muscle and hepatocytes by an mTORindependent mechanism.84–86 A recent study, using C2C12 myotubes in vitro, was the first to report that the amino acids alone, or additively with insulin, decrease muscle protein degradation in an mTOR dependent manner by acting on the ubiquitin-dependent proteasome pathway.87 This suggests that the mTOR pathway is influenced by amino acids other than the BCAA group. Several of the other amino acids may be involved. For example, it has been proposed that glutamate and glutamine have suppressive effects on proteolysis in the muscle88 and studies in other cell types have demonstrated that these amino acids influence the mTOR signaling pathway.89,90 The studies’ findings and conclusions are as follows: . . . . . . . . Varying the concentration of leucine while maintaining all other amino acids at a low level does not mimic the effect of varying all amino acid concentrations suggesting that the response to amino acids is more than a response to alterations in leucine as has been previously suggested.83 Depletion of a mixture of amino acid increases proteasome-dependent proteolysis and the expression of the components of the ubiquitin-proteasome pathway. These effects of amino acids were additive with those of insulin, not simply related to changes in leucine, and appear to be mediated by the mTOR pathway since the effects of amino acids on proteolysis, but not insulin and leucine, are blocked by inhibition of the mTOR signaling pathway. Amino acids, but not leucine, regulate protein degradation and 14 kDa E2 expression through the mTOR signaling pathway. Leucine can modulate the expression of components of the ubiquitin-proteasome pathway. Leucine increases the inhibitory effect of insulin on protein degradation. Amino acids and insulin downregulate expression of components of ubiquitin-proteasome pathway. Amino acids and insulin act additively to regulate total proteolysis. ADAPTIVE RESPONSE TO EXERCISE Exercise has profound effects on the skeletal muscle, which increases its contractile protein content resulting in muscular hypertrophy as it successfully adapts to increasing workloads. Studies have shown that certain stimuli produce muscle hypertrophy. In a review on the effects of exercise on protein turnover in man, Rennie et al.91 concluded the following: 1. Exercise causes a substantial rise in amino acid catabolism. 2. Amino acids catabolized during exercise appear to become available through a fall in whole-body protein synthesis and a rise in whole-body protein breakdown. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 16 29.10.2007 7:46pm Compositor Name: JGanesan 16 Amino Acids and Proteins for the Athlete: The Anabolic Edge 3. After exercise, protein balance becomes positive through a rise in the rate of whole-body synthesis in excess of breakdown. 4. Studies of free 3-methylhistidine in muscle, plasma, and urine samples suggest that exercise decreases the fractional rate of myofibrillar protein breakdown, in contrast with the apparent rise in whole-body breakdown. In contrast to exercise, most of the increased proteolysis during fasting is due to the degradation of myofibrillar proteins (contractile proteins) in skeletal muscle.92,93 A number of studies have examined the influence of exercise on protein synthesis and protein degradation. In general, it seems that exercise suppresses protein synthesis and stimulates protein degradation in skeletal muscle proportional to the level of exertion. In one early study using rats, mild exercise decreased protein synthesis by 17%. More intense treadmill running reduced synthesis by 30% and an exhaustive 3 h run inhibited synthesis by 70%.94 Another study examined the effects of exhaustive running on protein degradation and found that exhaustive running stimulates protein degradation in skeletal muscle.95 Other studies have also shown that exercise produces a catabolic condition. In a study on aerobic exercise in humans, six male subjects were made to exercise on a treadmill for 3.75 h at 50% VO2 max and the rates of protein synthesis and degradation were measured.96 During exercise there was a 14% decrease in protein synthesis and a 54% increase in the rate of degradation. This is one of the few studies to make measurements during recovery after exercise. The authors of this study found that after exercise protein synthesis increased above the initial resting levels while protein degradation decreased returning eventually to preexercise levels. Any gains in the recovery phase, therefore, seem to be due to increases in protein synthesis rather than to decreases in protein catabolism. In another study, multiple amino acid tracers were used to further elucidate the changes in protein synthesis and degradation which occur during prolonged, aerobic exercise.97,98 In these studies male subjects were made to exercise on a bicycle ergometer at 30% VO2 max for 1.75 h. The results of the study showed that while exercise inhibited protein synthesis, the degree of inhibition varied depending on the amino acid tracer used. For example, the authors found 48% decrease in protein synthesis using labeled leucine and 17% using lysine. With the relatively light workloads used in these studies, there were no changes in protein degradation or increases in urea production. Even though the acute effect of exercise on protein turnover is catabolic, the long-term effects are an overall increase in protein synthesis and lean body mass. Routine exercise produces maintenance or hypertrophy of muscle mass. There are few studies which have looked at the postexercise recovery of protein turnover. The above report and a later report by Devlin et al.99 suggest that recovery occurs through stimulation of protein synthesis. Preliminary studies by Layman et al.100 reported in the second edition of Nutrition in Exercise and Sport provide additional support for this recovery pattern. The authors found that after a 2 h bout of running on a motor-driven treadmill at 26 m=min, protein synthesis in the gastrocnemius muscle was suppressed by 26%–30% in fasted male rats. Recovery of protein synthesis occurred during the next 4–8 h even if the animals are withheld from eating. These data suggest that muscles have a very high capacity for recovery even during conditions of food restriction. Several recent studies have shown that exercise training and protein intakes before and after exercise are important for maximizing the effects of exercise on muscle protein synthesis and on subsequent recovery.101–107 Just what factors control protein synthesis during exercise is being worked out although there are some general trends and associations that have been recognized. For example, high-intensity, exhaustive bouts of exercise produce a transient catabolic effect on protein synthesis. This effect is controlled presumably at the translation level of protein synthesis. Transcription is depressed but RNA concentrations are unchanged during the relatively brief period of the exercise bout. At the Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 17 29.10.2007 7:46pm Compositor Name: JGanesan Proteins and Amino Acids 17 level of translation, potential regulatory controls include (1) availability of substrates, (2) hormones, (3) energy states, and (4) initiation factors.100 Recent studies have shown that in part the decline in protein synthesis is a result of repressed signaling through a protein kinase referred to as the mTOR, which in turn affects translation initiation and elongation phases of mRNA translation.108–110 Amino acids impact on macronutrient metabolism in many ways. One of the ways is by activating a nutrient sensitive, mTOR-mediated signaling pathway in synergy with insulin resulting in a stimulation of protein synthesis and inhibition of proteolysis.111 EXERCISE-INDUCED AMINO ACID FLUX The changes that occur in the BCAAs, alanine, and glutamine in the liver, plasma, and muscle during exercise suggest that individual amino acids may be limiting as substrates for protein synthesis and that they may be an important source of energy for various tissues and organs in the body. In general, decreases in protein synthesis and increases in protein degradation produce a net release of amino acids into the intracellular free pool which may or may not be reflected by increased plasma levels. Exercise is accompanied by changes in anabolic and catabolic hormones. It would appear that the molecular mechanism for the action of these hormones on translation remains equivocal, but most evidence points to changes in the initiation phase of translation. 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Biophys. Acta, 414(1), 71–84, 1975. 55. Yoshizawa, F., Sekizawa, H., Hirayama, S., Hatakeyama, A., Nagasawa, T., and Sugahara, K., Time course of leucine-induced 4E-BP1 and S6K1 phosphorylation in the liver and skeletal muscle of rats, J. Nutr. Sci. Vitaminol. (Tokyo), 47(4), 311–315, 2001. 56. Yoshizawa, F., Kido, T., and Nagasawa, T., Rapid dephosphorylation of eIF4E by dietary protein in the skeletal muscle and liver of food-deprived rats, Biosci. Biotechnol. Biochem., 65(4), 958–961, 2001. 57. Yoshizawa, F., Kido, T., and Nagasawa, T., Stimulative effect of dietary protein on the phosphorylation of p70 S6 kinase in the skeletal muscle and liver of food-deprived rats, Biosci. Biotechnol. Biochem., 63(10), 1803–1805, 1999. 58. Lynch, C.J., Fox, H.E., Vary, T.C., Jefferson, L.S., and Kimball, S.R., Regulation of amino-acid sensitive TOR signaling by leucine analogs in adipocytes, J. Cell. Biochem., 77, 234–251, 2000. 59. Anthony, J.C., Yoshizawa, F., Anthony, T.G., Vary, T.C., Jefferson, L.S., and Kimball, S.R., Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway, J. Nutr., 130, 2413–2419, 2000. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 20 29.10.2007 7:46pm Compositor Name: JGanesan 20 Amino Acids and Proteins for the Athlete: The Anabolic Edge 60. Lynch, C.J., Hutson, S.M., Patson, B.J., Vaval, A., and Vary, T.C., Tissue-specific effects of chronic dietary leucine and norleucine supplementation on protein synthesis in rats, Am. J. Physiol. Endocrinol. Metab., 283, E824–E835, 2002. 61. Vary, T.C. and Lynch, C.J., Meal feeding enhances formation of eIF4F in skeletal muscle: Role of increased eIF4E availability and eIF4G phosphorylation, Am. J. Physiol. Endocrinol. Metab., 290(4), E631–E642, 2006. 62. Benson, D.W., Foley, N.T., Chance, W.T., Zhang, F.S., James, J.H., and Fischer, J.E., Decreased myofibrillar protein breakdown following treatment with clenbuterol, J. Surg. Res., 50(1), 1–5, 1991. 63. Rooyackers, O.E. and Nair, K.S., Hormonal regulation of human muscle protein metabolism, Annu. Rev. Nutr., 17, 457–485, 1997. 64. Welle, S., Human Protein Metabolism, Springer, New York, 1999. 65. Jackman, R.W. and Kandarian, S.C., The molecular basis of skeletal muscle atrophy, Am. J. Physiol. Cell. Physiol., 287(4), 834–843, 2004. 66. Szewczyk, N.J. and Jacobson, L.A., Signal-transduction networks and the regulation of muscle protein degradation, Int. J. Biochem. Cell. Biol., 37(10), 1997–2011, 2005. 67. Attaix, D., Aurousseau, E., Combaret, L., Kee, A., Larbaud, D., Ralliere, C., Souweine, B., Taillandier, D., and Tilignac, T., Ubiquitin-proteasome dependent proteolysis in skeletal muscle. Reprod. Nutr. Dev., 38, 153–165, 1998. 68. Davies, K.J., Degradation of oxidized proteins by the 20S proteasome, Biochem., 83, 301–310, 2001. 69. Holecek, M., Muthny, T., Kovarik, M., and Sispera, L., Proteasome inhibitor MG-132 enhances wholebody protein turnover in rat, Biochem. Biophys. Res. Commun., 345(1), 38–42, 2006. 70. Smith, I.J. and Dodd, S.L., Muscle: Calpain activation causes a proteasome-dependent increase in protein degradation and inhibits the Akt signalling pathway in rat diaphragm muscle, Exp. Physiol., 92(3), 561–573, 2007. 71. Zhang, P., Chen, X., and Fan, M., Signaling mechanisms involved in disuse muscle atrophy, Med. Hypotheses, 69(2), 310–321, 2007. 72. Lassar, A.B., Buskin, J.N., Lockshon, D., Davis, R.L., Apone, S., Hanschka, S.D., and Weintraub, H., MyoD is a sequence specific DNA binding protein requiring a region of Myc homology to bind to the muscle creatine kinase enhancer, Cell, 58, 823–831, 1989. 73. Weintraub, H., The MyoD family and myogenesis: Redundancy networks and thresholds, Cell, 75, 1241–1244, 1993. 74. Blommaart, E.F., Luiken, J.J., Blommaart, P.J., van Woerkom, G.M., and Meijer, A.J., Phosphorylation of ribosomal protein S6 is inhibitory for autophagy in isolated rat hepatocytes, J. Biol. Chem., 270, 2320–2326, 1995. 75. Pullen, N. and Thomas, G., The modular phosphorylation and activation of p70 S6 kinase, FEBS Lett., 410, 78–82, 1998. 76. Xu, G., Kevon, G., Marshall, C., Lin, T.A., Lawrence, J.C., and McDaniel, M.L., Branched chain amino acids are essential in the regulation of PHS-1 and p70 S6 kinase by pancreatic beta cells: A possible role in protein translation and mitogenic signalling, J. Biol. Chem., 273, 28178–28184, 1998. 77. Busquets, S., Alvarez, B., Llovera, M., Agell, N., Lopez-Soriano, F.J., and Argiles, J.M., Branched chain amino acids inhibit proteolysis in rat skeletal muscle: Mechanisms involved, J. Cell. Physiol., 184, 380–384, 2000. 78. Fawcett, J., Hamel, F.G., and Duckworth, W.C., Characterisation of the inhibition of protein degradation by insulin in L6 cells, Arch. Biochem. Biophysics, 385, 357–363, 2001. 79. Crown, A.L., He, X.L., Holly, J.M.P., Lightman, S.L., and Stewart, C.E.H., Characterisation of the IGF-1 system in a primary adult human skeletal cell model and comparison of the effects of insulin and IGF-1 on protein metabolism, J. Endocrinol., 167, 403–415, 2001. 80. Kadowaki, M. and Kanazawa, T., Amino acids as regulators of proteolysis, J. Nutr., 133, 2052S–2056S, 2003. 81. Kee, A.J., Comaret, L., Tilignac, T., Souweine, B., Aurousseau, E., Dalle, M., Taillandier, D., and Attaix, D., Ubiquitin-proteasome dependent muscle proteolysis responds slowly to insulin release and refeeding in starved rats, J. Physiol., 546(3), 765–776, 2003. 82. Proud, C.G., Regulation of protein synthesis by insulin, Biochem. Soc. Trans., 34, 213–216, 2006. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 21 29.10.2007 7:46pm Compositor Name: JGanesan Proteins and Amino Acids 21 83. Mordier, S., Deval, C., Bechet, D., Tassa, A., and Ferrara, M., Leucine limitation induces autophagy and activation of lysosome-dependent proteolysis in C2C12 myotubes through a mammalian target of rapamycin independent signalling pathway, J. Biol. Chem., 275, 29900–29906, 2000. 84. Dann, S.G. and Thomas, G., The amino acid sensitive TOR pathway from yeast to mammals, FEBS Lett., 580, 2821–2829, 2006. 85. Nakashima, K., Ishida, A., Yamazaki, M., and Abe, H., Leucine suppresses myofibrillar proteolysis by down-regulating ubiquitin-proteasome pathway in chick skeletal muscles, Biochem. Biophys. Res. Commun., 336, 660–666, 2005. 86. Kanazawa, T., Taneike, I., Akaishi, R., Yoshizawa, F., Furuya, N., Fujimura, S., and Kadowaki, M., Amino acids and insulin control autophagic proteolysis through different signaling pathways in relation to mTOR in isolated rat hepatocytes, J. Biol. Chem., 279, 8452–8459, 2004. 87. Sadiq, F., Hazlerigg, D.G., and Lomax, M.A., Amino acids and insulin act additively to regulate components of the ubiquitin-proteasome pathway in C2C12 myotubes, BMC Mol. Biol., 8, 23, 2007. 88. Holecek, M., Relation between glutamine, branched-chain amino acids, and protein metabolism, Nutrition, 18, 130–133, 2002. 89. Xia, Y., Wen, H.Y., Young, M.E., Guthrie, P.H., Taegtmeyer, H., and Kellems, R.E., Mammalian target of rapamycin and protein kinase A signaling mediate the cardiac transcriptional response to glutamine, J. Biol. Chem., 278, 13143–13150, 2003. 90. Tokunaga, C., Yoshino, K., and Yonezawa, K., mTOR integrates amino acid- and energy-sensing pathways, Biochem. Biophys. Res. Commun., 313, 443–446, 2004. 91. Rennie, M.J., Edwards, R.H., Krywawych, S., Davies, C.T., Halliday, D., Waterlow, J.C., and Millward, D.J., Effect of exercise on protein turnover in man, Clin. Sci., 61(5), 627–639, 1981. 92. Lowell, B.B., Ruderman, N.B., and Goodman, M.N., Regulation of myofibrillar protein degradation in rat skeletal muscle during brief and prolonged starvation, Metabolism, 35, 1121–1127, 1986. 93. Li, J.B. and Goldberg, A.L., Effects of food deprivation on protein synthesis and degradation in rat skeletal muscle, Am. J. Physiol., 246, E32–E37, 1984. 94. Dohm, G.L., Tapscott, E.B., Barakat, H.A., and Kasperek, G.J., Measurement of in vivo protein synthesis in rats during an exercise bout, Biochem. Med., 27, 367–372, 1982. 95. Dohm, G.L., Williams, R.T., and Kasperek, G.J., Increased excretion of urea and N tau-methylhistidine by rats and humans after a bout of exercise, J. Appl. Physiol., 52(1), 27–33, 1982. 96. Millward, D.J., Davies, C.T., Halliday, D., Wolman, S.L., Matthews, D., and Rennie, M., Effect of exercise on protein metabolism in humans as explored with stable isotopes, Fed. Proc., 41(10), 2686–2691, 1982. 97. Wolfe, R.R., Wolfe, M.H., Nadel, E.R., and Shaw, J.H.F., Isotopic determinations of amino acid–urea interactions in exercise, J. Appl. Physiol., 56, 221–229, 1984. 98. Wolfe, R.R., Goodenough, R.D., Wolfe, M.H., Royle, G.T., and Nadel, E.R., Isotopic analysis of leucine and urea metabolism in exercising humans, J. Appl. Physiol., 52, 458–466, 1982. 99. Devlin, J.T., Brodsky, I., Scrimgeour, A., Fuller, S., and Bier, D.M., Amino acid metabolism after intense exercise, Am. J. Physiol., 258(2 Pt 1), E249–E255, 1990. 100. Layman, D.K., Paul, G.L., and Olken, M.H., Amino acid metabolism during exercise, in: Wolinsky, I. and Hickson, J.F. Jr. (Eds.), Nutrition in Exercise and Sport, CRC Press, Boca Raton, FL, 1994, p. 127. 101. Flakoll, P.J., Judy, T., Flinn, K., Carr, C., and Flinn, S., Postexercise protein supplementation improves health and muscle soreness during basic military training in Marine recruits, J. Appl. Physiol., 96(3), 951–956, 2004. 102. Levenhagen, D.K., Carr, C., Carlson, M.G., Maron, D.J., Borel, M.J., and Flakoll, P.J., Postexercise protein intake enhances whole-body and leg protein accretion in humans, Med. Sci. Sports. Exerc., 34(5), 828–837, 2002. 103. Esmarck, B., Andersen, J.L., Olsen, S., Richter, E.A., Mizuno, M., and Kjaer, M., Timing of postexercise protein intake is important for muscle hypertrophy with resistance training in elderly humans, J. Physiol., 535(Pt 1), 301–311, 2001. 104. Levenhagen, D.K., Gresham, J.D., Carlson, M.G., Maron, D.J., Borel, M.J., and Flakoll, P.J., Postexercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis, Am. J. Physiol. Endocrinol. Metab., 280(6), E982–E993, 2001. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C001 Final Proof page 22 29.10.2007 7:46pm Compositor Name: JGanesan 22 Amino Acids and Proteins for the Athlete: The Anabolic Edge 105. Willoughby, D.S., Stout, J.R., and Wilborn, C.D., Effects of resistance training and protein plus amino acid supplementation on muscle anabolism, mass, and strength, Amino Acids, 32(4), 467–477, 2007. 106. Bird, S.P., Tarpenning, K.M., and Marino, F.E., Liquid carbohydrate=essential amino acid ingestion during a short-term bout of resistance exercise suppresses myofibrillar protein degradation, Metabolism, 55(5), 570–577, 2006. 107. Tipton, K.D., Elliott, T.A., Cree, M.G., Aarsland, A.A., Sanford, A.P., and Wolfe, R.R., Stimulation of net muscle protein synthesis by whey protein ingestion before and after exercise, Am. J. Physiol. Endocrinol. Metab., 292(1), E71–E76, 2007. 108. Gautsch, T.A., Anthony, J.C., Kimball, S.R., Paul, G.L., Layman, D.K., and Jefferson, L.S., Availability of eIF-4E regulates skeletal muscle protein synthesis during recovery from exercise, Am. J. Physiol., 274, C406–C414, 1998. 109. Williamson, D.L., Kubica, N., Kimball, S.R., and Jefferson, L.S., Exercise-induced alterations in extracellular signal-regulated kinase 1=2 and mammalian target of rapamycin (mTOR) signalling to regulatory mechanisms of mRNA translation in mouse muscle, J. Physiol., 573(Pt 2), 497–510, 2006. 110. Atherton, P.J. and Rennie, M.J., Protein synthesis a low priority for exercising muscle, J. Physiol., 573(Pt 2), 288–289, 2006. 111. Meijer, A.J. and Dubbelhuis, P.F., Amino acid signalling and the integration of metabolism, Biochem. Biophys. Res. Commun., 313(2), 397–403, 2004. 112. Morgan, H.E., Earl, D.C.N., Broadus, A., Wolpert, E.B., Giger, K.E., and Jefferson, L.S., Regulation of protein synthesis in heart muscle. I. 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Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 23 2.11.2007 12:16pm Compositor Name: VAmoudavally and Protein 2 Exercise Metabolism INTRODUCTION Exercise has been shown to have a profound influence on statural, hypertrophic, and reparative growth throughout the human life span in both men and women.1,2 Exercise can facilitate statural growth and provides the necessary mechanical and metabolic stimuli (through the secretion of GH and other anabolic hormones) that are necessary for hypertrophy of the musculoskeletal system and for reparative growth.3 Exercise reverses the ultrastructural changes seen with immobilization (loss of myofilaments and shrinkage of muscle fibers),4 and is such a potent anabolic stimulus that muscle hypertrophy can occur even under nutritionally unfavorable conditions or in the presence of wasting diseases such as AIDS5 and malignancy.6 It appears that increased tension development (either passive or active) is the critical event in initiating compensatory growth and that this process appears to be independent of GH and insulin, as well as testosterone and thyroid hormones.7 One study looked at the combined effects of exercise and energy restriction on muscle mass.8 The findings of this study suggest that protein synthesis is stimulated by exercise even with energyrestricted diets and that energy intake and exercise have independent effects on the regulation of muscle mass and protein synthesis. Another study looked at protein turnover rates of two human subjects during an unassisted crossing of Antarctica.9 During the Austral summer of 1992–1993, two men walked 2300 km across Antarctica in 96 days, unassisted by other men, animals, or machines. During the journey they ate a high-energy diet of freeze-dried rations containing 56.7% fat, 35.5% carbohydrate, and 7.8% protein. Despite this high-energy intake both men lost more than 20 kg in body weight due to their extremely high energy expenditures. Studies of protein turnover using [15N]glycine by the single-dose end-product method were made before, during, and after the journey, and these demonstrated considerable differences in the metabolic responses of the two men to the combined stresses of exercise, cold, and undernutrition. However, both men maintained high and relatively stable levels of protein synthesis during the expedition. Thus even when one does not take in enough calories (usually intentionally while trying to lose weight) one can increase or at least maintain functioning muscle mass through resistance exercise.10–12 Exercise, or more correctly, the right kind of exercise is absolutely essential in producing muscle hypertrophy, and for increasing lean body mass and strength. For example, in one study two groups of obese females were compared; one group exercised while the other group did not.13 Both groups were on a calorie-reduced diet that contained 80 g of protein. The weight-training group showed a significant muscle hypertrophy in the muscles exercised although both groups lost the same weight. Although muscle hypertrophy in certain muscles can occur under hypocaloric conditions, significant muscle hypertrophy cannot occur if there is an inadequate intake of protein. Dietary protein levels must be sufficiently high in order to provide the substrate needed for muscular hypertrophy. In general, in order to increase muscle mass, increased exercise intensity must be 23 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 24 2.11.2007 12:16pm Compositor Name: VAmoudavally 24 Amino Acids and Proteins for the Athlete: The Anabolic Edge accompanied by an increased dietary protein intake. When intensity of effort is high and the body is stimulated to adapt by increasing muscle mass, dietary protein intake must also be high. Moreover, once a certain threshold of work intensity is crossed, dietary protein, and certain protein and amino acid supplements, become extremely important in augmenting the anabolic effects of exercise, not only by providing necessary substrates but also by directly influencing protein synthesis and catabolism and by influencing the hormonal milieu. EFFECTS OF EXERCISE ON PROTEIN SYNTHESIS AND DEGRADATION Acute bouts of exercise can induce measurable changes in protein, carbohydrate, and lipid metabolism. These changes are characterized by a change in protein catabolism and synthesis and an increased utilization of protein for gluconeogenesis and lipids for oxidative fuel.14–17 Chronic daily exercise leads to adaptive processes that result in a net increase in total body as well as peripheral nitrogen stores.18 Exercise has profound effects on protein synthesis and degradation, and on the endogenous anabolic and catabolic hormones, which in turn modulate the adaptation response to exercise.19–30 In general protein synthesis is suppressed during exercise while protein degradation appears to be increased; whereas in the recovery period, during which hypertrophy occurs, protein synthesis is increased while protein degradation is suppressed in those muscle bearing the greatest load.31–36 Shortly after training, protein synthesis increases with a peak in synthesis rate of between 3 and 24 h, with elevated rates lasting as long as 48–72 h after exercise.37–44 EFFECTS OF PROTEIN AND AMINO ACIDS ON EXERCISE PERFORMANCE An increase in protein intake by itself has been shown to not only increase protein synthesis and decrease muscle breakdown but also to increase both aerobic and anaerobic performance.45–48 Amino acid supplementation can have dramatic effects on various aspects of exercise, increasing protein synthesis and=or reducing degradation, and has been shown to attenuate muscle damage and enhance recovery.49–51 Data from one study showed that amino acid supplementation attenuates muscle damage during the initial high stress of overreaching, and the authors felt that this effect may partially explain the enhanced ability of the subjects to maintain muscle strength.52 Oral or infused amino acids increase amino acid availability and protein synthesis by influencing several pathways. Amino acids are important intermediary metabolites and also function as signaling molecules with insulin-like effects. For example, supplementation with only 6 g of essential amino acids has been shown to elevate protein synthesis.53 BCAAs have been shown to reduce proteolysis in rat skeletal muscle (SM)54 and leucine has been shown to elevate protein synthesis via posttranscriptional mechanisms (phosphorylation of p70S6 and eIF4E-binding protein 1).55 HORMONAL RESPONSE TO EXERCISE Before discussing hormonal levels and their response to exercise it is important to realize that variations in these hormones may or may not reflect an increase or decrease in their biological actions. For example, a decline in testosterone after exercise may be due to decreased synthesis or excretion and=or increased cellular intake. As such, a decrease in serum testosterone levels may actually represent an increase in secretion but an even greater increase in cellular uptake, so that in balance, the serum levels are lower even though, effectively, more testosterone is being produced Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 25 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 25 and there is more of an anabolic effect produced. This should be kept in mind when discussing any hormonal changes in response to stimuli such as exercise and nutrition. Although some studies show clear and consistent hormonal responses to exercise (as outlined below), others do not.56–68 Some of the conflicting results may be the result of different exercise protocols. The kind of exercise stimulus, whether maximal, submaximal, acute, constant, etc., has an important effect on the hormonal response. For example, in one study that found an absence of a specific serum androgenic response with strength development, the authors noted that the reason for this absence may be related to the fact that the constant exercise stimulus is not conducive to establishing clear-cut hormone–strength relationships.69 In another study, there was an attenuated GH response to resistance exercise if it was preceded by sprint exercise.70 As we shall see below, the effect of exercise on serum testosterone and GH depends, among other things, on the intensity and duration of exercise, with regimens using moderate exercise intensity and shorter rest periods increasing GH, testosterone, and cortisol secretions.71 However, it has also been shown that levels of these two hormones vary in response to the fitness and expertise of the athlete.72 Both heavy-resistance exercise and prolonged stressful exercise produce significant changes in serum levels of many potentially anabolic and catabolic hormones including testosterone, GH, IGF-1, the catecholamines, insulin, and cortisol. The nature and extent of the hormonal responses can be quite different depending on many factors including the type and length of exercise, age, and training status. Unless the exercise is prolonged or fatiguing, the general hormonal trend is an increase in GH and cortisol, and a decrease or less commonly an increase in testosterone during exercise. During the recovery period there is a fall in cortisol, a rise in GH, and a rise in testosterone. If there is a successful adaptive response to training over several weeks, the basal levels of testosterone and GH increase, while those of cortisol decrease. Overtraining, however, seems to have an adverse effect on serum hormone levels,73 although chronic overtraining appears to decrease the detrimental effects of stressful training on the endocrine system.74 Age is definitely a factor although the authors of one study concluded that older individuals are capable of similar hormonal responses to submaximal exercise of identical durations and intensities as their young and middle-aged counterparts, and that chronic endurance training can enhance the hormonal response to exercise in all age groups.75 In women, overtraining or endurance training may lead to hypothalamic-pituitary-ovarian axis suppression and exerciseinduced amenorrhea.76,77 Hormonal responses may also differ during training and competition in the same sport. Thus in some sports, there appears to be significant differences in plasma levels and urinary excretion of androgen hormones (testosterone, epitestosterone, androsterone, etiocholanolone, 11-hydroxyandrosterone, and 11-hydroxy-etiocholanolone) and plasma sex-hormone binding globulin (SHBG) after training and after competition. These differences may be due to the different stressors involved in competition in comparison to training. For example, in one study involving 16 professional racing cyclists, the urinary concentrations of androgen hormones decreased during the period of training and increased during competition, this being the reverse of what happened to SHBG plasma concentrations.78 Detraining has significant effects on hormonal parameters. A study, which investigated the effects of 14 days of resistive exercise detraining on 12 power athletes, found significant changes in serum hormones. Levels of plasma GH, testosterone, and the testosterone to cortisol ratio increased, whereas plasma cortisol and creatine kinase enzyme levels decreased.79 The authors remarked that changes in the hormonal milieu during detraining may be conducive to an enhanced anabolic process, but such changes may not materialize at the tissue level in the absence of the overload training stimulus. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 26 2.11.2007 12:16pm Compositor Name: VAmoudavally 26 Amino Acids and Proteins for the Athlete: The Anabolic Edge HORMONES We have known for decades that testosterone, insulin, GH, glucocorticoids, IGF-1, thyroid hormones, and other hormones regulate body protein metabolism.80 These hormones are especially important in relation to exercise and their effects is at times relative to the availability of amino acids as is seen in the case of insulin. The eight main hormones to be considered when attempting to maximize the anabolic effects of exercise are: . . . . . . . . Testosterone (and in women especially, other androgens) Growth hormone Insulin-like growth factor-I Insulin Thyroid Cortisol Glucagon Catecholamines Generally testosterone, GH, IGF-1, insulin, catecholamines, and physiological levels of thyroid are anabolic in nature while cortisol, glucagon, and high levels of thyroid hormone are catabolic. However, we must be careful in making absolute statements about the actions of specific hormones, since these actions are complex and interdependent. The hormones can act differently and sometimes contradictory depending on the metabolic environment and the presence of other hormones. When a number of influences coexist, the effect on any one hormone or group of hormones is difficult to predict. The complex relationship among the gonadal and adrenal steroids, including the glucocorticoids, testosterone, and the hypothalamic-pituitary hormones, has yet to be fully elucidated. It is now known that many of the hypothalamic-releasing and pituitary-stimulating hormones cross react, stimulating more than just the supposed target glands. For example, there is some evidence to show that protirelin (TRH), besides stimulating thyrotropin (TSH) and thyroid secretion, also stimulates endogenous GH, prolactin, and luteinizing hormone (LH), and therefore endogenous testosterone secretion.81 Although protein metabolism is felt to be a complicated balance between catabolic (glucocorticoids and thyroid hormones) and anabolic (insulin, GH, and androgens) hormones,82 this is not necessarily always the case. For example, at certain levels thyroid hormone can either be anabolic (physiological levels) or catabolic (higher levels) in its effect. Also some hormones have more than one mode of action. Insulin, for example, is a potent anticatabolic and anabolic hormone and is one of the most important factors controlling overall protein metabolism. Moreover, increasing serum insulin may cause a rise in endogenous androgens.83 Thus we see a direct anabolic effect by the insulin and a possible indirect anabolic action by the increase in serum androgens. A hormone can also regulate the secretion of other hormones and thereby produce a more complex spectrum of actions, an important instance being the stimulation by catecholamines of insulin and glucagon release. Therefore, when considering the physiological influence of alterations in certain circulating hormones, attention must be given to the impact these alterations have on other regulatory and counterregulatory hormones. The metabolic roles of hormones that affect protein synthesis have been uncovered by a variety of studies including those on the catabolic effects of surgery, disease, and trauma and the ways to counteract these effects. For example, several adjuvant therapies such as anabolic steroids,84,85 pharmacologic doses of GH,86–88 insulin,89 and amino acids that influence the hormones such as ornithine a-ketoglutarate,90,91 BCAAs,92,93 dipeptides,94 and glutamine95 have been instituted, supplementing postoperative nutrition to decrease protein catabolism and improve nitrogen retention. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 27 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 27 Before we discuss the effects of dietary protein and protein and amino acid supplements on the anabolic and catabolic hormones, it is important to discuss the individual and collective roles of these hormones on protein metabolism. GROWTH HORMONE GH, a polypeptide (also known as adenohypophyseal growth hormone, hypophyseal growth hormone, anterior pituitary growth hormone, phyone, pituitary growth hormone, somatotropic hormone, STH, and somatotropin), is one of the hormones produced by the anterior portion of the pituitary gland (situated at the base of the brain under the hypothalamus) under the regulation of the hypothalamic hormones, GH-releasing hormone (GHRH, somatoliberin), somatostatin, and galanin.96–98 The anterior pituitary also secretes other hormones that have an effect on protein metabolism including LH, FSH, prolactin (PRL), adrenocorticotrophin hormone (ACTH), and thyroid-stimulating hormone (TSH).99 The metabolic effects of human GH include promotion of protein conservation, stimulation of lipolysis, and fat oxidation. GH infusion increases lipolysis as evidenced by increases in glycerol and nonesterified fatty acid concentrations100 and also increases the incorporation of amino acids into protein.101 In adults, the GH=IGF-1 (somatomedin C) axis has been shown to be important in muscle homeostasis.102,103 Therapeutic administration of GH has been shown to reverse muscle loss associated with the aging process.104 GH administered to humans and animals results in nitrogen retention with an increase in protein synthesis and a decrease in protein catabolism. Quantitatively, the attenuation of losses of glutamine and alanine appear to dominate. This has led to the question of whether this anabolic agent should be used in connection with specific substrates, such as glutamine or glutamine precursors (such as a-ketoglutarate) in therapeutic applications.105 Although the presence of GH is important for physiological actions,106 it is not merely a permissive effect since it appears that the total amount and pattern of GH secretion are the determining factors of the growth rate.107,108 In one study, the effects of low and high GH on body composition, muscle protein metabolism, and serum lipids were studied in seven fit adults without GH deficiency.103 The authors concluded that GH alters body composition and muscle protein metabolism, and decreases stored and circulating lipids in fit adults with a preexisting supranormal body composition and that the magnitude of these effects were dose dependent. In a study done to determine whether doubling the GH dose would enhance its anabolic effects and facilitate fat loss, the results showed that GH can induce significant anabolic responses even when caloric intake is decreased and that the degree and duration of these anabolic responses are dependent on the GH dose given.109 Although GH has been shown to increase protein synthesis if adequate caloric intake and dietary protein is provided,110 the anticatabolic and anabolic effects of GH are evident even with calorie deprivation. In several studies, it has been shown that increased levels of endogenous and exogenous GH result in nitrogen conservation despite restriction of dietary intake.111,112 GH has been found to be useful for decreasing the catabolic effects resulting from surgery and trauma,86–88,113–123 conditions in which GH levels are decreased.124 In one study the protein kinetic response to exogenous GH in trauma patients fed parenterally was measured.121 After 7 days of total parenteral nutrition (TPN) nitrogen balance was less negative, protein synthesis efficiency was higher, whole-body protein synthesis rate was higher and deficient (because of trauma) anabolic GH and IGF-1 and insulin levels were significantly elevated. While GH is effective in most catabolic conditions, it is not in some instances. For example, GH has not been found to have significant anabolic or anticatabolic effects in the early stages of some acutely catabolic conditions including trauma. In one study, the effect of GH on hormone and nitrogen metabolism in 14 patients with multiple injuries in the early phase of injury was evaluated.125 In this study, administration of GH evoked a significant increase in plasma concentrations Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 28 2.11.2007 12:16pm Compositor Name: VAmoudavally 28 Amino Acids and Proteins for the Athlete: The Anabolic Edge of GH, IGF-1, and IGF binding-protein-3 (IGFBP-3). No significant differences were found for either daily or cumulative nitrogen balances in patients receiving GH and those receiving placebo. GH therapy did not affect SM extracellular water, nor did it affect plasma or muscle concentrations of total free amino acids or glutamine. GH administration in exercising elderly men does not augment muscle fiber hypertrophy or tissue GH–IGF expression and suggests that deficits in the GH–IGF-1 axis with aging do not inhibit the SM tissue response to training.126 Although useful in reasonable dosages, it would appear that it can be overdone. A review paper found that there is an approximate twofold increase in mortality in critically ill patients receiving large doses of GH.127 GH and IGF-1, especially the former, have immunoregulatory effects in addition to anabolic effects. The hormones may act to protect the host from lethal bacterial infection by promoting the maturation of myeloid cells, stimulating phagocyte migration, priming phagocytes for the production of superoxide anions and cytokines, and enhancing opsonic activity.128 Growth Hormone and Athletic Performance Although GH is effective in certain catabolic states, the question still remains: does GH have an effect on protein synthesis and muscle hypertrophy secondary to exercise? It is felt by some that GH has a permissive effect on protein anabolism. But do increased levels of GH increase protein synthesis and have an anabolic effect? Although the significance of GH release during exercise has yet to be fully elucidated, the overall actions of GH, including increasing growth, sparing glucose and utilizing fat as an energy source, affect the adaptive response to exercise. Prolonged intensive exercise causes elevated levels of somatotropin and glucagon, and a reduced level of insulin in the blood. The somatotropin response exercise seems to depend on the severity of the exercise and on other factors such as emotional strain and the environmental temperature. It seems that the more adverse the conditions and the harsher the exercising, the more the GH produced. Exercise is a potent stimulator of several hormones including GH.129 However, of all the hormonal changes that take place with exercise, GH levels seem to be most affected especially after an acute bout of resistive exercise.130 We know that the GH response to exercise is workload dependent, and that with increasing intensity, GH secretion increases.131 We also know that the GH response is greater with acute anaerobic exercise bouts involving large muscle masses and less with continuous aerobic activity.132–134 Almost all forms of exercise increase GH secretion, although there appears to be a higher increase in GH level after maximal exercise than after submaximal exercise.135,136 Some studies, however, have not shown this relationship.137 The results of one study suggests that the balance between oxygen demand and availability may be an important regulator of GH secretion during exercise.138 Several studies have documented the increased GH secretion that occurs during weight lifting, and during the subsequent recovery phase.133 It appears that the secretion of GH varies according to the muscle group that is exercised, with smaller muscle groups leading to increased GH release. For example, in one study, arm work elicited a greater serum GH response than leg work.139 The GH response, however, may be attenuated under certain conditions, including bouts of exercise done consecutively without sufficient recovery between them.70,140 As with insulin and glucagon, the availability of carbohydrates modulates the response of GH to exercise.141 The administration of glucose excludes the response while during a carbohydratedeficient state the response is exaggerated. One study examined the effect of sprint and endurance training on GH secretion.142 In this study 23 highly trained sprint (100–400 m) or endurance (1,500–10,000 m) athletes were tested for GH levels after a performance of a maximal 30 s sprint on a nonmotorized treadmill. There was a Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 29 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 29 marked GH response for all athletes but the response was three times greater for the sprint-trained athletes than for the endurance-trained athletes. The sprint-trained athletes reached their peak GH values 20–30 min following exercise, whereas the endurance athletes reached a peak GH value 1–10 min postexercise. One hour following exercise, the GH level was still approximately 10 times baseline in the sprint-trained group. The authors concluded that the GH response to the sprints was secondary to the metabolic responses (high blood lactate and plasma ammonia, low blood pH) and the power-output generated during this type of exercise. The authors also speculated that long-term increase in GH in the sprinttrained group may result in an increase in protein synthesis and a decrease in protein catabolism resulting in retention or increase of SM mass. The results of this study further corroborate the effect on high-intensity exercise on GH secretion, whether the stimulus for this increased secretion is lactic acid, serum pH, ATP depletion, low serum glucose, oxygen deficit, or some other factors. Although it is important to determine just what affects GH secretion, the real question is whether the increased GH response results in an anabolic response or whether the increased GH secretion simply allows the more efficient use of stored body fat with no long-term effects on protein synthesis. The increased secretion of GH during exercise may be physiologically important for one or all of the following: glucoregulatory, lipolytic, and anabolic actions. Its anabolic and glucoregulatory effects include the increase of protein synthesis in all cells (through an increased transcription of DNA leading to increased quantities of RNA), an increase in the metabolism of fatty acids, a decrease in glucose utilization throughout the body and an increase in glucose formation (by the stimulation of hepatic gluconeogenesis).143 GH increases the movement of amino acids from intracellular spaces into the cytoplasm and increases ribosomal protein synthesis.144 Moreover, GH has significant anticatabolic effects. Human GH is a major lipolytic factor in the body that stimulates the release of fatty acids from adipose tissue into circulation and increases intracellular conversion of fatty acids into acetylCoA.145 In one study, the lipolytic effect of exogenous GH during caloric restriction, manifested by increases in glycerol concentrations, and body fat loss, persisted well beyond its anabolic effects.146 Thus, through the increased use of fatty acids for energy contributions, protein and glucose supplies are spared, permitting circulating levels of glucose to increase. The anticatabolic effects of GH (decrease in the breakdown of cell protein) may be due to its effects on lipid metabolism. GH stimulates the release of fatty acids from adipose tissue into the circulation and increases the intracellular conversion of fatty acids into acetyl-CoA with subsequent utilization for this energy.147 The increased supply of free fatty acids for energy metabolism has the effect of sparing protein and glucose stores.148 Moreover, lipogenesis is decreased by GH. Chronic administration of GH results in inhibition of glyceride synthesis and decreased body fat content.149 The overall effect is an increase in fat metabolism, decreased body fat, and a decrease in protein catabolism. In one study, administration of GH in a therapeutic dose for 2 weeks, despite apparently normal daytime levels of major metabolic hormones, induced significant increases in circulating lipid fuel substrates, increased energy expenditure, and lipid oxidation.150 Indirect promotion of growth can be seen in the enhanced action of other growth factors in the presence of GH. Kelley and Ruvalcaba observed that the presence of GH improves the anabolic action and tissue responsiveness to androgens.151 In addition, Alen et al. demonstrated that androgens and anabolic steroids could increase the responsiveness of GH secretion during various secretion challenges.152 The synergistic effects of GH and IGF-1 will be examined in more detail below. In addition to the better-recognized roles of GH as mentioned above, GH was proposed to play a major role in acid–base homeostasis.153 GH accelerates renal acid secretion and thereby facilitates elimination of acid from the body fluids, a potentially important role during strenuous exercise when acidogenesis may limit performance. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 30 2.11.2007 12:16pm Compositor Name: VAmoudavally 30 Amino Acids and Proteins for the Athlete: The Anabolic Edge Problems with Exogenous Growth Hormone Increasing our own natural endogenous levels of GH is an effective way to get the benefits of increased GH levels and for reasons outlined below may be a natural alternative to the use of exogenous GH. The use of recombinant or synthetic GH (the only kind available since 1985 when the possibility of prion infection resulting in Creutzfeldt–Jakob disease,154 a variant of mad cow disease, halted the use of GH harvested from the pituitaries of cadavers) only provides limited GH exposure. That is because human GH represents a family of proteins rather than a single hormone. In fact, the circulation contains over 100 GH forms,155 and because we have yet to discover enough about the various forms, the net biological activity of this mixture is difficult to predict since the exogenous recombinant GH represents only 20% or so of the mix. Thus far, most of the research has been largely confined to monomeric 22-kDa, the same GH that is available for exogenous use. However, while it is certainly effective for the original intended purpose, namely growth promotion, it is not known if this is sufficient for optimal growth. It is unlikely that it can fulfill all the functions of the GH family that are naturally present in the body. Moreover, the use of one GH variant likely decreases the production of the other variants in the body156 thus limiting the normal biological activity of GH. On the contrary, the use of exogenous GH shuts down our own GH production and may produce some dysfunction in the endogenous production of GH once the therapy is stopped. This may simulate the situation of hypogonadotropic hypogonadism that sometimes occurs secondary to the use of anabolic steroids.157–163 Since it takes a variable amount of time for our body to ramp up the GH machinery once GH therapy is discontinued, for a certain period of time, a GH deficiency (GHD) may exist. In some cases it may be possible that some dysfunction might exist as a result of which the production of GH is below that which was produced before the GH therapy. Effects of Growth Hormone on Body Composition and Athletic Performance GH and IGF-1 have various anabolic and ergogenic effects including increasing protein synthesis, decreasing protein catabolism, increasing lipolysis and utilization of fatty acids for energy, increasing SM blood flow, and increasing noncontractile musculoskeletal tissue. The various mechanisms involved are still being worked out and there are several theories as to their modes of action. For example, a recent paper postulated that the ergogenic effects of GH, and the associated levels of IGF-1 are due to their effects on connective tissue.164 In this paper, the authors state that ‘‘a strengthened connective tissue would give a stronger and more strain-resistant muscle and tendon and this could, in part, fit with the claimed effect of rhGH on athletic performance. Furthermore, an anabolic effect of rhGH in connective tissue could also suggest a potential for rhGH in treatment of muscle and tendon injuries, which are common problems in many sports.’’ This is an interesting hypothesis and the authors do make a creditable presentation. However, in light of the many studies showing the anabolic and other metabolic effects of GH and IGF-1, I believe there is much more involved. Their views, however, on the therapeutic potential of GH and IGF-1 for the prevention and treatment of injuries are more probable. Adults with GHD who receive GH replacement have been reported to significantly increase total lean body mass and muscle mass.102,165–167 It has also been shown that long term continuous replacement therapy in patients with GHD results in a sustained increase in total body nitrogen168 and that even low-dose regimens of GH result in improvements in body composition, fatigue, bone mineral density, and lipid profiles.169 Other studies have also shown that the use of GH stimulates amino acid uptake and protein synthesis in SM and enhances nitrogen retention.170–172 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 31 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 31 However, until recently, there have only been a few studies documenting the anabolic effects of GH in normal adults,173 even though increases in GH production are known to have significant effects on carbohydrate, fat, and protein metabolism, and have been shown to enhance recovery after exercise and stimulates IGF-1 production in muscle. Several studies have shown that GH and IGF-1 have significant anabolic, anticatabolic, lipolytic, regenerative, and other effects that impact body composition and athletic performance.174 Although there is evidence from animal models that the effects of GH may be mediated through circulating or locally produced IGF-1, there is additional evidence to suggest that the mechanisms by which IGF-1 and GH promote protein anabolism are distinct. Moreover, although studies have shown that insulin and IGF-1 decrease protein degradation and GH results in enhanced protein synthesis, it is also shown that under certain circumstances both enhance protein synthesis and decrease protein degradation.175 For example, one study looked at the effects of an acute dose of GH on amino acid metabolism in 15 young healthy males while controlling for the effects of other confounding hormones (i.e., via simultaneous infusion of somatostatin and replacement of insulin, glucagon, and GH).176 This study demonstrated direct effects of GH in terms of increased anabolic effects via inhibition of amino acid oxidation and stimulation of whole-body protein synthesis. The direct effect of GH for this action was supported by the fact that insulin, glucagon, cortisol, IGF-1, catecholamine, and glucose concentrations were not different between the control and GH treatment groups. We know that increases in local IGF-1 expression are associated with increased muscle protein and DNA synthesis, and exhibit both GH and insulin-like effects.175 Local production of IGF-1 also stimulates satellite cells to multiply and fuse with existing muscle fibers to enlarge muscle mass.177 GH, IGF-1, Insulin, and Amino Acid Synergism It has been known for decades that insulin, GH, glucocorticoids, IGF-1, thyroid hormones, and other hormones regulate body protein metabolism. It is also well known that insulin,178 IGF-1,179 and GH173 each has acute, anabolic actions on SM protein while the glucocorticoids (e.g., cortisol) have a catabolic effect. Increasing levels of GH, IGF-1, and increasing insulin production and sensitivity (by the use of arginine,180 ornithine,181 a-lipoic acid, chromium, and zinc182) results in powerful synergistic effects on body composition.80,179,183–185 This is evidenced by the exogenous use of all four by power athletes and bodybuilders. The massive increase in muscle mass, decrease in body fat, and increase in strength and performance attests to their effectiveness. Unfortunately their use results in significant and sometimes debilitating side effects both while taking them and after, including permanent dysfunction in the regulating systems. Reproducing their synergistic effects by increasing endogenous production and effects produces significant results without the side effects. For example, a combination of certain amino acids not only increases the effectiveness of endogenous GH, IGF-1, insulin, and thyroid, but also provides other boosting effects secondary to the interaction of the select amino acids in the formulation. For example, GH levels are increased by specific amino acids and other compounds, and in turn the GH acts synergistically with amino acids, such as arginine, ornithine, glutamine, GABA, lysine, glycine, tyrosine, and taurine to increase IGF-1 levels even further.186 Also IGF-1 levels, which are controlled by GH and insulin, rise even higher because of the increase in insulin sensitivity. This synergistic effect of several ingredients results in an increase in IGF-1 levels that is out of proportion to the increase in GH levels, and thus provides a more potent anabolic and fat loss stimulus than either one alone.187–189 Moreover, increasing levels of GH has been shown to decrease the catabolic effects of cortisol by decreasing its formation from inactive cortisone,190 to decrease myostatin expression and increase testosterone effects in the body. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 32 2.11.2007 12:16pm Compositor Name: VAmoudavally 32 Amino Acids and Proteins for the Athlete: The Anabolic Edge Along with the anabolic benefits of the combined effects of GH and IGF-1, there are also significant adverse effects secondary to their excessive use. One of these effects has been seen in elite bodybuilders starting in the early 1990s, a time when these bodybuilders started making more extensive use of GH, IGF-1, and insulin. At that time there was not only a dramatic change in the level of muscle mass seen in these elite bodybuilders, but also increasing abdominal girth. Although the abdomens were muscular and defined, the waist looked bloated and much bigger in girth than prior to that time. The reasons are obvious once you realize the extensive use that was being made of GH, IGF-1, and insulin. All three can contribute to growth of the visceral organs, including the small intestines, liver, spleen, and kidneys,191–202 and their use, especially insulin, may increase visceral adipose tissue (the fat in the abdomen between the muscle and backbone) adding to the abdominal distension. Nutrient and Hormone Delivery to Muscle On top of the direct anabolic, fat burning, performance and body composition effects, GH, insulin, and IGF-1 also increase nutrient delivery to SM. While exercise is known to dramatically increase muscle blood flow, studies have shown that IGF-1,175 GH,173 and an improvement in insulin sensitivity203,204 can also increase muscle blood flow. The improvement in blood flow by the nutrient route (as against the nonnutrititive route) can have anabolic and performance-enhancing effects by increasing oxygen, hormonal, and nutrient delivery to the exercising muscle.205 Growth Hormone and Exercise Both aerobic and anaerobic exercise have a dramatic effect on serum GH and IGF-1 levels, and tissue IGF-1 levels.206 This effect is influenced by several variables, including the type of exercise, training state, body composition, age, and gender. For example, all-out rowing performance results in dramatic postexercise increases in GH,66 and studies have shown that a single 30 s sprint elicits a marked GH response.207 However, repeated exercise can result in an attenuation of that response. For example, repeated 10 min bouts of high-intensity submaximal exercise has been shown to result in a dramatic attenuation of the exercise-induced GH response.208 It has also been shown that although a single bout of intense exercise results in a marked increase in serum GH, repeated bouts separated by 60 min of recovery results in a lower GH response.209 A later study found that repeated bouts separated by up to 4 h of recovery still resulted in an attenuated GH response whereas there was no adverse effect on serum GH levels in bouts of intense exercise done on consecutive days.210 Another study looked at the GH responses to two consecutive 30 min cycling sessions at 80% of individual maximal oxygen consumption (V_ O2max) in amateur competitive cyclists.211 Subjects were tested on three occasions with different time intervals between the two bouts: 2, 4, and 6 h. The GH response to the second bout was significantly lower at the 2 and 4 h time intervals between bouts but not with the 6 h interval. Other studies have shown that even repeated bouts of aerobic (submaximal) exercise resulted in an increase in serum GH that was greater when the recovery period between bouts increased.212 The reason for the attenuated serum GH and IGF-1 responses (likely also tissue levels of IGF-1) seen in these studies are not known but it could be due to several factors, including autoinhibition at the level of the pituitary secondary to increased levels of GH and IGF-1,213 to an increase in serum free-fatty acid levels214 secondary to exercise, and GH effects on lipolysis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 33 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 33 Growth Hormone and Endurance Athletes Several recent studies have shown that GH is an effective anabolic and ergogenic aid. Although many of the studies center on anaerobic training such as resistance training, GH is also effective in increasing aerobic performance in all sports and activities, including ultraendurance events. In fact, levels of GH (and cortisol) increase more dramatically in endurance events than with anaerobic events.215 Aerobic capacity is increased by GH. It has been shown that GH treatment displays multiple actions through several components of the oxygen transport system including increasing red cell mass, blood volume, and cardiac function.216,217 Cardiac systolic function, in particular, is well documented to improve with GH treatment of both male and female GH-deficient subjects.218 One study looked at the effects of exogenous GH in male endurance athletes. The use of GH resulted in reduced leucine oxidation by more than 50%, which suggests that the metabolic effects of GH include an anabolic effect in SM.219 Another recent study found that GH use resulted in lower exercise oxygen consumption without a drop-off in power output.220 The authors concluded that the use of GH may improve exercise economy. Moreover, GH decreases the perception of fatigue in adults with untreated GHD and therefore may be useful in this regard as well in endurance athletes. Effects of GH on Core Temperature and Performance GH has other effects that can result in an increase in performance in endurance athletes. An increase in sweat rate is associated with a greater heat loss from evaporation and a lower core temperature. Investigators have shown that GH stimulates sweat production and heat loss from evaporation during exposure to heat in either the absence or presence of exercise.221 GH deficiency, on the other hand, is associated with reduced sweat secretion and an increase in heat storage.222 A recent study of Ironman triathletes found that those with higher GH expression had lower rectal temperatures which in turn correlated with increased performance.223 Effects of GH on Intramyocellular Triacylglycerol=Triglyceride Content GH has been shown to stimulate fat breakdown, increase the use of fat, and decrease the use of carbohydrates as fuel.224–226 It has also been shown to increase the amount of fat in muscle cells in the form of intramyocellular triacylglycerols=triglycerides (IMTG), perhaps through the involvement of hormone sensitive lipase.227,228 Fat and carbohydrate are the primary fuels for muscle cells. Both of these macronutrients are stored in the cells as quick fuel sources—carbohydrates are mainly stored as glycogen and fat mainly as triacylglycerols. Both of these sources are dynamic and are used and replenished constantly both at rest, and more so with activity. These dynamic processes allow the body to modulate their levels constantly depending on rate of use and rate of depletion. IMTGs are the storage form of fat, basically fat droplets inside the muscle cells, and are equivalent to glycogen, which is the storage form of glucose (carbohydrates) inside muscle cells (and other cells in the body). These intramyocellular lipid aggregates are usually located adjacent to the muscle mitochondria, which suggest that they function as a readily available substrate source.229 It has now been demonstrated that hormone-sensitive lipase (HSL) exists in muscle, and is considered to be the rate-limiting enzyme for IMTG hydrolysis. HSL is believed to be regulated by intramuscular (contraction) and extramuscular (epinephrine=adrenalin) factors.230,231 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 34 2.11.2007 12:16pm Compositor Name: VAmoudavally 34 Amino Acids and Proteins for the Athlete: The Anabolic Edge Muscle HSL content correlates well with the known concentrations of IMTG storage and the oxidative capacity of SM fiber types. The activity of this lipase is increased with exercise duration, however, once 2 h of activity have passed, the level returns back to baseline. The metabolism of IMTG is inhibited by the release of insulin. Although in the general population it appears that an increase in IMTG content of SM correlates with insulin resistance, obesity, and diabetes, this is not the case in endurance athletes where increases in IMTG levels are associated with an increase in insulin sensitivity.232 Regenerative and Cognitive Effects of GH and IGF-1 As we have seen, the anabolic, anticatabolic, fat-burning effects of GH are well established. However, there is also evidence that the HG=IGF-1 axis affects cognition and the biochemistry of the adult brain. The findings of various studies suggest that both GH and IGF-1 have cognitive, neuroprotective, and regenerating effects on the central nervous system (CNS).233 It seems that GH and IGF-1 can act directly on their own and also have shared effects, likely due to cross talk between the GH and IGF-1 transduction pathways. Effects of GH on Aging As we have seen, GH levels decline dramatically from age 20 to 60. This decline leads to a progressive GH deficiency. Since GH has beneficial effects on muscle mass, bone density, and body fat, it counters some of the age-related changes in body composition. This reduction in GH contributes to physical and psychological changes seen with aging, including decreased SM, increased body fat, thinning skin, and decreased well-being and quality of life. Studies have shown that restoring GH levels results in improvements in all of these areas as well as boosting the immune system, improving protein, lipid, and carbohydrate metabolism, improving recovery, and decreasing inflammation in the body.104,234–236 Some of these effects from increasing GH are secondary to increases in IGF-1 and may also be secondary to subsequent changes in other hormones and growth factors. For example, as we have seen, GH has a direct effect on decreasing myostatin and an indirect effect of increasing the effects of testosterone in the body. A recent study found that while exercise has been shown to downregulate myostatin expression in adults, it does not do so in the elderly. The authors concluded that this impaired capacity may play a role in limiting hypertrophy in older females.237 Growth Hormone Synthesis and Secretion Many psychological and physiological parameters affect GH release. Among the many factors that increase GH secretion are the various amino acids especially arginine, L-dopa, L-tryptophan, tyrosine, and ornithine. The GH-stimulating effects of the various amino acids are discussed in detail below. High-protein diets also seem to have an increased GH response as against high carbohydrate diets. In one study, measuring the effect of isocaloric (500 kcal) protein and carbohydrate ingestion found that after both diets, GH increased after an initial decline, the increase was greatest after protein intake and maximum was reached at 180 min.238 Endogenous control of GH secretion has still not been fully elucidated although somatostatin and GHRH seem to be the ultimate mediators of most of the changes in GH secretion, including the feedback effects from GH itself and IGF-1.239–243 GH dysfunction is often associated with changes in one or both of these regulatory hormones.244,245 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 35 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 35 For example arginine-induced GH release is mainly mediated by a decrease in somatostatinergic tone, while GH responses to insulin stress are probably mediated by both an increase in hypothalamic GHRH release and inhibition of somatostatin.246,247 The mechanisms by which insulin-induced hypoglycemia, L-dopa, and arginine stimulate GH secretion are all different; a suppression of somatostatin release may be partially involved in the stimulatory mechanism of GH secretion by L-dopa, arginine suppresses somatostatin release from the hypothalamus to cause GH secretion.248 Cholinergic pathways play an important role in the regulation of GH secretion mediated through their effects on somatostatin and GHRH.249–251 The adrenergic system is involved in the neural control of GH secretion with both stimulatory and inhibitory influences mainly mediated via GHRH and somatostatin modulation.252 The secretion of GH, and its plasma levels, as well as their response to GHRH is severely reduced in old people because of an increased somatostatin secretion by the hypothalamus.253,254 Yohimbine, an a2-antagonist, suppresses the pulsatile hGH secretion either through the elimination of GHRH release or via enhanced somatostatin release.255 In obesity, the basis for the derangements in GH secretion (an impairment of GH secretion elicited by all stimuli known to date) is associated with a state of chronic somatostatin hypersecretion.256 The decrease in GH response after repeated stimulation with GHRH is partly caused by an elevation of somatostatin tonus.257 Tamoxifen, a partial competitive antagonist to the estrogen receptor, is widely used clinically in the treatment of breast cancer. In vitro data shows that tamoxifen suppresses serum IGF-1 levels by acting at the pituitary to inhibit GH release.258 Another study has shown that the blunting of GH pulse amplitude by tamoxifen is mediated at least in part by increased release of endogenous somatostatin.259 Myostatin Myostatin, a member of the transforming growth factor (TGF)-b superfamily and a cytokine that works by inhibiting the proliferation of satellite cells and the differentiation of myoblasts, inhibits muscle growth and is a negative regulator of muscle mass.260 It also decreases adipogenesis secondary to a reduction in the secretion of leptin.261 Inhibiting myostatin, either at the genetic level or by inhibiting its effects in the body, can result in marked increases in muscle hypertrophy and improvements in body composition. Information on myostatin was first published in 1997 in a paper describing enhanced muscle hypertrophy in myostatin-deficient mice.262 Later that year myostatin was reported as being responsible for double-muscled cattle.263 Double muscling, a recessive trait in cattle, results in extreme muscularity. These cattle produce heavy-muscled, lower-fat carcasses that have higher muscle to bone ratios than normal-muscled cattle. This study reports on the myostatin sequences of nine other species and the identification of mutations in the coding sequence of bovine myostatin in two breeds of double-muscled cattle, Belgian Blue and Piedmontese. In both, the mice and cattle there were reported negative effects of the myostatin deficiency. The myostatin-deficient animals show some weaknesses and deficiencies in spite of their hardy appearance and excessive muscularity. Like the mice, the myostatin-deficient cattle are docile and have some genetic disadvantages. For example, they have cardiorespiratory weaknesses, as well as a loss of oxidative capacity of the muscles, factors that may decrease the appeal of using myostatin inhibition as therapy for muscle loss.264–267 Moreover, their performance is nothing to write home about. For example, it has been shown that these double-muscled cattle have a poor metabolic and cardiopulmonary response to Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 36 2.11.2007 12:16pm Compositor Name: VAmoudavally 36 Amino Acids and Proteins for the Athlete: The Anabolic Edge exercise.268 It is important to know whether all this muscle is functional. Just how strong are these behemoths? Unfortunately, there was nothing in the literature about their strength. Since then research on myostatin has validated its role in regulating muscle mass, regeneration of muscle tissue, and in regulating body fat, with myostatin promoting increased formation of adipose tissue.269 Decreases in myostatin levels or effects results in increased SM mass and strength, and decreased levels of body fat while increases result in decreased muscle and increased body fat.261,269–273 Various degrees of natural inhibition of myostatin secondary to genetic polymorphisms likely explains some of the differences naturally seen in muscular hypertrophy and in the ability to attain significant increases in muscle mass. An extreme example of this is seen in a child that was born in Berlin with a natural loss of function mutation that turns off the myostatin gene.274 The base pair alteration in the gene results in the missplicing of myostatin mRNA, thereby reducing the production of mature myostatin protein. At 4.5 years, this boy is reported to have the physique of a mini-bodybuilder with markedly increased muscle mass and reduced levels of body fat. At 7 months of age muscle hypertrophy was pronounced in all muscles. A picture shown of the lower body of this child shows the protruding muscles of the patient’s thigh and calf. In a recent study done on double-muscled Belgian Blue bulls, muscle biopsies revealed that this deletion in the myostatin gene altered the muscle fiber composition by increasing the fast-twitch glycolytic muscle fiber proportion inducing a higher proportion of fast-twitch glycolytic fibers and, to a lesser extent, a lower proportion of slow-twitch fibers.275 An important function of myostatin is inhibition of satellite cells. Thus one would expect that in the absence of myostatin there would be enhanced muscle regeneration. This appears to be true from studies of acute and chronic muscle injury.276,277 For example, muscle from myostatin null animals that were acutely injured regenerated large-diameter myofibers earlier than injured controls with normal myostatin.278 A recent review of the physiology and pharmacological relevance of myostatin279 concluded that numerous studies have established the physiological relevance of myostatin in myogenesis and in muscle maintenance during adulthood. Moreover, regulation of myostatin expression levels also appears to be involved in muscle wasting of different origin. Nowadays, the myostatin pathway is better understood and several myostatin signaling partners have been identified, in particular myostatin-binding proteins that compete for receptor binding or prevent myostatin activation. Consequently, these proteins can be considered as new potential targets to improve the behavior of muscle tissue after injury or in degenerative diseases, as suggested by experiments performed in mdx mice. In addition, the observation that a monoclonal antibody raised against myostatin could increase SM mass and strength provides a promising way to improve the behavior of diseases characterized by muscle wasting. In addition to using neutralizing antibodies, there are a number of ways of inhibiting myostatin, including the myostatin propeptide, follistatin, a secreted glycoprotein, which inhibits the activity of myostatin, and other TGF-b family members.276,280 Some elite bodybuilders, taking these conclusions and similar ones in other papers to heart and using the information for their own purposes, have validated the real world results of inhibiting myostatin through various means and have thus achieved massive levels of muscle hypertrophy in part through this inhibition. If not already being used (many are), it will not be long before these and other methods, including the use of various growth factors and cytokines, and genetic modifications, of increasing muscle mass, strength, and performance will be used by other aspiring athletes. GH and Myostatin Recently studies have shown a negative correlation between GH and myostatin, It appears that deficits in GH may result in increased myostatin expression and a disassociation in autocrine IGF-1 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 37 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 37 effects on muscle protein synthesis and that myostatin likely represents a potential key target for GH- and IGF-1-induced anabolism.281 Interestingly enough, decreasing myostatin can also result in an increase in the expression of the androgen receptor. This results in an increased anabolic stimulus due to increased binding of testosterone to the androgen receptor. GH potentially can have multiple effects on body composition due to its direct action, and actions secondary to increases in IGF-1, decreases in myostatin, and subsequently, increased testosterone activity. INSULIN-LIKE GROWTH FACTOR 1 IGF-1, with a structure close to insulin, has both similar functions to insulin and distinct properties of its own.282–285 However, the interaction among insulin, GH, and IGF-1, as well as the sex hormones and other hormones and factors has yet to be fully worked out.286 For example, recent research has shown that while IGF-1 is recognized as an insulin sensitizer at the liver and muscle, IGF-binding protein-3 (IGFBP-3) acts as an insulin antagonist.287 IGFBPs bind to IGFs with high, but variable, affinities.288 While they are modulators of endocrine, paracrine, and autocrine actions of IGFs, IGFBPs exert GH-independent effects on various tissues in vivo and in vitro.289,290 Although the details of their synthesis and metabolic effects are not fully understood, it has been shown that IGFBP-1 is regulated by insulin and IGF-1, IGFBP-2 is also related to insulin levels, increases in IGFBP-3 are closely correlated with those in IGF-1 in response to GH, and IGFBP-4 and -5 inhibit and stimulate, respectively, IGF-mediated effects on bone.286,291–293 Although there are two IGFs, IGF-1 and IGF-2, IGF-2 has more of a developmental role while IGF-1 accelerates satellite cell proliferation and increases muscle mass.294,295 IGF-1 has been implicated in protein metabolism and in growth and growth processes of many tissues in animals.296–300 IGF-1 has a structure similar to insulin301,302 and is thought to mediate most of the anabolic effects of GH in the body. The IGF peptides are bound tightly to plasma proteins (IGFBPs). Because of this binding their activity is extended to several hours as against the 20–30 min of the unbound forms. IGF-1 is produced in the liver, chondrocytes, kidney, muscle, pituitary, and gastrointestinal tract. The liver is the main source of circulating IGF-1.303 Levels of IGF-1 and IGFBP-3 (a GH-dependent protein that binds IGF-1) are tied in with GH secretion and increase when GH levels increase. Levels are also age dependent, with low levels in early childhood, a peak during adolescence, and a decline after 50 years. As a consequence of protein binding and thus controlled release, the concentration of IGF-1 remains relatively constant throughout the day, in contrast to the fluctuating levels of GH itself. IGF-1 seems to have a split personality in that it exerts both GH and insulin-like actions on SMs175 by increasing protein synthesis and decreasing protein breakdown. Both GH and IGF-1 seem to shift the metabolism to decreasing fat formation and increasing protein synthesis. Since both GH and IGF-1 have been shown to have anabolic and anticatabolic effects under certain stressful and catabolic conditions and since IGF-1 is an important mediator of the anabolic effects of GH, it has been suggested that IGF-1 may be used as a substitute for GH in catabolic states because IGF-1 is not felt to have certain side effects seen secondary to GH use. IGF-1 and IGF-2 have been observed in vitro to increase glucose uptake and to possess antilipolytic activity in cell cultures,304 but with a molar potency that is considerably less than that of insulin.305,306 These findings have been reproduced in vivo during IGF-1 and -2 and insulin infusions in rats,307 and after the administration of IGF-1 and insulin bolus injections to human subjects.308 Thus when infused into humans, IGF-1 produces a rapid lowering of blood glucose (hypoglycemia) but with much less potency comparable to insulin. High rates of infusion of IGF-1 induce hypoglycemia and decrease estimates of whole-body proteolysis, suggestive of a predominant insulin-like effect. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 38 2.11.2007 12:16pm Compositor Name: VAmoudavally 38 Amino Acids and Proteins for the Athlete: The Anabolic Edge IGF-2 is thought to be a less effective anabolic agent than IGF-1.309,310 In one study the in vivo effects of 5 h infusions of recombinant IGF-1 and -2 on glucose and protein metabolism were investigated in awake, fasted lambs.311 The effects were compared with an insulin infusion that had the same hypoglycemic potential as the high dose IGF-1 infusion. IGF-1 lowered blood glucose by increasing the rate of glucose clearance, in contrast to insulin, which both increased clearance and reduced glucose production. Net protein loss was reduced after infusion of low and high dose IGF-1 and insulin. IGF-2 infusion did not alter the rate of net protein loss. In contrast to insulin, high-dose IGF-1 infusion increased the rate of protein synthesis in skeletal and cardiac muscle and in hepatic tissue. The authors of this study concluded that (1) protein metabolism is more sensitive than glucose metabolism to IGF-1 infusion, as protein loss was reduced by an IGF-1 infusion that did not alter glucose kinetics, (2) protein synthesis is increased by IGF-1 infusion but not by insulin infusion, and (3) IGF-2 is a less effective anabolic agent than IGF-1. The authors also speculated that the effects of IGF-1 on protein metabolism are not mediated by insulin receptors. Unlike GH, IGF-1 by itself, even at levels associated with pharmacological levels of GH, does not seem to significantly affect protein metabolism,312 but may play a role as a fat repartitioning agent.313 However, if confused with TPN,314 or if amino acids are added to even low-dose IGF-1 infusions, protein synthesis increases, but there may be little effect on protein catabolism (see below). IGF-1 plays an important role in mediating the somatotrophic effects of GH, induces the liver to synthesize IGF-1 and other somatomedins, and the GH=IGF axis plays an important role in growth regulation and improvements in muscle protein synthesis and whole-body nitrogen economy in humans. IGF-1 not only exerts some insulin-like actions but is structurally similar to proinsulin (the immediate precursor to insulin). GH and insulin are in many ways interdependent. GH stimulates insulin release directly and also facilitates its release in response to various other factors. Moreover, GH-deficient individuals have impaired insulin release to glucose challenge. Because of the various actions and interactions, one might consider insulin, GH, and IGF-1 as part of the same family. Investigations into the anabolic and anticatabolic effects of IGF-1 and any synergism shown with GH and insulin on protein metabolism have yielded rather conflicting results. These studies, while showing the significant anabolic and anticatabolic effects of IGF-1 especially when acting synergistically with GH and insulin, are somewhat contradictory when delegating which actions are due to GH, IGF-1, and insulin. Although somewhat confusing, a review of the literature is presented below. The purpose of one study was to examine the effects of a heavy-resistance exercise protocol known to dramatically elevate immunoreactive GH, on circulating IGF-1 after the exercise stimulus.315 Seven young men were asked to perform an eight-station heavy-resistance exercise protocol consisting of three sets of ten repetition maximum resistances with 1 min rest between sets and exercises followed by a recovery day. In addition, a control day followed a nonexercise day to provide baseline data. According to the authors of the study, the subjects were required to have engaged in resistance training 2–3 times a week for the last 1 year. The subjects were screened in this way to ensure that the participants were better able to tolerate the training performed. Despite postexercise elevations in GH, significantly above resting levels, there were no significant differences between pre- and postexercise total serum IGF-1 concentrations, and total serum IGF-1 concentrations did not increase the day after the exercise period. It is possible explained the authors that the amount of muscle damage sustained by the subjects during the training period does not require alterations in circulating IGF-1 and that local IGF-1 release may have been sufficient to provide for repair or adaptational processes. Thus, these data demonstrate that a high-intensity bout of heavy-resistance exercise that increases circulating GH did not appear to affect IGF-1 concentrations over a 24 h recovery period in recreationally strength-trained and healthy young men. The conclusion was that IGF-1 concentrations following exercise may be independent of GH stimulatory mechanisms. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 39 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 39 Another study examined the effects of infusing recombinant human GH, IGF-1, the truncated IGF-1 analogue, des(1–3)IGF-1, and insulin over a 7-day period in streptozotocin-induced diabetic rats. The authors concluded that IGF peptides stimulate muscle protein synthesis and improve nitrogen balance in diabetes without obviously influencing the abnormal carbohydrate metabolism. Moreover, des(1–3)IGF-1 is at least as potent as the full-length IGF-1.316 Carcass fat increased substantially following insulin administration. This did not occur with the IGF peptides, suggesting that IGF predominantly stimulates the growth of lean tissue. In another study, the effects of giving either infusions of IGF-1 alone, or combined with insulin and amino acids were examined.317 The use of IGF-1 alone lowered insulin levels, and produced no increases in muscle protein synthesis while the use of IGF-1 combined with insulin and amino acids increased protein synthesis. Thus it appears that the protein-synthesizing effect of IGF-1 is limited and only becomes significant in the presence of adequate amounts of insulin and amino acids. In this study low-dose infusions of IGF-1 failed to increase muscle protein synthesis in rats because it either suppressed insulin secretion or lowered plasma amino acid levels. Adding amino acids not only prevented the fall in plasma amino acid levels, but also prevented the drop in insulin that occurs with IGF-1 alone. This study also shows that insulin is an important modulator of IGF-1 action in regard to protein synthesis. Providing GH with IGF-1 also prevents the decline in insulin common when IGF-1 is given alone. This may explain the apparent synergistic effect shown by some studies when GH is combined with both IGF-1 and insulin. In a series of studies318–322 it was shown that: . . . . . . Although GH exerts some direct effects, some of its growth and anabolic effects are mediated by IGF-1. The muscle hypertrophic stimuli of work-overload and passive stretch are associated with significantly increased muscle IGF-1 mRNA levels. GH significantly decreased body fat and increased the protein to fat ratio only in the animals with the restricted intake. SM weight was increased by GH regardless of food intake. Thus the anabolic effect of GH administered to hypophysectomised rats was independent of dietary intake. Muscle IGF-1 mRNA levels were also elevated by GH but unaffected by food intake. In contrast serum IGF-1 levels were markedly reduced by undernutrition. GH increased IGF-1 mRNA concentration similarly in SM in both rats with food available ad libitum and in pair-fed rats. Serum concentrations of insulin and IGF-1 were increased by GH in the rats with food available ad libitum but not in the pair-fed rats. Decreased nutrition therefore modulated the action of GH but emphasized its nutrient partitioning effect, thus increasing the anabolic drive toward SM growth; this appeared to be mediated by the local production of IGF-1 within the muscle. These data suggest that the anabolic action of GH on muscle can be mediated through the autocrine=paracrine action of the IGF-1 hormone. In contrast, insulin dramatically affected muscle protein synthesis rates but had no measurable effect upon muscle IGF-1 mRNA levels, which suggests that the anabolic action of this hormone is not mediated through the autocrine=paracrine action of IGF-1. These studies suggest that IGF-1 may mediate growth in muscle in response to variety of stimuli by autocrine=paracrine action or in response to certain stimuli possibly by endocrine action. It has been shown that IGFs stimulate protein synthesis and inhibit protein degradation at physiological concentrations.323 Studies done on normal humans comparing IGF-1 action to insulin action, suggest that insulin-like effects of IGF-1 in humans are mediated in part via IGF-1 receptors and in part via insulin receptors.324 IGF-1 infusion has also been shown to directly increase protein synthesis in normal humans.325 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 40 2.11.2007 12:16pm Compositor Name: VAmoudavally 40 Amino Acids and Proteins for the Athlete: The Anabolic Edge In low doses it appears that the use of IGF-1 may be counterproductive in that not only it is anabolic but also it inhibits insulin and GH secretion326 and therefore decreases any anabolic effect from these hormones. Although IGF-1 in high doses positively affects nitrogen balance, use of IGF-1 alone may also be somewhat counterproductive since it induces hypoglycemia while simultaneously decreasing the production of GH and insulin. However, when GH and IGF-1 are administered simultaneously nitrogen balance is remarkably improved.188 Only a few studies have looked at the effects of IGF-1 on protein synthesis in athletes. One study has shown that neither GH nor IGF-1 treatment results in increases in the rate of muscle protein synthesis or reduction in the rate of whole-body protein breakdown, metabolic alterations that would promote muscle protein anabolism in experienced weight lifters attempting to further increase muscle mass.327 In another study, however, larger doses of IGF-1 had significant anabolic effects in humans.328 This study investigated whether recombinant human IGF-1 could serve as a protein-sparing nondiabetogenic agent. As many as 21 healthy volunteers were studied in 3 similar clinical models: IGF-1 alone, IGF-1 and prednisone, and prednisone alone. Prednisone, being a potent catabolic agent, was used to determine the anticatabolic effects of IGF-1. The authors of this study concluded that 100 mg=kg IGF-1, given twice daily 1. Has GH-like effects on whole-body protein metabolism 2. Markedly diminishes the protein catabolic effect of glucocorticosteroids 3. Is nondiabetogenic in prednisone-treated humans As a result of this and previous studies, the authors feel that IGF-1 offers promise in the treatment of protein catabolic states. IGF-1 may also be useful in treating various GH-resistant and insulin-resistant disorders and perhaps as adjunctive treatment in insulin (such as diabetes) and GH-deficient disorders.329 IGF-1, because of its short half-life in the body and its effects in mediating metabolic, mitogenic, and anabolic cellular responses, especially during altered energy states, is receiving increasing attention as a biomarker reflecting metabolic status.330–333 Unlike many hormones used as biomarkers of nutritional=metabolic status such as GH, IGF-1 concentrations exhibit minimal circadian variability through the day;334 therefore, single-time samples are valid indicators of IGF-1 status occurring as a result of intervention and not due to day-to-day variability. Overall, the majority of studies support the fact that IGF-1 has significant anabolic and anticatabolic effects especially when acting with insulin and GH, and when there is an adequate amount of certain amino acids. There is also considerable evidence that IGF-1 is involved in the maintenance of the CNS and this may be the reason why regular exercise is claimed to improve cognitive function. Thus increasing endogenous levels of IGF-1 could be useful for maximizing the effects of exercise on muscle mass and strength. Increasing amino acid intake and increasing endogenous levels of GH and insulin would increase the anabolic and anticatabolic effects of IGF-1. All of the anabolic and anticatabolic effects and the hormonal synergism can be mediated in part by the judicious use of protein, and protein and amino acid supplements. GH and IGF-1 Synergism GH and IGF-1 have some similar and disparate functions in the body, with IGF-1 having some insulin-like and other effects and mediating some but not all the effects of GH.335–340 The use of GH as an anabolic agent is limited by its tendency to cause hyperglycemia and by its inability to reverse nitrogen wasting in some catabolic conditions. IGF-1 has a tendency to cause hypoglycemia. A hypothesis has been proposed that administration of GH with IGF-1 to patients Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 41 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 41 after severe injury could be more effective than GH alone in promoting anabolic protein metabolism, resulting in synergistic nitrogen retention and improved wound healing and immunologic function.341 A combination of GH and IGF-1 has been shown to be much more potent in improving nitrogen balance than either one alone and also attenuates the hyperglycemia caused by GH alone and the hypoglycemia caused by IGF-1 alone.342 One study looked at the effects of giving both IGF-1 and GH simultaneously to tube-fed rats.343 The rats were catabolic, having been previously subjected to surgical stress. Results showed that weight gain on the combined hormone regime was doubled to that when given either hormone separately. IGF-1 selectively increased visceral organ mass, while GH increased calf muscle mass. Both hormones increased carcass protein and water content while reducing fat. When IGF-1 was given with GH, it reversed the elevated insulin level commonly observed when GH is taken alone. In another study by the same author, the fractional rate of protein synthesis (Ks) in SM, jejunal mucosa and muscularis, and liver were compared to investigate the differential effects of GH and IGF-1 on tissue protein synthesis in surgically stressed rats.343 Body weight gain, nitrogen retention, and serum IGF-1 concentrations confirmed that GH plus IGF-1 additively increased anabolism. Serum insulin concentrations were significantly increased by GH and decreased by IGF-1. GH significantly increased Ks in SM and jejunal muscularis; IGF-1 significantly increased Ks in jejunal mucosa and muscularis; and neither GH nor IGF-1 altered Ks in liver. The authors concluded that GH and IGF-1 differentially increase tissue protein synthesis in vivo. In another study, seven calorically restricted normal volunteers were treated with a combination of GH and IGF-1, and the effects on anabolism and carbohydrate metabolism were compared to treatment with IGF-1 alone.187 The GH=IGF-1 combination caused significantly greater nitrogen retention compared to IGF-1 alone. GH=IGF-1 treatment resulted in substantial urinary potassium conservation suggesting that most protein accretion occurred in muscle and connective tissue. GH attenuated the hypoglycemia induced by IGF-1 as indicated by fewer hypoglycemic episodes and higher capillary blood glucose concentrations on GH=IGF-1 compared to IGF-1 alone. These results suggest that the combination of GH and IGF-1 treatment is substantially more anabolic than either IGF-1 or GH alone. GH=IGF-1 treatment also attenuates the hypoglycemia caused by IGF-1 alone. A report demonstrated in a group of calorically restricted (20 kcal=kg ideal body weight=day) normal volunteers that the combination of GH and IGF-1 treatment is substantially more anabolic than either GH or IGF-1 alone.187 Thus there is data that demonstrates that recombinant human IGF-1 has GH-like effects on protein metabolism, selectively stimulating whole-body protein synthesis. Additionally, studies suggest that the combination of recombinant human GH and IGF-1 might be more anabolic than either compound alone when administered to calorically deprived subjects. Although these studies examined the effects of GH and IGF-1 in surgically stressed or caloriedeprived subjects, a study using normally fed healthy individuals did not find a synergistic anabolic effect of GH and IGF-1.344 In this study, subjects receiving combination treatment had a significant increase in plasma IGF-1 concentrations. As expected, there was a significant decrease in leucine oxidation in subjects given combination therapy as well as a significant increase in leucine turnover; hence the nonoxidative leucine disposal, a measure of whole-body protein synthesis, was significantly increased. However, when the absolute changes in all three parameters of leucine kinetics were compared in the three treatment groups (GH alone, IGF-1 alone, and combined GH and IGF-1), there was no additive effect of combination treatment on whole-body protein anabolism. The authors concluded that, contrary to the calorically deprived model, in normally fed individuals, the coadministration of IGF-1 and GH at these doses is not more anabolic on wholebody protein than either compound given alone. This difference in observed effects in the fed and partly fasted state may be related to the difference in total insulin output and suggests a saturable capacity of the body to accumulate protein during normal substrate availability while the body is exposed to individual anabolic hormones. These data suggest a common pathway for GH and IGF-1 to enhance whole-body protein anabolism in the normally fed state. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 42 2.11.2007 12:16pm Compositor Name: VAmoudavally 42 Amino Acids and Proteins for the Athlete: The Anabolic Edge It would appear from this study that the simultaneous use of GH and IGF-1, while synergistically effective in increasing protein synthesis in catabolic states, does not have the same effects in normally fed healthy individuals. However, this does not necessarily imply that this is the case in athletes. Both the catabolic effects of intense exercise and increasing amino acid availability, by increasing the capacity of the body to accumulate protein and increasing substrate availability, may be mitigating factors in determining any synergistic effects of GH and IGF-1 in athletes. Interestingly, in the natural state, GH and IGF-1 are part of a feedback loop (with IGF-1 receptors present in the pituitary345–349) so that while IGF-1 levels may be dependent on GH, elevated IGF-1 levels inhibits GH release.239,350–355 Conversely a decrease in IGF-1, as induced by starvation=fasting, leads to a compensatory increase in GH release.356–358 Thus it appears that in the natural state both peptides are not present together in elevated levels for any significant period of time. As shown above, amino acids increase the anabolic effects of both GH and IGF-1 so that protein and amino acid supplementation may allow an increased synergism between these two hormones. For example, it is been shown that increasing IGF-1 does not seem to decrease GH stimulation by secretogogues such as arginine.359 However, while certain amino acids have been shown to increase GH (and insulin) secretion, and indirectly IGF-1, amino acids per se have not been shown to directly increase plasma IGF-1 levels.360 Although in a recent study, creatine supplementation was shown to increase IGF-1 mRNA in SM.361 Dietary proteins, on the other hand, especially casein at higher versus lower levels and casein versus gluten and soy, have been shown to increase IGF-1 levels and IGF-1 mRNA in rat liver.362–365 Moreover, the restriction of protein,366 and even a single essential amino acid, results in a decrease in IGF-1 production.367 Mechano Growth Factor Although the sequencing of the human genome has found that there are only about 40,000 genes, there are many more proteins because some genes are spliced to produce isoforms, different protein=peptides which usually have somewhat different biological functions, systemically and locally. While the GH=IGF-1 axis is an important systemic regulator of muscle mass, there is also local paracrine=autocrine control, which augments the systemic regulation and may even be the main determinants.368 Some of the regulating local factors involve IGF-1 isoforms which are specifically affected by exercise. Variation in IGF-1 gene expression is based on site-specific alternative splicing of the gene, producing different isoforms of the precursor peptide of IGF-1 (IGF-1Ea, IGF-1Eb, and IGF-1Ec or MGF in human SM), which also undergo posttranslational modification.369 In the past decade there has been increasing interest in differential expression and implication of IGF-1 isoforms in the regulation of muscle fiber regeneration and hypertrophy following mechanical overloading and damage.284 Details of splice variants has recently been identified,370 one of them linked with mechanical stimuli has been named mechano growth factor (MGF).371,372 MGF, a growth=repair factor cloned from exercised or damaged muscle and that has an unique C-terminal peptide, differs from other splice variants of the IGF-1 gene, and has been found to initiate muscle hypertrophy and local repair process of SM by activating muscle stem cells, as well as upregulating protein synthesis.373–377 The local expression of the IGF-1 isoform MGF is felt to be an important local growth factor that is the link between the mechanical stimulus and the activation of gene expression. From various experiments it has been found that the muscle forms of IGF-1 have different functions than the systemic IGF-1, and that in the case of MGF its unique C-terminal peptide has a special function of activating and replenishing the muscle stem (satellite) cell pool.373,374 This activation is one of the early steps in muscle hypertrophy with subsequent fusing of satellite cells with existing muscle fibers, thus providing muscle fibers with the extra nuclei required for growth and repair, and the means by which strength adaptation occurs after exercise or local muscle damage. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 43 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 43 MGF is not the only IGF-1 influence on SM. IGF-1Ea, which has the same sequence as the main liver IGF-1 isoform and, therefore, has been assumed to have mainly a systemic action, is also expressed in SM as well as liver and several other nonhepatic tissues. However, muscle expresses several of the major binding proteins for the systemic form of IGF-1 and their expression tends to be upregulated, as is IGF-1Ea, by exercise.378 Exercise is also known to elevate the serum levels of IGF-1.376 Interestingly, during intensive exercise not only do the muscles produce more systemic IGF-1 than the liver, but they also utilize more of the circulating IGF-1.379 The uptake from serum seems to be a consequence of the greater abundance of specific binding proteins in trained muscle.378 Therefore, not only does the IGF-1Ea have more effect on the muscles that produce it, but also their uptake of circulating IGF-1Ea will be increased as a result of exercise. Studies have shown, however, that MGF is expressed earlier than IGF-1Ea in response to exercise suggesting that the two isoforms were differentially regulated and that the function of MGF is to initiate the hypertrophy process by activating the proliferation of satellite cells.377 Thus the probable reason why MGF is so effective in producing muscle hypertrophy in comparison to IGF-1Ea is that it is responsible for the initial activation of the muscle satellite (stem) cells and thus ‘‘kick starts’’ the hypertrophy=repair process. These in vitro experiments showed that the C terminal peptide of MGF does have a separate action to the C termini of the other splice variants. MGF is also expressed in other postmitotic tissue and is neuroprotective as well as cardioprotective but its expression is much reduced in certain diseases and during aging, leaving open possible therapeutic uses for IGF-1 and MGF.285,380–384 It also opens up therapeutic abuses. In a recent paper, one of the researchers involved in discovering MGF warns of the potential misuse of MGF by athletes because of it is potential ergogenic effects.385 In this paper he states: We also know that MGF as well as IGF-1 exist as class 1 and class 2 isoforms and that the ratios of these in serum change if they are introduced as the peptide or by gene transfer. Therefore there is the possibility of detecting the misuse of these strength generating substances even if delivered in the form of ‘‘gene doping.’’ INSULIN Insulin is produced in the pancreas and is made up of two amino acid chains connected together by a disulfide bridge. It is secreted in the blood where it is bound to carrier proteins and carried to muscle, liver and other tissues. The main action of insulin is to regulate the glucose level in our blood. In order to do this insulin influences the metabolism of not only sugars or carbohydrates, but also fats and protein. Nondiabetic people maintain their plasma glucose concentration within a narrow range at all times despite episodic and erratic food intake. During and after meals, insulin, along with other hormones such as glucagon, epinephrine, and GH, regulates the amount of glucose in the blood so that it does not go out of bounds since both high and low serum glucose can cause problems. When a meal is eaten, an immediate rise in insulin release occurs such that absorbed carbohydrate is rapidly transported into the liver, muscles, and other tissues. After meals, insulin levels drop as the amount of glucose in the blood decreases. Insulin helps ensure smooth control of plasma glucose throughout the day, regardless of how hectic and erratic our day is. Insulin exerts a permissive effect on nutrient storage processes including increasing the rate of amino acid uptake by muscle.386–391 Pancreatic b-cells increase their rate of insulin secretion within 30 s of exposure to increased glucose concentrations, and can shut down just as quickly. Insulin secretion follows a biphasic secretion pattern. An initial burst first occurs, thought to prime the pancreas for a later sustained secretion. While the first phase involves only stored insulin in the pancreas, the second, more Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 44 2.11.2007 12:16pm Compositor Name: VAmoudavally 44 Amino Acids and Proteins for the Athlete: The Anabolic Edge sustained phase includes both stored and newly synthesized insulin. Type II diabetes often features an absence of the initial insulin release phase. Amino acid signaling also occurs in b-cells and promotes insulin production by stimulating b-cell proliferation.392–394 This further adds to the anabolic properties of amino acids. Also in these cells, leucine is effective in stimulating the phosphorylation of mTOR downstream targets. In b-cells the combination of glutamine and leucine is effective in stimulating both signaling395 and insulin production.396 Insulin acts as a nutrient storage hormone involving protein, fat, and carbohydrate synthesis and storage. It promotes processes that build up stores of these macronutrients and inhibits all processes that break down or release those same nutrients. It would appear that insulin has anabolic properties on par with testosterone, GH and IGF-1, although likely by different mechanisms,179 and may be responsible for up to 30% of protein synthesis in SM. Insulin promotes glucose and amino acid uptake and increases protein synthesis.397 It has been known for decades that there is a dose-dependent suppressive effect of insulin and amino acids on protein degradation178,398–405 and the stimulatory effect of insulin and insulin=amino acids on protein synthesis.406–416,408 An earlier study looked at the contribution of dietary amino acids and endogenous hyperinsulinemia to prandial protein anabolism.417 The results of this study demonstrate that by increasing whole-body protein synthesis and decreasing proteolysis, dietary amino acids account for the largest part (approximately 90%) of postprandial protein anabolism. In an early review of the hormonal regulation of protein degradation in skeletal and cardiac muscle, the authors felt that insulin is probably the single most influential factor for regulating and maintaining positive nitrogen balance in heart and SM.418 Insulin promotes protein synthesis by directly stimulating the uptake of amino acids by muscle cells as well of the uptake of those substrates (e.g., glucose) required to support the process.402 One study looking at how insulin exerts its protein anabolic and anticatabolic effects in muscle tissue used human type I diabetic male patients as subjects, in both insulin-deprived and insulin-treated states.419 Among the many other findings was that insulin augmented protein anabolism by decreasing leucine transamination to a-ketoisocaproate. The issue has also been investigated using healthy volunteers with the use of stable isotopic tracers of amino acids.397 Calculations of muscle protein synthesis, breakdown, and amino acid transport were made. Insulin infusion into the femoral artery caused tissue concentrations of free essential amino acids to decrease, and an increase in the fractional synthesis rate of muscle protein. While insulin did not significantly modify the catabolic release of phenylalanine, leucine, and lysine it did increase the rates of inward transport of leucine, lysine, and alanine in leg muscle tissue. The authors concluded that hyperinsulinemia, within the physiological range, promoted muscle protein anabolism primarily by stimulating protein synthesis independently of any effect on transmembrane transport. Other early evidence showed that insulin may exert its protein anticatabolic effect through stimulating cell hydration.420 Insulin stimulates cell transporter systems that result in a cellular accumulation of potassium, sodium, and chloride leading to cell water retention. The resultant swelling, in turn, signals increased protein synthesis in the cell. Glucagon produces opposite effects (cell dehydration), and thus promotes protein catabolism. Other studies have shown that insulin increases SM protein synthesis and decreases SM protein breakdown especially if hypoacidemia is not present. It appears that adequate amino acids must be present to maximize insulin’s effects on protein metabolism.421 Interestingly, the use of certain amino acids may increase endogenous insulin as well as providing the needed amino acid substrates for protein synthesis. In one study the effects of insulin were measured in vivo.422 In vitro, insulin has been shown to increase SM protein synthesis and decrease SM protein breakdown. Whether these same effects are found in vivo in humans is less clear. The study of the effect of hyperinsulinemia (INS) on SM protein turnover (SMPT) is complicated by hypoaminoacidemia, which can obviate the true effect of insulin on SMPT. To prevent this, the authors studied the effect of INS on SMPT in the human Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 45 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 45 forearm with amino acid (AA) infusion to ensure adequate substrate for full evaluation of insulin’s effect. They found that protein balance became positive with INS þ AA infusion. Insulin can influence other anabolic hormones including GH,423 IGF-1,424,425 and testoster426 one to further increase the overall anabolic effect. A study looked at the effects of insulin suppression after long-term diazoxide (an inhibitor of pancreatic insulin secretion) administration on testosterone and SHBG blood concentrations in two groups of obese and healthy normal weight men.427 The results of this study demonstrated that insulin stimulates testosterone production while at the same time inhibiting SHBG in adult normal weight healthy controls and obese individuals. The decrease in SHBG, without a parallel drop in total testosterone, increases the serum level of free testosterone, the active form of testosterone. The authors of this study felt that insulin may increase testosterone levels through one or several mechanisms that include: 1. It may work synergistically with LH, the pituitary hormone that dictates testicular testosterone production. It may do this by stimulating a testes enzyme called 11-b-hydroxysteroid dehydrogenase, which has been shown to relieve steroid-dependent inhibition of Leydig cell function and thus increase testosterone secretion.427 2. Insulin may suppress activity of a peripheral enzyme found in fat called aromatase. This enzyme converts testosterone into estrogen, thus decreasing plasma testosterone levels. Through suppressing this enzyme, insulin will raise free testosterone levels. Although insulin increases both glycogen and fat storage, there is a clear-cut dissociation between the effects of insulin on protein, lipid, and glucose metabolism.428 Thus while the effects of GH opposes the action of insulin on sugar uptake and fatty acid release, it complements the anabolic action of insulin on amino acid uptake.183 Thus insulin may have a synergistic protein-synthesizing effect with GH. Some believe that GH has a superior muscle protein synthesizing effect compared to insulin, while insulin has a superior anticatabolic effect. Some studies, however, show that GH interferes with insulin’s anticatabolic properties,429 as do corticosteroids. The effects of glucocorticoid on basal muscle protein turnover are minimized by compensatory hyperinsulinemia, and glucocorticoids cause muscle resistance to insulin’s antiproteolytic action.430 Moreover, insulin can suppress the stimulation by glucocorticoids of muscle protein catabolism.431 In one study it was found that the administration of insulin and testosterone and feeding a highprotein high-fat diet minimized muscle protein wasting by counteracting insulin resistance caused by corticosterone, and masking the corticosterone receptor.432 Insulin resistance has also been found to increase muscle catabolism by activation of the ubiquitin-proteasome proteolytic pathway.433 Not only does insulin have potent anabolic effects either alone or in concert with other anabolic hormones but insulin also increases the anabolic effect of certain amino acids. For example, the effect of glutamine on SM includes the stimulation of protein synthesis which occurs in the absence or presence of insulin, the response being greater with insulin.434 On the basis of this capacity of insulin to stimulate testosterone production and inhibit SHBG production, it is likely that amino acids (with and without added carbohydrate) which stimulate insulin secretion, particularly after training, may result in an enhanced anabolic state. We present more information on posttraining nutrition below. Insulin’s Controversial Effects on Protein Synthesis and Degradation Insulin is a potent anabolic hormone that affects both protein synthesis and protein catabolism. The popular view, however, is that insulin is an anabolic hormone in muscle but has little anticatabolic effects. As such, insulin’s anticatabolic effects in humans remain controversial, with contradictory reports showing either stimulation of protein synthesis or inhibition of protein breakdown, or both, by insulin. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 46 2.11.2007 12:16pm Compositor Name: VAmoudavally 46 Amino Acids and Proteins for the Athlete: The Anabolic Edge Insulin deficiency results in protein catabolism and loss of muscle, which can only be reversed by the use of insulin.435 Insulin has been shown to decrease protein catabolism in some studies,178,436–438 and to increase protein synthesis in others when combined with increases in systemic amino acid levels.397,413,422,439 A recent study on humans has shown that insulin can acutely stimulate muscle protein synthesis by increasing the initiation of mRNA translation.440 However, if the physiological increase in insulin secretion is pharmacologically suppressed during feeding in rats, the stimulation of translation initiation is abolished and muscle protein synthesis is suppressed.441 On the other hand, insulin can also reduce protein breakdown by stabilizing lysosomes and reducing the activity of the ubiquitin-proteasome pathway.442–445 Another recent study looked at whether the response of muscle protein synthesis to insulin depends on insulin-induced changes in amino acid delivery and availability for the muscle tissue.446 In summary, the results of the study indicate that ‘‘in healthy young individuals, physiological hyperinsulinemia can stimulate muscle protein synthesis provided that it increases blood flow and amino acid delivery to and availability for the muscle tissue.’’ As such, it seems that amino acid availability by a combination of adequate amino acid levels as well as adequate perfusion are critical factors in the insulin regulation of muscle protein turnover. Still another study found that intra-arterial insulin infusion into healthy human subjects, which increased insulin to normal physiological postprandial levels, suppressed muscle protein breakdown with no significant effect on muscle protein synthesis but significantly suppressed muscle protein breakdown.447 The authors concluded that ‘‘normal physiological levels of insulin achieve muscle protein anabolism by inhibiting muscle protein breakdown. This conclusion does not contradict results demonstrating stimulatory effect of insulin on muscle protein synthesis when amino acid levels are elevated or when supraphysiological doses of insulin are administered.’’ Although the studies are conflicting, the consensus seems to be that insulin and amino acids are required for the postprandial and exercise-induced stimulation of protein synthesis in SM. The anabolic action of insulin and amino acids on muscle protein metabolism involves a stimulation of translation initiation which, in part, involves the phosphatidylinositol 3-kinase (PI3-kinase)=mTOR pathway.448 A study looking at the roles played by insulin and amino acids on muscle protein synthesis449 concluded that the stimulatory effect of insulin was associated with an increase in 4E-BP1 and eIF4E phosphorylation states leading to a dissociation of the inactive 4E-BP1–eIF4E complex. Insulin also activates other signal transduction pathways parallel and upstream to 4E-BP1–eIF4E through phosphorylation of p70S6k and PKB, respectively. Dietary amino acids induced a significant improvement of protein synthesis in adult rats along with an increase in the phosphorylation states of 4E-BP1 and p70S6k. Phosphorylation of p70S6k increases the phosphorylation of ribosomal protein S6 and facilitates the synthesis of some ribosomal proteins, initiation factors, and elongation factors that play important roles in protein synthesis.450 Insulin and Nutrient Delivery Insulin has been shown to increase both total SM blood flow as well as capillary recruitment and thus increase flow to the individual muscle cells, in NO-dependent and -independent fashions.451–461 Infusion of L-NAME, a competitive NOS inhibitor, completely blocks the effect of insulin to increase flow and partially blocks insulin stimulated leg glucose uptake.462,463 Pretreatment with L-NAME attenuates insulin-enhanced capillary volume by 50%–70%, suggesting that these effects are partially NO-dependent.464,465 Insulin resistance, the metabolic syndrome, and diabetes can counteract the vascular effects.466 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 47 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 47 As such, insulin has significant effects not only on nutrient uptake but also on nutrient delivery secondary to its effects on increasing both bulk and microvascular blood flow to SM. This effect of insulin improves the ability to redistribute blood flow between working and recovering muscle and other tissues so as to increase the delivery of hormones, nutrients, and other factors essential to growth and repair. In addition to effects on endothelium-derived NO, direct actions of insulin on vascular smooth muscle cells, release of other vasoactive factors, changes in muscle metabolism, and alterations in sympathetic activity may contribute to insulin-stimulated capillary recruitment. In the last few decades we have seen that insulin’s action to increase microvascular (nutritive) perfusion of muscle, which is enhanced by exercise,467,468 is crucial to maximizing the anabolic effects of exercise and targeted nutrition. The major hemodynamic effect of insulin is to increase nutritive capillary blood flow in muscle. It remains unclear whether changes in capillary recruitment and total blood flow are independent or functionally coupled. For example, a recent review looked at the effects of insulin on the vascular system and on nutrient delivery to muscle.469 The paper points out the fact that there are two flow routes in muscle: one in intimate contact with the muscle cells (myocytes) and able to exchange nutrients and hormones freely and thus regarded as nutritive, and a second with essentially no contact with myocytes and regarded as nonnutritive (felt to provide blood to muscle connective tissue and adjacent fat cells, but not muscle cells). It seems plausible that in the absence of increases in bulk flow to muscle, say after a training session, insulin may act to switch flow from nonnutritive to the nutritive route. This capillary recruitment would result in an increase in nutritive blood flow so that muscles that have been stressed and are undergoing an adaptive response will have what they need to recover and grow. A comprehensive review published online and waiting to be published in the journal Endrocrine Reviews looked at the cardiovascular effects of insulin.470 The review discusses molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases. Pertinent areas of discussion in this review include: . . . . . . Insulin signaling in vascular endothelium and smooth muscle Insulin-stimulated capillary recruitment and blood flow Effects of insulin resistance on hemodynamic and cardiovascular parameters Role of insulin in coupling hemodynamic and metabolic physiology Insulin resistance and pro-inflammatory signaling and adipocytokines The salutary effects of calorie restriction and exercise on vascular actions of insulin Insulin, GH, and IGF-1 Synergism In addition to the GH=IGF-1 combination mentioned above, there are other studies supporting the synergistic effects on protein metabolism of combinations of other hormones. In one study, GH and insulin separately produced an increase in whole-body and SM protein net balance. GH plus insulin was associated with a higher net balance of protein than was insulin alone.183 There thus appears to be a synergistic improvement in whole-body and SM protein kinetics when GH and insulin are used together. For example, together they reduce whole-body and SM protein loss in cancer patients.423 It has also been shown that insulin and IGF-1 have additive effects on protein metabolism.471 An increase in whole-body and SM protein net balance was produced with separate administration of GH and insulin in postabsorptive normal volunteers, and a combination of GH and insulin was associated with a higher net balance of protein than either given separately.183 Such a therapy of a combination of anabolic hormones or of specific nutrients would be expected to be of substantial value in limiting protein catabolism. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 48 2.11.2007 12:16pm Compositor Name: VAmoudavally 48 Amino Acids and Proteins for the Athlete: The Anabolic Edge A study compared the metabolic and cardiovascular effects of recombinant human IGF-1 and insulin in six normal subjects.472 Results showed that IGF-1 has a similar effect to insulin on glucose metabolism, except with a slower onset and longer duration of action. IGF-1 had no effect on free-fatty acid levels in blood. IGF-1 also inhibits insulin secretion, and increases cardiac output, heart rate, and stroke volume. Although other studies show that IGF-1 acutely reduces free-fatty acid levels, this study showed no effect on fatty acids. This may have been due to the small number of study subjects. IGF-1 may directly inhibit insulin release at the level of the pancreatic b-cells, where insulin is produced. In contrast to insulin, IGF-1 has a definite heart stimulating action. Insulin only affects the heart in high doses, an effect thought to occur through a cross-reaction with IGF-1 cell receptors in the heart. This study underscores the fact that since IGF-1 can decrease blood glucose levels in a manner similar to that of insulin, large amounts taken under uncontrolled conditions may produce hypoglycemia as a common side effect. While IGF-1 is recognized as an insulin sensitizer at the liver and muscle, recent evidence suggests that IGFBP-3 induces insulin resistance in adipocytes.473 IGFBP-3, the major binding protein for IGF-1, has been shown to be able to act directly and has recently been demonstrated to play a role in glucose metabolism.474,475 THYROID HORMONES The thyroid gland, located in the lower front part of the neck, secretes two iodinated amino acids, levothyroxine (T4, L-thyroxine, thyroxine), and liothyronine (T3, triiodothyronine), that are the active thyroid hormones. Triiodothyronine possesses five times the biological activity of L-thyroxine. The thyroid gland is the only source of endogenous T4. Normally small amounts of T3 (about 15%–20% of circulating T3) are secreted from the thyroid while the majority comes from the peripheral (mainly in the liver and kidneys) monodeiodination of T4. Thyroid hormones are essential for the normal growth and development of many tissues and exert most of their effects by increasing the basal metabolic rate, controlling protein synthesis, and enhancing the lipolytic response of fat cells to other hormones. Thyroxine increases the rate of metabolism of all cells, affects protein metabolism, and modulate energy production. The activity of the citric acid cycle (tricarboxylic acid cycle or Krebs cycle) is diminished in the presence of high plasma levels of thyroid hormones and enhanced by the hypothyroid state.476 Under conditions of insufficient carbohydrates and fats, thyroxine increases protein catabolism in order to provide substrates for gluconeogenesis.477 On the other hand, under normal dietary conditions, including adequate protein intake, thyroxine, in concert with other hormones, can increase the rate of protein synthesis. Thyroid hormone levels can be influenced by a number of factors. For example, too little or too much iodine in the diet can adversely affect thyroid hormone production. So can excess stress, lack of sleep, and the abuse of alcohol478 or cocaine.479 In animals and man hypothyroidism is associated with growth impairment, and hyperthyroidism with the development of a hypercatabolic state and SM wasting.480 Both states adversely affect muscle function. Thyrotoxicosis produces myopathy caused by net protein catabolism, accelerated basal metabolic rate, and impaired carbohydrate metabolism.481 Although T4 has some hormonal actions itself, it is primarily a prohormone, and most of the actions of thyroid hormones are mediated by T3. In the liver, and many other tissues including muscle, T3, within parenchymal cells, appears to be derived from (or to be in equilibrium with) T3 in plasma, whereas about 50% of intracellular T3 in the anterior pituitary and most T3 in brain is locally produced.482 Thus, the actions of thyroid hormones in liver and muscle, as compared with the pituitary and the brain, might be expected to relate more closely to circulating levels of T3. Under some conditions such as fasting, a significant amount of T4 is converted peripherally (the thyroid gland is a minor source of both T3 and rT3) to reverse T3 (rT3) at the expense of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 49 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 49 T3 formation, resulting in lower serum levels of T3.483–485 Artificially increasing levels of T3 in fasting results in protein catabolism. In one study with grossly obese subjects, T3 administration resulted in mostly body protein loss and only to a small extent loss of body fat.486 T3, on the other hand, increases with overfeeding and with increased carbohydrate intake. rT3 is more biologically inactive and more calorigenically inactive than either T4 or T3, so diverting T4 metabolism=conversion into making rT3 instead of T3 can save the organism a significant expenditure of energy. Alterations in the metabolism of T4 to rT3 appear to be important in the adjustment of energy expenditure in various tissues during different metabolic and nutritional states.487 A recent study found that lower T3 levels were a predictor of weight gain. Moreover, the findings of this study indicated that the control of T4 to T3 conversion may play a role in weight regulation.488 In acute nonthyroidal illness there are profound alterations in thyroid homeostasis. Most frequently, the ‘‘sick euthyroid syndrome’’ is produced in which the person does not experience clinical symptoms of hypothyroidism, but is physiologically hypothyroid and thus may experience a lowering effect on metabolism. This includes a reduction in serum T3 levels, low or normal T4, normal free T4, and elevated rT3 while TSH is normal.489 The low serum T3 may be an adaptive mechanism of the body to lower the basic metabolic rate (BMR) and therefore conserve energy and minimize wasting.490 The increased conversion of T4 to rT3 and decreased T3 formation has been observed in severe systemic diseases, including starvation and fasting,491–494 hepatic disease,495 after surgery,496 during treatment with corticosteroids,497 and in high catecholamine states, such as burns.498 In postoperative critically ill patients, Zaloga et al.499 observed that although the serum thyroidstimulating hormone (TSH) response to thyrotropin-releasing hormone (TRH) was normal after surgery, the maximal TRH-induced increase in serum TSH and the integrated serum TSH response to TRH were suppressed in the early postoperative period. Starvation impairs T4 conversion to T3 and can increase rT3 formation both in humans and animals.500,501 When T3 is administered in physiologic amounts to fasting subjects, plasma T3 increases, T4 and rT3 are unchanged, TSH response to TRH is diminished, and nitrogen excretion increases. This suggests that the low T3 levels during fasting spare muscle protein and that there is a lower set point for regulation of TSH secretion in fasting subjects.493 In humans, these changes in thyroid metabolism are more readily reversed by carbohydrate than by protein or fat.500,501 The relative contributions of various tissues to T4 metabolism and where the effects of starvation and other influences of T4 conversion actually occur are not known, but the liver is undoubtedly a major site and has been most widely studied. In an attempt to explain reverse T3 changes during protein supplemented semistarvation, rT3 and T3 changes have been evaluated in regard of bodyweight changes and changes of nutrient combustion.502 The authors concluded that although no definite clue could be found for the rT3 increase during semistarvation, a combined effect of lipolysis and lean mass preservation is suggested. Overfeeding results in an adaptive thermogenic response (a process whereby excess calories are converted to heat rather than stored as fat) that attenuates any resulting weight gain. Part of this adaptive response is related to an increase in thermogenesis manifested as an increase in the resting metabolic rate. Thyroid hormone, a well-known regulator of activity and biogenesis of mitochondria in many tissues,503 is also the major effector of the increased mitochondrial biogenesis in brown adipose tissue seen secondary to adaptive changes.504 Overeating leads to increased plasma levels of T3 and decreased levels of rT3 while a converse effect is seen in starvation, with decreased T3 and increased rT3 levels.485 Starvation produces a rapid fall in BMR of 20% or more, and impairs thermogenesis in parallel with the drop in thyroid hormone. Besides the adverse physiological effects of large amounts of thyroid hormone (with associated neurological and cardiovascular complications), the significant muscle loss, and the suppression of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 50 2.11.2007 12:16pm Compositor Name: VAmoudavally 50 Amino Acids and Proteins for the Athlete: The Anabolic Edge the hypothalamic-pituitary-thyroid axis, there has been some evidence that both hyperthyroidism and thyroxine replacement therapy may be associated with subclinical liver damage.505 Thyroid Hormones and Protein Metabolism The effects of low or high thyroid hormone levels on protein synthesis and SM have been well documented. However, the studies looking at changes in thyroid levels during exercise are not consistent and give rise to some confusion. Thyroxine and triiodothyronine stimulate both protein synthesis and degradation—depending on their levels and the presence and levels of other hormones and regulators. In physiological concentrations thyroid hormones stimulate the synthesis as well as the degradation of proteins, whereas in higher or supraphysiological doses protein catabolism (breakdown) predominates—as part of the overall hypermetabolism that occurs. Thus hypothyroidism suppresses muscle growth while hyperthyroidism increases protein catabolism. The rate of protein degradation is significantly higher in hyperthyroid patients. In one study, a significant positive correlation was found between serum T3 and rate of protein degradation in SM.506 This study demonstrated for the first time increased proteolysis in SM tissue from patients with high serum T3 concentrations. Hyperthyroidism also increases the rate of glutamine release from SM.507 Thus in hyperthyroidism or with the use of high doses of thyroid hormone, the net effect is muscle wasting (with an increase in the turnover of body protein) rather than hypertrophy. Prolonged use of large doses of thyroid may result in extensive muscular wasting. In contrast, an excess in thyroid hormone can produce cardiac muscle hypertrophy.508 One study investigated the responsiveness of protein metabolism to insulin as a mediator of the protein catabolic response to hyperthyroidism in humans.509 Six healthy volunteers were studied in a postabsorptive state before and after oral intake of thyroid hormones. Insulin was infused for 2.33 h under euglycemic and eukalemic clamps. An appropriate amino acid infusion was used to blunt insulin-induced hypoaminoacidemia. Hyperthyroidism induced a significant proteolysis and in the oxidation and nonoxidative disposal of leucine. Insulin lowered proteolysis and hyperthyroidism improved the ability of insulin to inhibit proteolysis. Insulin also moderately lowered protein synthesis in both control and hyperthyroid states. These changes in insulin action may provide a mechanism to save body protein during hyperthyroidism. To study the role of thyroid hormones in exercise-induced muscle growth and protein synthesis, Katzeff et al.510 measured skeletal and cardiac muscle protein synthesis and MHC gene expression in hypothyroid rats allowed to exercise voluntarily. The rats without exercise experience significant SM atrophy while the exercise group did not. Exercise, therefore seems to mediate the effects of hypothyroidism on SM protein synthesis. Calorie-reduced diets, when combined with exercise, can result in an increase in protein synthesis which helps maintain lean body mass, aided by a decrease of T3 which protects the organism against energy deficit.511 The authors of one study investigating diurnal changes in whole-body protein turnover associated with the increasing fasting body nitrogen (N) losses and feeding gains with increasing protein intake in normal adults, found that thyroid hormones (free and total triiodothyronine) did not change in any state.512 Resting metabolic rate in the fasted and fed state was not influenced by dietary protein intake, but was increased by feeding with no influence of dietary protein concentration. A study investigated the relations between changes in plasma insulin and triiodothyronine (T3), and muscle growth and protein turnover in the rat in response to diets of varying protein concentrations.513 The authors found that dietary energy had no identifiable influence on muscle growth. In contrast, increased dietary protein appeared to stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, as well as increasing insulin and free T3 levels. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 51 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 51 They also found that changes in rates of muscle growth were accompanied by parallel changes in rates of protein synthesis and degradation, as well as parallel changes in concentrations of plasma insulin and free T3, to the extent that all these variables were highly correlated with each other. Insulin and free T3 stimulated each other as well as muscle protein turnover. On the other hand, a study in rats found that high protein diets decreased T3 levels. This study looked at the effects of a high-protein (30%, HP) and a medium-protein (15%, MP) diet in rats after 21 days of hindlimb unweighting.514 With respect to pair-fed animals, a significant reduction in protein synthesis occurred (33%) in MP non-weight-bearing rats, whereas it was of lesser magnitude and not significant in HP rats. The authors concluded that because the circulating level of free triiodothyronine was reduced by 14% with the HP diet, this effect on protein synthesis may involve thyroid hormones. It seems that with strenuous exercise, thyroid function decreases.515 In some cases, one way to stop the decrease is to eat more.516 However, some lowering of the T3 levels may be beneficial and lead to increased protein synthesis and a decreased energy deficit. Thyroid Hormone and Other Hormones Thyroid growth and function is affected by various other hormones. For example, insulin or IGF-1 needs to be present for the stimulation of thyroid hormone production by TSH.517 Thyroid hormone interacts with, and may act synergistically or antagonize, the anabolic effects of GH, IGF-1, insulin, and testosterone,518–520 and may be involved in the regulation of leptin synthesis.521 The effect of thyroid hormone seems to vary in different species. For example, in both rat and fish, thyroid hormone increases transcription of GH mRNA in the pituitary and increases protein synthesis.522 In vitro studies on goat testicular cells have shown that T3 resulted in a dose-dependent increase in androgen release from testicular Leydig cells.523 T3 also had additive stimulatory effects on LH-augmented androgen release from Leydig cells. A number of studies and reviews of thyroid hormones and their role in testicular steroidogenesis524–530 found that: . . . T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. T3 stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. It has also been shown to affect testicular steroidogenesis by optimizing mitochondrial function.531 In an in vitro study using sheep thyroid, the cells were found to be a site for IGF action, binding, and production.532 In this study IGF synthesis was enhanced consistently by recombinant GH. Thyroid hormone has been shown to have a role in secretion of GH and the expression of IGF-1. A recent study found that the thyroid status affects GH=IGF axis.533 Hypothyroidism is associated with significant reductions of IGF-1 and IGFBP-3, and IGFBP-1 is elevated in both hypothyroidism and hyperthyroidism. In man, however, thyroid hormone seems to decrease GH effects possibly by reducing the bioavailability of IGF-1, and antagonize the anabolic effects of testosterone and insulin. Thyroid hormones in excess, like glucocorticoids, increase the catabolism of fast glycolytic fibers, whereas androgens, catecholamines, and b-agonists have been shown to be anabolic and produce an enlargement.534 Moreover, thyroid hormone activity is reduced by androgens.535 Decreases in thyroid hormone secretion and serum levels can occur secondary to the use of anabolic steroids.152,536 A study measured urea nitrogen synthesis rates and blood alanine levels concomitantly before, during, and after 4 h constant intravenous infusion of alanine.537 Eight normal male subjects were Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 52 2.11.2007 12:16pm Compositor Name: VAmoudavally 52 Amino Acids and Proteins for the Athlete: The Anabolic Edge randomly studied four times: (1) after 10 days of subcutaneous saline injections, (2) after 10 days of subcutaneous GH injections, (3) after 10 days of T3 administration, and (4) after 10 days given (2) þ (3). The results of the study show that GH decreased functional hepatic nitrogen clearance and that T3 has no effect on functional hepatic nitrogen clearance. However given together with GH, T3 abolishes the effect of GH on functional hepatic nitrogen clearance. A possible mechanism is the known effect of thyroid hormones in reducing the bioavailability of IGF-1. A decline in urea excretion is seen following long-term GH administration, reflecting overall protein anabolism. Conversely, hyperthyroidism is characterized by increased urea synthesis and negative nitrogen metabolism. These seemingly opposite effects are presumed to reflect different actions on peripheral protein metabolism. The extent to which these hormonal systems have different direct effects on hepatic urea genesis has not been fully characterized. In one study, therapy with GH for 6 months to adult patients with GHD increased lean body mass, decreased fat mass, improved the sense of general well-being in most patients, increased bone turnover without a measurable increase in bone density, caused some minor changes in lipid and carbohydrate metabolism, and increased the metabolism of thyroxine to T3.538 In the same study the authors felt that the mechanism by which thyroid hormone produces cardiac hypertrophy and myosin isoenzyme changes remains unclear but may involve IGF-1, another anabolic compound. In this study, levels of T4 and free T4 as well as rT3 decreased, while T3 increased during GH treatment. It has been shown that physiological doses of insulin selectively attenuate the stimulatory effect of T3 on GH mRNA levels.539 Thus insulin decreased the thyroid-increased GH synthesis. This suppressive effect of insulin occurs independently of protein synthesis and is presumably mediated both at a transcriptional and posttranscriptional level. Insulin has previously been shown to inhibit basal and stimulated rat GH (rGH) secretion as well as basal GH transcription in rat pituitary cells. It appears that in man, thyroid hormone changes are largely independent of changes in insulin and glucagon. The effect of isocaloric (500 kcal) protein and carbohydrate ingestion was studied in a crossover study in nine healthy humans.238 The physiological increase in insulin after carbohydrate ingestion, and the physiological increase in glucagon after protein ingestion were not associated with any changes in total T4, free T4, T3, free T3, or rT3. TESTOSTERONE The androgenic and anabolic actions of substances produced by the testes have been known for centuries. Two case reports by Sacchi,540 and Rowlands and Nicholson541 comprise the early evidence of the influence of testicular androgens, specifically testosterone, on growth and musculoskeletal development. Kenyon, in a series of studies extending from 1938 to 1944,542–545 investigated and elucidated the metabolic effects of testosterone. He was one of the first to outline possible clinical uses for testosterone. Studies have shown that testosterone increases muscle protein synthesis and enhances strength, size, and athletic performance.546 Adequate levels of endogenous testosterone are needed for successful adaptation to strength training and body composition, and suppressing endogenous testosterone production has been found to have an adverse effect on both.547 Studies have also shown that supraphysiologic doses of testosterone significantly increases both muscle size and strength in normal men and that this increase is dose dependent.548,549 In one study,550 43 normal men were assigned to four different groups: placebo with no exercise, testosterone with no exercise, placebo plus exercise, testosterone plus exercise. The men received injections of 600 mg of testosterone enanthate or placebo weekly for 10 weeks. The results of this study conclusively shows that supraphysiologic doses of testosterone increases muscle mass and strength both on its own and more so when combined with exercise. In another study with healthy eugonadal men it was found that muscle volume increased in proportion to the dosage of testosterone given, and that the increased muscle mass was associated Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 53 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 53 with an increase in the cross-sectional areas of both type I and type II muscle fibers and myonuclear number.551 Although increasing muscle mass testosterone also decreases fat mass and thus significantly improves body composition,552 a recent comprehensive study in men found dose-dependent changes in overall, subcutaneous, intra-abdominal, and intermuscular fat with testosterone administration.553 In a recent review,554 the authors concluded: ‘‘Testosterone increases muscle mass by multiple mechanisms; it promotes muscle protein anabolism and the differentiation of pluripotent stem cells toward the myogenic lineage and inhibits adipogenic differentiation.’’ Stress and Testosterone Levels Stress has an adverse effect on serum testosterone levels. Decreased plasma testosterone levels have been observed after surgery555,556 and myocardial infarction.557 Woolf et al.558 observed that critically ill men and women have decreases in plasma concentrations of testosterone and estradiol, respectively. The latter explains the amenorrhea seen during such stressful situations. The exact cause is not clear but may be due to decreased or altered secretion of intrapituitary and suprapituitary substances (e.g., FSH and LH) or decreased response of the pituitary to gonadotropin-releasing hormone.559 Exercise and Testosterone Levels Testosterone levels can increase or decrease with exercise. Some studies have shown that serum testosterone concentrations tend to increase if the resistance training session lasts for a relatively short period of time, say from 30 to 60 min,63–66,560,561 and that repetitive, intense resistance training can reduce resting serum testosterone levels when the overall training stress is exceptionally high for too long a period.562 For instance, it has been shown that 2 h of resistance training twice a day can decrease resting testosterone concentrations.563 This would be expected to lower the testosterone to cortisol ratio, an indicator of changes in the anabolic–androgenic activity of the body,564 in favor of a catabolic state. Studies have found that women with low average testosterone to SHBG ratios (an index of the serum-free testosterone concentration) did not increase their muscle mass to the same degree as those with higher ratios.565 In women, the level of serum testosterone seems just as important for the development of muscle strength and hypertrophy during resistance training as in men. Androgens such as testosterone (and their regulating hormones such as LH) and GH increase with short bursts of intense exercise as long as the bursts are not taken over too long a period of time so as to become exhausting. The level of testosterone, after a certain point in the exercise session, decreases with increased duration of intense exercise. Thus repetitive and prolonged heavy exercise can be counterproductive. Overtraining also results in chronically lowered serum levels of GH. We thus need to strike a balance between GH and testosterone increases both during and between exercise sessions. In general, studies have shown a rise in testosterone levels and GH (both during and after the exercise) during short-term high volume, high strength level exercises (such as weight lifting, sprinting, etc.), while with strenuous prolonged exercises (such as marathon running and other endurance events, or with overtraining in high strength level exercise), the testosterone and GH level are depressed while the cortisol levels are elevated.61,566 Prolonged stressful exercise can lead to distinct hormonal adaptations that offset some of the changes involving the catecholamines, the hypothalamic-pituitary-testicular axis, and the hypothalamic-pituitary-adrenal axis.567 Acute resistance training has been shown to increase serum testosterone levels. A study by Falkel et al.568 measured the time course for SM adaptations to heavy-resistance training in men and Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 54 2.11.2007 12:16pm Compositor Name: VAmoudavally 54 Amino Acids and Proteins for the Athlete: The Anabolic Edge women. The authors found that absolute and relative maximal dynamic strength was significantly increased after 4 weeks of training in both sexes. Moreover, there was an increase in the resting levels of serum testosterone after 4 weeks and a decrease in cortisol after 6 weeks in men. It has been shown that prolonged physical stress can decrease the serum levels of testosterone and increase serum levels of cortisol and dihydroepiandrosterone sulfate (DHEA-S).569 In this study the circadian rhythm of hormones was abolished by the adverse conditions and physical stress. Testosterone is felt to play a major role in the muscle hypertrophic response to exercise although its exact role has yet to be fully defined. On the contrary, the implications of changes in total and free testosterone and in SHBG have not been adequately investigated and there are many discrepancies in the literature. Although more studies are needed to define the role of testosterone in protein synthesis and the response to exercise, a number of studies over the last few decades are providing us with some useful information. One study570 tested the response of total testosterone (T), free testosterone (fT), SHBG, and non-SHBG-bound testosterone (NST) to the same exercise protocol in two different experiments on long-distance runners. The two experiments were carried out at the end of the general preparation of the second 4-month macrocycle of the annual training season. In each experiment, each athlete ran for 1 h at a predetermined speed, assessed during a previously performed field test. The first experiment was performed at 9.00 a.m., the second at 3.00 p.m. Total testosterone and SHBG decreased after the first experiment, but the value after the recovery period was significantly lower than the initial value only for SHBG. In the second experiment, both total testosterone and SHBG increased after exercise, but only total testosterone decreased significantly to the initial concentration after the rest period. In both experiments, NST and fT increased postexercise and decreased in the rest period to the initial values. The authors referred to previous studies in which it has been shown that the decrease in SHBG keeps the biologically active fraction of testosterone at a fixed level. As the current study found that SHBG variations showed the same trend as total T and the values of the two parameters postexercise were positively correlated, the authors claimed that their results provide further support for these earlier findings. In addition, they concluded that their results are suggestive of a compensatory mechanism, operating during exercise, which maintains adequate concentrations of biologically active testosterone when total testosterone concentrations decrease. Several factors appear to influence the acute serum total testosterone responses to resistance exercise, including the muscle mass involved, the type of exercise, exercise intensity and volume, nutrition intake, and training experience.571 For example, large muscle mass exercises such as the squat and deadlift result in larger elevations in testosterone compared to small muscle mass exercises. Studies have also shown an increase of GH and IGF-1 with increasing levels of serum testosterone. In a study it was shown that there is an association between serum levels of freetestosterone and SHBG on the one hand, and serum IGF-1 and IGFBP-1 on the other.572 In this study serum levels of IGF-1 decreased both with increasing age and increasing SHBG levels, but increased with increasing free testosterone. Testosterone may decrease muscle catabolism. This effect of testosterone can be seen in the decreased exercise induced alanine levels seen with hypertestosteronemia.573 The anabolic and catabolic hormones influence each other and their combined effects can be synergistic or antagonistic depending on the hormones involved and on the overall environment. As we have seen GH, IGF-1, testosterone, and insulin can act synergistically in increasing protein synthesis and decreasing protein catabolism, and without GH and IGF-1, testosterone has limited anabolic effects; however, the combined use of testosterone and IGF-1,574 of testosterone and GH,575 results in a synergistic effect on protein metabolism. Moreover, testosterone and anabolic steroids increase GH secretion152,576 partly secondary to their aromatization to estradiol,577 and also increase serum IGF-1 levels.578 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 55 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 55 CATABOLIC HORMONES Testosterone, insulin, GH, and IGF-1 can generally be considered anabolic hormones as they increase protein synthesis or decrease protein degradation or both. Corticosteroids (cortisol, cortisone and other similar endogenous and exogenous glucocorticoid compounds), glucagon, and the catecholamines are usually considered the main catabolic hormones. However, although studies have shown that simultaneous physiologic elevations of the concentrations of the three hormones, cortisol, glucagon, and epinephrine, can accelerate net SM proteolysis,579 catecholamines have been found to actually exert anabolic effects on SM protein metabolism as against their catabolic effects on glucose and lipid metabolism.580 The anabolic and catabolic hormones interact in complex and intricate ways and their serum levels often reflect the influences of their hormonal counterparts. An example is the interaction between insulin and cortisol. Glucocorticoids and insulin are major, antagonistic, long-term regulators of energy balance.581 Studies have shown that insulin and corticosteroids interact to affect cellular metabolism in various ways depending on their respective serum levels. There are marked diurnal rhythms in function of the hypothalamic-pituitary-adrenal (HPA) axis under both basal and stress conditions. The HPA axis controls corticosteroid output from the adrenal and, in turn, forward elements of this axis are inhibited by feedback from circulating plasma corticosteroid levels. One review proposed the hypothesis that normal diurnal rhythms in the HPA axis are highly regulated by activity in medial hypothalamic nuclei to effect an interaction between corticosteroids and insulin such that optimal metabolism results in response to changes in the fed or fasted state of the animal.582 Corticosteroids interact with insulin on food intake and body composition, and corticosteroids also increase insulin secretion. Corticosteroids stimulate feeding at low doses but inhibit it at high doses; however, it is the high levels of insulin, induced by high levels of corticosteroids, which may inhibit feeding. Corticosteroids both synergize with and antagonize the effects of insulin. In the presence of elevated insulin stimulated by glucocorticoids and nutrition, stress causes less severe catabolic effects. The metabolic responses to SM stress and injury include accelerated net SM protein breakdown associated with increased stress hormone and decreased insulin concentrations. In one study, temporary insulin suppression during physiologic increases in stress hormone concentrations amplified whole-body nitrogen loss and led to the development of accelerated net SM protein breakdown.583 Therefore increasing or maintaining insulin levels and amino acid availability would likely decrease the catabolic response to exercise and other stressors. Effect of the Catabolic Hormones on Skeletal Muscle Catabolism The mobilization of amino acids from SM is an adaptive response to stress. The liberated amino acids are oxidized and used as fuel, and they are taken up by the immune system, liver, and other visceral organs. The proteolysis of muscle under physical stress thus enables the body to shift protein resources from muscle to the more urgent needs of the immune system and visceral organs. This shift is in large part mediated by circulating and local levels of various compounds, including cortisol, glucagon, epinephrine, GH, and cytokines. The involvement of cytokines such as interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-a) patients with a systemic inflammatory process probably play a major role in sparing the visceral protein compartment at the expense of the SM; it is not known if these two factors significantly affect protein metabolism under physical stress. Increased plasma levels of the hormones glucagon, epinephrine, and cortisol secondary to trauma and sepsis affect the metabolic response to these stressors. Infusion of the three hormones causes significant negative nitrogen and potassium balances, glucose intolerance, hyperinsulinemia, insulin resistance, sodium retention, and peripheral leukocytosis.584,585 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 56 2.11.2007 12:16pm Compositor Name: VAmoudavally 56 Amino Acids and Proteins for the Athlete: The Anabolic Edge In a study set up to determine the part that these hormones have and whether they act synergistically, normal subjects were infused with hydrocortisone, glucagon, and epinephrine at levels that simulated the plasma levels seen after moderate injury.586 The infusion produced increases in glucose production and decreases in glucose clearance. The effect was more pronounced when all three hormones were administered than when they were infused individually or in groups of two. The authors thus proposed that these hormones act synergistically. In one study physiological elevations of plasma cortisol levels, as are encountered in stress and severe trauma, were produced in six normal subjects by infusing them with hydrocortisone for 64 h. Physiological hypercortisolemia produced increases in proteolysis, and in glutamine and alanine production (secondary to proteolysis and de novo synthesis).587 In another study, accelerated muscle protein breakdown was found after a mixture of the catabolic hormones was infused into normal rats.579 The authors felt that although increased muscle proteolysis was mediated in part by catabolic hormones, reduced muscle protein synthesis, and amino acid uptake are probably signaled by other substances or mechanisms. In yet another study set up to determine the acute in vivo response of human muscle protein to stress, arterial infusion of the catabolic hormones, epinephrine, cortisol, and glucagon, resulted in a net catabolism of human muscle protein by increasing the rate of protein breakdown in excess of an increased protein synthetic rate.588 However, the degree of nitrogen loss observed during the infusion of the counterregulatory hormones (glucagon, epinephrine, and cortisol) into normal subjects is less than that observed during injuries causing comparable increases in the counterregulatory hormones.584 Studies have indicated that TNF may be the principal catabolic monokine, with IL-1 potentiating SM proteolysis.589,590 Increases or decreases in any or all of insulin, GH, IGF-1, and testosterone may affect the individual or combined catabolic effects of cortisol, glucagon, and epinephrine. For example, the catabolic hormonal effects of cortisol, glucagon, and epinephrine may be significantly attenuated by a concurrent increase in serum insulin concentration although how insulin may modify the catabolic hormonal effects remains to be fully elucidated. This modifying effect of insulin may be reduced if insulin concentration does not rise until after the catabolic hormonal effects have become established. CATECHOLAMINES Catecholamines exist in the circulation both as free catecholamines and as the sulfur conjugate that accounts for 60%–90% of the total catecholamines.591 The serum levels of the catecholamines norepinephrine592 and epinephrine593 have been observed to increase after a variety of stresses, including anxiety, hypotension,594 hypothermia, hypercarbia, and accidental injury.595 Epinephrine is secreted by the adrenal medulla in response to sympathetic nervous system activation while norepinephrine spills over into the plasma after release from the sympathetic nerve endings. The sympathetic nervous system, in turn, is controlled by the hypothalamus, the same area of the brain responsible for the excretion of releasing factors that initiate the secretion of other endocrine hormones. Plasma levels of epinephrine and norepinephrine do not necessarily increase concurrently.596 In a study of major accidental injury, plasma epinephrine concentrations were increased for only a short period (about 48 h) while norepinephrine levels remained elevated for periods of up to 8–10 days.597 Plasma epinephrine concentrations reflect adrenomedullary secretion, whereas plasma norepinephrine concentrations are used as an index of sympathetic nervous system activity. It is important to realize that most of the norepinephrine released by sympathetic ganglia is removed from the synapse by reuptake into the nerve ending.598 Epinephrine in physiologic doses results in glycogenolysis, increased hepatic gluconeogenesis with mobilization of gluconeogenic precursors from peripheral tissues, inhibition of insulin release, peripheral insulin resistance, and lipolysis.599,600 Epinephrine is a potent stimulator of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 57 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 57 gluconeogenesis. This is demonstrated by the observation that during starvation there is no diminution in the ability of epinephrine to stimulate gluconeogenesis. This contrasts with glucagon’s gluconeogenic ability which is less in starved than fed subjects.601 Catecholamines are important in increasing glucose production rates during exercise. A study looked at defining the relative roles of catecholamine versus glucagon=insulin responses in stimulating glucose production rates and found that catecholamines are most important in the regulation of glucose production rates in intense exercise.602 Sympathoadrenal activation during physical and emotional stress results in release of epinephrine and norepinephrine into the circulation. Heavy exercise and trauma have been reported to result in a 10- to 20-fold elevation of plasma epinephrine concentrations above resting values.593,603 Increased plasma catecholamine concentrations are associated with well-recognized changes in glucose,604 fat,605 and ketone body metabolism.606,607 Effects of Catecholamines on Protein Metabolism The mediators of muscle breakdown may be many. Shaw et al.608 observed that a-adrenergic blockade reduces nitrogen losses. Thus, adrenergic activity may be involved in catabolism. However, it seems that the catecholamines are not catabolic with regard to whole-body protein breakdown, although they do induce muscle protein proteolysis. For example, an epinephrine infusion will decrease proteolysis and thereby reduce circulating amino acid (except alanine) levels.609,610 There are only a small number of studies that have looked at the effects of catecholamines on protein metabolism in humans.580,611 Epinephrine administration results in decreased plasma concentrations of BCAAs in normal and diabetic subjects.610 Epinephrine infusion into normal subjects has been shown to decrease whole-body proteolysis studied by tracer infusions of amino acids.609 However, epinephrine infusion resulted in increased plasma insulin concentrations which may have confounded epinephrine’s effects. Epinephrine seems to have a split personality in that it has been shown to both increase and inhibit protein catabolism. For example, epinephrine has adrenergic effects, and b-adrenergic stimulation has been reported to inhibit amino acid release from SM.612,613 Moreover, the results of some studies are consistent with an insulin-like effect of epinephrine.614 A more definitive study, however, has cleared up some of this confusion. This study looked at the effect of hyperepinephrinemia on leucine plasma flux, oxidation, and nonoxidative disappearance using 1-(13C) leucine infusions.615 The authors assessed the contribution of muscle proteins in epinephrine’s effect on whole-body leucine kinetics by measuring net plasma leucine balance across the forearm and determine the role of b-adrenergic receptors in mediating epinephrine’s effect on leucine kinetics. They eliminated confounding effects of altered insulin and glucagon concentrations by administration of somatostatin with replacement infusions of the two pancreatic hormones. Epinephrine was infused either alone or combined with propranolol (b-blockade) into groups of six subjects fasted overnight; leucine flux, oxidation, and net plasma leucine forearm balance were determined during 3 h. All effects of epinephrine were independent of alternations in plasma insulin and glucagon concentrations since constant plasma insulin and glucagon concentrations were maintained in all studies by infusing somatostatin combined with insulin and glucagon replacements. During epinephrine infusion, net forearm leucine release increased threefold, suggesting that the 22% fall of total body leucine flux during epinephrine was the result of a considerably greater fall of leucine appearance from nonmuscle tissues such as splanchnic organs. The results suggest that elevation of plasma epinephrine concentrations similar to those observed in severe stress results in redistribution of body proteins and exerts a whole-body protein-sparing effect, but a catabolic effect on SM tissue. Net forearm release of leucine increased during epinephrine suggesting increased muscle proteolysis. The fall of total body leucine flux was therefore due to diminished proteolysis in nonmuscle tissues, such as splanchnic organs. The effect Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 58 2.11.2007 12:16pm Compositor Name: VAmoudavally 58 Amino Acids and Proteins for the Athlete: The Anabolic Edge of epinephrine on forearm blood flow agrees with a previous paper,616 and largely explains the increase in net leucine forearm balance. On the contrary, epinephrine infusion resulted in a significant increase in plasma glucose concentrations. This effect was mediated via b-adrenergic receptors. Hyperepinephrinemia also results in a significant increase in plasma FFA concentrations due to its lipolytic effect. This effect was also b-receptor mediated.617 Elevation of plasma FFA in dogs decreased leucine plasma flux, oxidation, and concentration.618 The inhibitory effect of epinephrine on proteolysis is at least in part mediated through the increase in FFA. Studies have demonstrated that epinephrine administration results in increased circulating plasma insulin concentrations in man.619 These may have decreased plasma leucine flux and amino acid concentrations.620 To exclude the confounding effect of insulin these studies were performed using somatostatin with insulin and glucagon replacements to maintain constant plasma insulin and glucagon concentrations. Moreover, the hypothesis that epinephrine reduces protein synthesis by virtue of its stimulatory effect on BCAA oxidation621 may be true in SM but not in the splanchnic bed. Thus, the present data suggest that acute hyperepinephrinemia results in redistribution of amino acids, with a catabolic effect on SM. While muscles such as those of forearm release amino acids, protein breakdown is inhibited in other tissues. Whether prolonged b-adrenergic stimulation or blockade influence whole-body protein metabolism remains to be determined. However, although increases in catecholamines are generally considered catabolic, some work has pointed out the possible anabolic effects of these hormones. A study accepted for publication but not as yet published, has shown that epinephrine directly inhibits proteolysis of SM via the b-adrenoceptor.622 However, the decrease in proteolysis may be offset by a decrease in protein synthesis, and in fact may result in an overall catabolic response. In the last decade, however, there have been several studies showing that b-adrenergic receptor agonists have a beneficial effect on muscle protein balance and induce SM hypertrophy. One study found that the effect of epinephrine on amino acid metabolism is not detrimental and that epinephrine allows amino acid metabolism to proceed normally but at lower concentrations of amino acids.623 Other studies have found that oral administration of b2-adrenergic agonists, such as clenbuterol, formoterol, and cimaterol, induces hypertrophy of SM.624–628 GLUCAGON Glucagon, a hormone synthesized and secreted by the a2-cells of the pancreatic islets of Langerhans, is not chemically or structurally related to insulin. Glucagon increases blood glucose concentration by stimulating hepatic glycogenolysis. Besides stimulating glycogenolysis and gluconeogenesis, glucagon stimulates hepatic ketogenesis.629 Some of the metabolic effects produced by glucagon in various tissues (e.g., liver, adipose tissue) are similar to those of epinephrine. Glucagon stimulates the formation of adenylate cyclase, which catalyzes the conversion of ATP to cAMP, particularly in the liver and in adipose tissue. Formation of cAMP initiates a series of intracellular reactions including activation of phosphorylase, which promote the degradation of glycogen to glucose. The increase in blood glucose concentration occurs within minutes. Endogenous secretion of glucagon is stimulated when blood glucose concentration is low or when serum insulin concentration is increased. In general, the actions of glucagon are antagonistic to those of insulin; however, glucagon has been reported to stimulate insulin secretion in healthy individuals and in patients with type II diabetes mellitus. In nondiabetic subjects, variable elevations of glucagon levels have no effect on postprandial blood glucose because of the rapid increase in insulin levels which renders ineffective the stimulatory effects of glucagon on endogenous glucose production.630 Glucagon also has been reported to enhance peripheral utilization of glucose. The intensity of the hyperglycemic effect of glucagon depends on hepatic glycogen reserve and the presence of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 59 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 59 phosphorylases. The hyperglycemic effect of glucagon is increased and prolonged by concomitant administration of epinephrine. Glucagon produces extrahepatic effects that are independent of its hyperglycemic action. Glucagon has been shown to decrease plasma amino nitrogen concentration and decrease synthesis of protein and fat. Despite the strong association between protein catabolic conditions and hyperglucagonemia, and enhanced glucagon secretion by amino acids (alanine, arginine, and leucine), glucagon’s effects on protein metabolism remain less clear than on glucose metabolism. Physiological hyperglucagonemia during insulin resistance is catabolic in the short term and indicates that glucagon may influence muscle protein metabolism acutely in man.631 A study looking at the effects of physiological hyperglucagonemia on protein synthesis found that glucagon may reduce phenylalanine availability for protein synthesis.632 Another study concluded that glucagon is the pivotal hormone in amino acid disposal during an amino acid load and, by reducing the availability of amino acids glucagon inhibits protein synthesis stimulated by them.633 Glucagon and Insulin Glucagon and insulin are both secreted by the pancreas, the former by a-cells and the latter by b-cells. These endocrine secretions enter the portal vein so that the liver is exposed to high concentrations of these hormones. Glucagon increases hepatocyte cyclic AMP and promotes gluconeogenesis;634 insulin has the opposite effect: it decreases intracellular cyclic AMP concentration and prevents gluconeogenesis. In addition, glucagon increases glycogenolysis, lipolysis, and hepatic ketogenesis in the liver during starvation and diabetic ketoacidosis. The receptor mechanism that increases cyclic AMP concentrations is not the same as used by adrenergic mediators.635 Stimulators of glucagon secretion include hypoglycemia, protein ingestion, amino acid infusion, endorphins, exercise, GH, epinephrine, and glucocorticoids.636,637 Suppressors of glucagon secretion include the infusion and ingestion of glucose, somatostatin, and insulin.638 Insulin is an anabolic hormone with a multitude of effects. In addition to its role in increasing glucose transport across the cell membranes of adipose tissue and muscle, it stimulates glycogen production, inhibits lipolysis in adipose tissue, inhibits hepatic ketogenesis, and increases the rate of amino acid transport and protein synthesis in muscle, adipose tissue, and liver. The hormonal milieu of low insulin with elevated counterregulatory hormones, however they are brought about, is thought to be a stimulus to gluconeogenesis. The interaction of the counterregulatory hormones (glucagon, catecholamines, and cortisol) in response to injury has been of great interest. It is likely that catecholamines and glucagon work synergistically because either infused alone into normal subjects only causes transient elevation in gluconeogenesis, whereas more prolonged gluconeogenesis is seen when they are infused together.586 Nevertheless, in both normal and high protein diets, it is the glucagon to insulin ratio that is the major determinant of the degree of gluconeogenesis.639 During starvation the ratio is increased (increased glucagon and decreased insulin levels) and gluconeogenesis is promoted, whereas the reverse is true in the fed state. After most types of major surgery638 there is an increase in plasma glucagon, although some640 have failed to observe an increase after abdominal hysterectomy. Like starvation, there is an increased glucagon to insulin ratio. The amino acid composition of the diet influences the postprandial levels of plasma amino acids along with the hormones insulin and glucagon in humans fed single test meals identical in composition except for protein source.641 Soy protein induces a low postprandial insulin to glucagon ratio in both hypercholesterolemic and normocholesterolemic subjects while casein induces a high postprandial insulin to glucagon ratio. Individual and groups of amino acids have varying effects on both insulin and glucagon.642 In a study, the effects of intravenous infusion of 17 amino acids, each at a dose of 3 mmol=kg over 30 min, on the secretion of insulin, glucagon, and GH were studied in six castrated male sheep.360 Leucine was the most effective amino acid in stimulating insulin secretion but did not produce any Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 60 2.11.2007 12:16pm Compositor Name: VAmoudavally 60 Amino Acids and Proteins for the Athlete: The Anabolic Edge increase in glucagon and GH secretion. Alanine, glycine, and serine induced a greater enhancement of both glucagon and insulin secretion than other amino acids. No amino acid was able to specifically stimulate glucagon secretion without also increasing insulin or GH secretion. With regard to insulin and glucagon secretion, amino acids could be divided into groups according to their R groups. Neutral straight-chain amino acids stimulated both insulin and glucagon secretion, with a greater secretory response to shorter C-chain amino acids. BCAAs tended to enhance insulin and suppress glucagon secretion. In another study the relationships between changes in the plasma levels of insulin and glucagon in response to the postprandial increments of circulating amino acids were studied under normal physiological conditions in healthy dogs.643 Multiple correlation analyses indicated that only BCAAs were significantly correlated with insulin increases and were devoid of a relationship to glucagon. Similarly, only ornithine, lysine, and glycine were significantly correlated with glucagon profiles and devoid of a relationship to insulin. The significance of individual glucagon stimulating effects of alanine and arginine were masked by other amino acid interactions, as significant intercorrelation was found among all 13 amino acids. CORTISOL Cortisol is a vital hormone because it diverts glucose utilization from muscles to the brain, facilitates the action of catecholamines, and prevents the overreaction of the immune system to injury.644 Cortisol has many actions, including stimulating gluconeogenesis, increasing proteolysis645 and alanine synthesis, sensitizing adipose tissue to the action of lipolytic hormones (GH and catecholamines) and anti-inflammatory action. In addition, it causes insulin resistance by decreasing the rate at which insulin activates the glucose uptake system, likely because of a postinsulin receptor block.646,647 Increased pituitary production of ACTH stimulates increased glucocorticoid production. Glucocorticoids have a negative feedback effect on ACTH production and can also stimulate the adrenomedullary secretion of catecholamines. The administration of 500 mg cortisol sodium succinate at the time of surgical incision attenuated the increase in plasma ACTH.648 ACTH release is itself stimulated by corticotrophin-releasing factor (CRF) and by catecholamines, vasopressin, and vasoactive intestinal peptide.649 Cortisol is increased in stress and is thought to be a major mediator of the response because adrenalectomized animals and patients with Addison’s syndrome fare poorly when stressed. This was well demonstrated by the increased mortality rate observed following the use of etomidate to sedate critically ill patients.650 It was subsequently discovered that etomidate blocks adrenal steroidogenesis, specifically by inhibiting 11-b-hydroxylation and 17-a-hydroxylation.651,652 It appears that any type of stress monitored by the CNS is relayed to the hypothalamus, which responds by starting the stress hormone cascade beginning with CRF release. Such stresses can include trauma, anxiety, severe infections, hypoglycemia, surgery, and even intense exercise. In a study looking at the effect of cortisol on energy expenditure and amino acid metabolism in humans,653 the authors found that: . . . . Acute hypercortisolemia increased protein breakdown 5%–20%, as measured by increases in leucine and phenylalanine appearance rates Normalizing insulin during hypercortisolemia did not alter phenylalanine flux but enhanced leucine appearance rate, the latter result indicating that insulin was affecting leucine metabolism during hypercortisolemia The fraction of the leucine flux that was oxidized was not significantly increased with hypercortisolemia, but disposal by the nonoxidative route of leucine uptake for protein synthesis was increased Hypercortisolemia increased cycling of amino acids by increasing protein breakdown and synthesis Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 61 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 61 Cortisol can increase body fat levels especially when present in supraphysiological amounts such as is seen in Cushing’s disease where fat distribution increases in the face and abdominal area. As well, cortisol dramatically stimulates lipoprotein lipase, a fat-synthesizing enzyme.654 Cortisol, Amino Acids, and Other Hormones Stress and increased cortisol levels have an adverse effect on serum testosterone levels. A primary anticatabolic effect of exogenous testosterone and the anabolic steroids lies in their interference with muscle cortisol metabolism.655 These compounds may prevent the growth-suppressing activity of cortisol.656 GH also inhibits the muscle catabolic actions of cortisol.657 In addition, a study found that lowering cortisol in the blood enhances the response to GHRH in healthy adults.658 High levels of cortisol blunt GH release during exercise and after taking GH-releasers, such as arginine. It does this by increasing the release of somatostatin, which blocks GH release in the brain. IGF-1 can diminish the catabolic effects of cortisol without the side effects noted with GH therapy.328 A study has shown that cortisol downregulates the IGF-1 mRNA levels, indicating that some of the catabolic effects of glucocorticoids in humans is mediated via a reduced autocrine=paracrine expression of IGF-1.659 One study found that glutamine directly prevents the cortisol-induced destruction of muscle contractile proteins.660 Animal studies have shown that a high-protein, high-fat diet coupled with the use of anabolic steroids will mitigate corticosterone-induced muscle breakdown.432 It has been shown that normal increases in cortisol results in stimulation of lipolysis, ketogenesis, and proteolysis and that circadian variation in cortisol concentration is of physiologic significance in normal humans in that it helps to regulate both anabolic and catabolic events.661 Even mild elevations in serum cortisol within the physiological range can increase the plasma glucose concentration and protein catabolism within a few hours in healthy individuals.645,662 Testosterone to Cortisol Ratio It has been shown that stress or disease-induced increases in plasma cortisol and corticosterone, the use of long-term systemic, and to a lesser extent inhaled, corticosteroid therapy663 results in diminished testosterone secretion from the testes. Glucocorticoids suppress testicular steroidogenesis (and adrenal steroidogenesis) in two ways: by inhibiting the anterior pituitary secretion of LH, perhaps secondary to alteration of hypothalamic GnRH secretion,664 and by directly inhibiting testicular function. The end result is catabolic with a breakdown of muscle and tendon tissue, counterproductive for athletic performance. It appears that the level of 11-b-hydroxysteroid dehydrogenase (11-b-OHSD) in Leydig cells dictates the level of intracellular glucocorticoid available to the glucocorticoid receptor and thus the potency of corticosteroid as an inhibitor of testosterone secretion.427 Thus lowered enzyme activity increases glucocorticoid dependent inhibition of testosterone production. One study examined the part played by the glucocorticoids in the ACTH inhibitory effect on plasma testosterone levels in men.665 Plasma androstenedione, testosterone, cortisol, and LH were measured in eight normal men, before and after the following tests: ACTH stimulation (2 mg in metyrapone administration (500 mg every 4 h, 6 times a day)) and dexamethasone suppression (8 mg=day for 3 days). In addition, androstenedione and testosterone were evaluated under human chorionic gonadotrophin (5000 IU HCG=day for 3 days) before and after dexamethasone suppression (8 mg=day for 6 days). In all patients, ACTH decreased plasma testosterone. In contrast, after metyrapone, the mean plasma testosterone was increased. This increase, though not statistically significant, was observed in all patients but one. Both tests resulted in a significant increase of plasma androstenedione. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 62 2.11.2007 12:16pm Compositor Name: VAmoudavally 62 Amino Acids and Proteins for the Athlete: The Anabolic Edge Dexamethasone suppressed both testosterone and androstenedione levels. None of the three tests had a significant effect on the LH concentration. HCG injection increased the mean plasma testosterone. Dexamethasone significantly depressed the testosterone response to HCG. These data are consistent with the following conclusions: 1. The decrease of plasma testosterone levels, observed in men after ACTH administration, is not observed after metyrapone-induced ACTH increase. This confirms that it is related to cortisol levels rather than to ACTH itself. 2. Glucocorticoids act directly on testicular biosynthesis since they do not induce any change in LH secretion and since dexamethasone reduces testosterone response to HCG. Testosterone to Cortisol Ratio and Exercise Cortisol (which induces the breakdown of cellular proteins) increases with increased duration of intense exercise. In athletes who are training effectively basal testosterone levels rise as do cortisol levels. Although exercise increases cortisol, well-conditioned athletes show less cortisol secretion during exercise compared to their out-of-shape peers.666 In overtrained athletes, however, cortisol levels rise more and testosterone levels decline. Thus, one measure of overtraining is the testosterone to cortisol ratio. An elevated cortisol in relation to testosterone suggests overtraining, although a recent study has shown that this may not necessarily be true with some athletes. In endurance-trained cyclists, the results of this study indicate decreased testosterone levels, increased cortisol levels, and a decreased testosterone to cortisol ratio does not automatically lead to a decrease in performance or a state of overtraining.667 In addition to their direct catabolic effects, cortisol, and ACTH have been shown to inhibit LH and testosterone secretion through an effect on both the hypothalamic-pituitary axis (decreasing the secretion of LH and GnRH) and directly on the testes.664,668–673 The end result is catabolic with a breakdown of muscle and tendon tissue, counterproductive for strength-related athletic performance, and for increasing lean body mass. Endogenous androgens and synthetic anabolic steroids are powerful anabolic compounds partly because of their antiglucocorticoid effects. From the literature it can be seen that these compounds may act as an antiglucocorticoid in at least two ways: (1) by competing with the glucocorticoid receptors and thus blocking their uptake and (2) by suppressing adrenal glucocorticoid production both directly and by suppressing ACTH. In rat there is some evidence that anabolic steroids can act via muscle glucocorticoid receptors, thereby antagonizing the catabolic activity of endogenous glucocorticoids, rather than via muscle androgen receptors.674 Interestingly, at least some of the protein anabolic effects of anabolic–androgenic steroids on SM have been suggested to be due to the inhibition of protein catabolism in this tissue.675 As noted above, anabolic–androgenic steroids have been hypothesized to enhance muscle hypertrophy via an antagonism of catabolic activity induced by endogenous glucocorticoids, such as cortisol. Thus, anabolic–androgenic steroids may reduce the activity of proteolytic enzyme pathways in SM that are dependent on the presence of glucocorticoids for activation. Endogenous hormones are essential for physiological reactions and adaptations during physical work and influence the recovery phase after exercise by modulating anabolic and catabolic processes. Testosterone and cortisol play significant roles in the metabolism of protein. Both are competitive agonists at the receptor level of muscular cells. The testosterone to cortisol ratio can be used as an indication of the anabolic–catabolic balance and for monitoring fitness, overtraining, and overstrain in strenuous and ultraendurance exercise.676,677 Decreases in the testosterone to cortisol ratio (and insulin to glucagon ratios; see above) seen during prolonged exercise are consistent with reduced muscle protein synthesis. This ratio decreases in relation to the intensity and duration of physical exercise (the depression in protein synthesis is Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 63 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 63 proportional to the duration and intensity of exercise), as well as during periods of intense training or repetitive competition, and can be reversed by regenerative measures. It seems likely that the testosterone to cortisol ratio indicates not only the actual physiological strain in training, but also overtraining especially if other factors are considered. These other factors could include the absolute values of both hormones and the hormonal response to a standardized exhaustive exercise test performed with an intensity of 10% above the individual anaerobic threshold.678 This response is typically a decreased maximum rise of pituitary hormones (corticotrophin, GH), catecholamines, cortisol, and insulin. Myofibrillar proteolysis appears to proceed by a pathway in which glucocorticoids (e.g., cortisol or hydrocortisone and to a much lesser degree, corticosterone) are essential for the activation.679 Thus, conditions which may tend to elevate the cortisol concentration, such as overtraining or undernutrition, could lead to an enhanced rate of myofibrillar proteolysis and consequently, muscle atrophy. In support of this view, it has been shown that the response in SM to undernutrition is predominantly a decrease in mass, whereas the response in visceral tissues is mainly a reduction in the rate of metabolic activity.680 An imbalance between the overall strain experienced during exercise training and the athlete’s tolerance of such effort may induce overreaching or overtraining syndrome. This failure to adapt to training load results in long-term hormonal and biochemical changes that are counterproductive. Overtraining syndrome is characterized by diminished sport-specific physical performance, accelerated fatigability, and subjective symptoms of stress. Overtraining results in chronically decreased levels of testosterone and GH and increased levels of cortisol. It has been shown that in muscle energy restriction, protein deficiency and corticosterone treatment induces parallel inhibitions of growth and protein synthesis, mediated by similar graded reductions in ribosomal capacity and activity.681 Exercise affects hormonal levels both during and after exercise including during sleep.682 In one study it was found that long-duration exercise (LDE) of moderate intensity, but not LDE of low intensity, during the daytime changes, the typical temporal patterns of hormone release during subsequent nocturnal sleep. During the no-exercise nights, the typical secretory patterns were present with peak concentrations of GH but nadir concentrations of cortisol during the first half of sleep but increased cortisol levels and minimum GH levels during the second part of sleep. Testosterone concentrations increased during the second half of sleep. LDE of moderate intensity reduced rapid-eye-movement sleep. Levels of testosterone decreased with increasing intensity of daytime exercise. Moderate, but not low intensity, LDE decreased GH levels in the first half and increased GH levels in the second half of sleep. Also, LDE of moderate intensity but not LDE of low intensity increased cortisol levels during the first half and decreased cortisol secretion during the second half of sleep. Results suggest that nocturnal profiles of GH and cortisol concentrations may serve to indicate the disturbance of normal anabolic functions of sleep due to daytime exercise. Although cortisol and GH often rise simultaneously as testosterone decreases, there are ways of exercising so as to minimize cortisol and maximize GH and testosterone.65,683,684 Effect of Dietary Nutrients on Testosterone and the Testosterone to Cortisol Ratio As we have seen, changing certain training variables such as exercise intensity, duration, and type of exercise can affect the levels of the various anabolic and catabolic hormones. Not much is known, however, about the effects of dietary nutrients and nutritional supplements on exercise-induced hormonal changes. A number of studies have shown that both testosterone and cortisol levels can be influenced by dietary manipulation. For example, one study has shown that a high-protein, high-fat diet may reduce corticosterone-induced muscle proteolysis.432 On the other hand, a study measured the Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 64 2.11.2007 12:16pm Compositor Name: VAmoudavally 64 Amino Acids and Proteins for the Athlete: The Anabolic Edge effects of dietary fat and fiber on plasma and urine androgens and estrogens in men. The study had a crossover design and included 43 healthy men aged 19–56.685 Men were initially randomly assigned to either a low-fat, high-fiber, or high-fat, low-fiber diet for 10 weeks and after a 2-week washout period crossed over to the other diet. The energy content of diets was varied to maintain constant body weight, but averaged approximately 13.3 MJ (3170 kcal)=day on both diets. The low-fat diet provided 18.8% of energy from fat with a ratio of polyunsaturated to saturated fat (P=S) of 1.3, whereas the high-fat diet provided 41.0% of energy from fat with a P=S of 0.6. Mean plasma concentrations of total SHBG-bound testosterone were 13% and 15% higher, respectively, on the high-fat, low-fiber diet and the difference from the low-fat, high-fiber diet was significant for the SHBG-bound fraction. Daily urinary excretion of testosterone in men was 13% higher with the high-fat, low-fiber diet than with the low-fat, high-fiber diet. Conversely, their urinary excretion of estradiol and estrone and their 2-hydroxy metabolites were 12%–28% lower with the high-fat, low-fiber diet. The beneficial effects of dietary fat on serum testosterone were also seen in another earlier study.686 This study found that in men, a decrease in dietary fat content and an increase in the degree of unsaturation of fatty acids reduce the serum concentrations of androstenedione, testosterone and free testosterone. During the low-fat period a significant negative correlation between serum PRL and androgens was observed. The study also found that with the exception of a small but nonsignificant decrease in serum estradiol-17b, the other hormone parameters were practically unaffected by the dietary manipulation. Other studies have also shown that dietary changes, including both the amount and composition of the various macronutrients, can significantly affect serum testosterone and cortisol.687,688,786 Results of these studies show that diet can alter endogenous levels of sex hormones and cortisol in both men and women. A study investigated the influence of dietary nutrients on basal and exercise-induced concentrations of testosterone (T) and cortisol (C).689 In this study, 12 men performed a bench press exercise protocol (5 sets to failure using a 10 repetitions maximum load) and a jump squat protocol (5 sets of 10 repetitions using 30% of each subject’s 1-repetition maximum squat) with 2 min of rest between all sets. A blood sample was obtained at preexercise and 5 min postexercise for determination of serum T and C. Subjects also completed detailed dietary food records for a total of 17 days. As expected there was a significant increase in postexercise T compared with preexercise values for both the bench press (7.4%) and jump squat (15 ¼ 2E1%) protocols; C was not significantly different from preexercise concentrations. Interestingly the authors found a significant correlation between preexercise T and percent energy protein, percent energy fat, saturated fatty acids, monounsaturated fatty acids, the polyunsaturated fat to saturated fat ratio, and the protein to carbohydrate ratio. There were no significant correlations observed between any nutritional variables and preexercise C or the absolute increase in T and C after exercise. Although these data confirm that high-intensity resistance exercise results in elevated postexercise T concentrations, they also show that dietary nutrients are capable of modulating resting concentrations of T and increasing resting testosterone to cortisol ratios. A recent study on the effects of amino acid supplementation on hormonal responses to resistance training overreaching looked at several hormonal concentrations including testosterone and cortisol.52 Further discussions on the effects of individual amino acids on endogenous hormonal levels and subsequently on the anabolic effects of exercise can be found under the individual amino acids. CYTOKINES AND MUSCLE PROTEIN SYNTHESIS Several studies have shown that the response to stress is mediated by complex interactions between the nervous, endocrine, immune, and hematopoietic systems. Thus, besides the anabolic and Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 65 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 65 catabolic hormones we have been discussing the many other factors and variables that are likely involved in protein metabolism. Not only is the neuroendocrine system operative but cytokines such as IL-1, IL-6, and TNF-a, also play important roles. Although their roles in response to physical stress have not been fully worked out we do know, however, that these processes can significantly affect protein metabolism and muscle catabolism.690–692 It seems that a common denominator for these cytokines is the downregulation of either IGF-1 or signaling components of the PI3K=Akt=mTOR pathway.693 IL-15 has been identified as an anabolic cytokine with beneficial effects on body composition and muscle mass694–702 (see below under myokines, including information on IL-6 and its role in exercise metabolism, for more information). Injury (surgical, traumatic, and burn) and sepsis result in accelerated protein breakdown.703 This is manifest by increased urinary nitrogen loss, increased peripheral release of amino acids, and inhibited muscle amino acid uptake observed in sepsis. The amino acids originate from both injured tissue and uninjured SM, and are transported to the liver for conversion to glucose (gluconeogenesis) and protein synthesis. The negative nitrogen balance observed in such patients represents the net result of breakdown and synthesis, with breakdown increased and synthesis either increased or diminished.704 Jahoor et al.705 observed in burned children that the amount of protein breakdown was consistently elevated to the same degree during the acute and convalescent phases, whereas synthesis increased in the latter phase and caused a positive nitrogen balance. Different muscle groups respond differently to injury and sepsis with some undergoing more proteolysis than others.706 Amino acid uptake by the liver is enhanced with resultant increased gluconeogenesis. Also, hepatic protein synthesis of selected proteins (acute phase reactants) increases while others decrease (such as transferrin, albumin, retinols, and prealbumin). The acute phase reactants include fibrinogen, complement, C-reactive protein, haptoglobin, a-1-acid glycoprotein, a-1-antitrypsin, a-1-antichymotrypsin, ceruloplasmin, ferritin, and serum amyloid A.707 The degree of acute phase response is proportional to the level of tissue injury. Stimulators of the acute phase reactants include cytokines, such as IL-1, TNF, IL-6, ciliary neurotrophic factor (CNTF), a member of the IL-6 superfamily, and IFN-g,708–712 neuroendocrine mediators, specifically glucocorticoids, and possibly toxic products such as bacterial lipopolysaccharides. It has been observed that after IL-1 stimulation the total amount of protein synthesized does not increase, but rather synthesis of the acute phase proteins is favored. Another study has found that in response to exercise, there is a stimulation of the synthesis of some acute phase proteins, which may be a mechanism whereby nitrogen resulting from muscle protein breakdown is spared.713 A study has shown that TNF has significant short-term and long-term effects on protein synthesis.714 Earlier work demonstrated that TNF can reduce food intake, weight gain, and enhance muscle catabolism.715 In one study, a 50 mg subcutaneous dose of TNF was shown to acutely alter amino acid transport with an early catabolic and later anabolic effect.716 As we can see, the various factors that regulate protein metabolism are complex and interrelated. For example, stimulation of the immune system results in a series of metabolic changes that are antagonistic toward growth. Cytokines including IL-1, IL-6, and TNF-a appear to mediate homeorhetic response, which alters the partitioning of dietary nutrients away from growth and SM accretion in favor of metabolic processes which support the immune response and disease resistance. These alterations include decreased SM accretion due to increased rates of protein degradation and decreased protein synthesis, use of dietary amino acids for gluconeogenesis and as an energy source instead for muscle protein accretion, for and the release of hormones such as insulin, glucagon, and corticosterone.717 Body composition is adversely affected by the pro-inflammatory cytokines resulting in decreased muscle mass and increased body fat. Weight gain has been found to be associated with an increase in pro-inflammatory signals from adipose tissue.718 These signals in turn may be Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 66 2.11.2007 12:16pm Compositor Name: VAmoudavally 66 Amino Acids and Proteins for the Athlete: The Anabolic Edge responsible for further increases in weight and body fat. What we have is a vicious circle where fat begets more fat, inflammation leads to insulin resistance and an increase in body fat which begins the cycle again and soon begins to affect our health.719 Several studies have shown links between inflammatory markers (such as C-reactive protein) and body fat, especially visceral adipose tissue.720–725 These and others also show a link between the pro-inflammatory cytokines and both insulin resistance and excess local glucocorticoid activity. It has been shown that an increase in pro-inflammatory cytokines results in an increase in insulin resistance, which in turn increases the formation of cortisol from cortisone by the enzyme 11-b-hydroxysteroid dehydrogenase. While the effects may not be measured systemically (blood cortisol levels are usually normal in obese people), the pro-inflammatory cytokines have been implicated in the local regulation of cortisol, by increasing its production in fat cells. For example, it has been shown that TNF-a causes insulin resistance and increased local cortisol production,726 and to add to the problem, it has also been shown that body fat in obese people secretes more TNF-a than the equivalent amount of fat in lean people. Studies show that an increase in inflammatory mediators predicts the future development of obesity and diabetes and, interestingly enough, depression. It would seem that the link between obesity and increased body fat, and increased inflammatory mediators might be considered as a cause–effect relationship. As such, decreasing levels of inflammation and especially the proinflammatory cytokines may decrease body fat and weight gain by both direct and indirect effects. The indirect effects would be by decreasing inappropriate cortisol production and by increasing insulin sensitivity. But there is more to the story. Reducing the pro-inflammatory cytokines results in an increase in levels of IGF-1, resulting in an increase in fat breakdown and oxidation of fatty acids, and anticatabolic effects on muscle mass.727,728 The overall result is an increase in fat loss while maintaining SM mass. EICOSANOIDS AND MUSCLE PROTEIN SYNTHESIS Eicosanoids are physiologically active metabolites of the essential fatty acids with important effects on the immune,729–735 cardiovascular,736–741 reproductive,742–748 and CNS,749–751 and include leukotrienes, lipoxins, prostaglandins (PGs), and the thromboxanes.752 The essential fatty acids, linoleic and a-linolenic acid, are converted to other fatty acids including eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), dihomo-gamma-linolenic acid (DGLA), and arachidonic acid, from which the eicosanoids are synthesized. Synthesis of eicosanoids is initiated by signal transduction cascades which result in the hydrolysis of free arachidonic acid from membrane phospholipids.753 Since eicosanoids act locally in and around the tissues in which they are produced, they are not hormones but autacoids, although they have hormone-like effects and influence hormonal function.754 Virtually all cells in the body can form some of the eicosanoids but tissues differ in enzyme profile and consequently in the products they form. They also differ in their ability to be affected by specific eicosanoids, which are not stored to any degree and must be synthesized in response to immediate need. They are rapidly synthesized, released, and degraded. The eicosanoids are both stimulated by and stimulate the production of other autocrine and paracrine messengers755,756 and hormones (such as insulin, glucagon, GH, IGF-1, cortisol, and the sex hormones)757–767 and are partly responsible for the effects of and work synergistically with other hormones and growth factors.768,769 For example, studies have indicated that TNF may be the principal catabolic monokine, with IL-1 potentiating SM proteolysis.589 Reports indicate that in vitro proteolysis could be blocked by cyclooxygenase inhibitors leading to the possibility that catabolism is mediated by PGs, specifically E2.770 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 67 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 67 In gastrectomy patients, the administration of indomethacin postoperatively resulted in reduced fever, attenuated cortisol and catecholamine increase, and reduced protein loss.771 It is unclear whether just the reduction of fever (central cylcooxygenase inhibition) or another action of the indomethacin, e.g., peripheral cyclooxygenase inhibition, is operative. On the other hand, other in vivo studies in burned and septic rats indicate that indomethacin is unable to block proteolysis.772,773 This implies that PGs may not be the mediators of proteolysis. Eicosanoids and cytokines released from one cell activate their receptors on neighboring cells, interact with each other, and affect each others production, release, and degradation.774,775 The cytokines IL-1 and TNF enhance arachidonic acid release, probably by inducing the synthesis of phospholipase A2 and possibly other enzymes involved in the metabolism of phospholipids.776 In turn, arachidonic acid itself may act as a second messenger, synergizing with other agents, as well as serving as the precursor for PGs and various other bioactive eicosanoids. Like all eicosanoids, PGs are autocrine and paracrine in behavior, and influence the metabolism and growth of the same and neighboring cells in which they are synthesized.777,778 PGs are known regulators of protein turnover in SM, with prostaglandin E2 (PGE2) and PGF2a stimulating protein degradation and protein synthesis rates, respectively.770,779–782 In one study, when rat skeletal or cardiac muscles were incubated with arachidonate, rates of protein breakdown rose and protein balance became more negative.783 Aspirin or indomethacin, which prevented synthesis of PGE2, markedly reduced this effect. PGF2a directly stimulates protein synthesis in SM, but does not effect protein degradation,784 whereas PGE2 enhances rates of protein degradation without affecting protein synthesis.785 PGs regulate the muscle hypertrophic response to insulin and tension–stretch.786 PGF2a appears to be involved in the stimulation of protein synthesis by insulin in muscle from fasted animals and to mediate stretch-induced increases in protein synthesis in muscle.787 The repletion of muscle protein after disuse atrophy is also a PGF2a-dependent event.788 PGs have also been reported to be involved in the catabolic effect of the synthetic glucocorticoid dexamethasone. The main effect of glucocorticoids on PG metabolism is to lower the availability of free arachidonic acid.789 PG synthesis in SM has been shown to increase during high-intensity exercise in both humans790 and animals.791 These findings suggest that cyclooxygenase inhibitors may be useful in the treatment of patients showing excessive protein breakdown. Although controversial data exist in the literature, there are a number of studies that implicate arachidonic acid and PGs in steroidogenesis, specifically in testosterone production and on LH. In one study the effects of single and repeated administration of PGF2-a on the secretion of testosterone and the effect of PGF2-a on the action of human chorionic gonadotropin (hCG) were investigated in adult male rats.792 The results of the study indicate that repeated stimulation by PGF2-a directly inhibits testosterone production in the rat testis without mediation by LH. In another study in rats arachidonic acid metabolites were found to be intratesticular factors which can regulate LH-stimulated testicular steroidogenesis.793 In this study the use of inhibitors of PG production led to a decrease in the LH-stimulated increase in secretion of testosterone, while the use of vitamin E (antioxidant and inhibitor of lipoxygenase) enhanced LH-stimulated androgen production. In one study arachidonic acid induced a dose-related increase of testosterone formation and, at the highest dose, stimulated the production of PGE2, leukotrienes B4 (LTB4), and C4 (LTC4) by purified rat Leydig cells.794 In this study the contemporary addition of the PG synthesis blocker, indomethacin, and arachidonic acid further increased testosterone formation and decreased PGE2 levels. The findings of this study suggest that (1) exogenous amino acid stimulates T secretion, (2) conversion of amino acid to cycloxygenated and lipoxygenated metabolites is not required for its steroidogenic effect and (3) cycloxygenated and lipoxygenated compounds play a diverse modulatory role on testicular steroidogenesis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 68 2.11.2007 12:16pm Compositor Name: VAmoudavally 68 Amino Acids and Proteins for the Athlete: The Anabolic Edge In one study aspirin was found to inhibit the stimulatory activity of naloxone on LH release, suggesting that the inhibitory tone of opioids on GnRH secretion may be caused by the block of hypothalamic PG biosynthesis with consequent inhibition of PGE2-induced GnRH secretion.795 Together, these results suggest that arachadonic acid effects on testosterone production are mediated by metabolites of the cyclooxygenase pathway. Other major metabolites of arachidonic acid such as the prostacyclins and thromboxanes also have effects on protein metabolism and cellular growth.796 For example, the vasoconstrictor eicosanoid thromboxane has a direct stimulatory effect on extracellular matrix protein production. In one study, two different thromboxane analogues resulted in increased production of components of the extracellular matrix including fibronectin and the basement membrane proteins laminin and type IV collagen.797 These results suggest a potential role for thromboxane as a mediator of the sclerotic and fibrotic responses to injury. In another study, the effects of the thromboxane A2 mimetic, U-46619, on the biosynthesis of human mesangial cell proteins was evaluated.798 Cellular tubulin and collagen type IV, but not actin, were markedly stimulated by U-46619. Although it would be advantageous to be able to direct eicosanoid production so that good eicosanoids would be produced deferentially to bad eicosanoids, it is difficult to do so because of the complexity of eicosanoid production, actions, and metabolism. Unfortunately we do not know a lot about the dietary influences that effect the known eicosanoids and thus can make only limited use of any knowledge we do have. For example, PGs can be both good and bad. Unfortunately, it is difficult to stimulate the good ones and not the bad ones. If we decrease the formation of PGs from arachidonic acid we inhibit the formation of both good and bad PGs. Of more relevance for dieters, it is not possible to differentially stimulate the production of PGI2 that has a lipolytic action, from PGE2 which has an antilipolytic action. Both PGs belong to the series 2 PGs and are formed from arachidonic acid. In humans, PGE1 seems to have an anabolic effect on SM.799 At present some treatment strategies using the essential fatty acids have tentatively been formulated to try and take advantage of the good eicosanoids. For example, Omega-3 fatty acids found in fish oils can decrease production of some arachidonate metabolites and increase levels of certain PGs. Feeding of these fatty acids has been used as a therapeutic strategy to diminish platelet aggregation by decreasing TXA2 and increasing TXA3 synthesis by platelets. The omega-3s like LNA and eicosapentaenoic and docosahexaenoic acid, known as EPA and DHA, respectively, are critical to anyone concerned with dieting. They increase fatty acid oxidation (burning of fat) and basal metabolic rates and lower cholesterol. Omega-3 fatty acids also provide an anabolic effect by increasing the binding of IGF-1 to SM and improving insulin sensitivity, even on diets high in fat which have a tendency to decrease insulin sensitivity.800 Moreover, fish oils may also have important implications for women prone to osteoporosis since they appear to decrease calcium excretion.801 Omega-3s also stimulate PG production.802 PGs are eicosanoids that regulate activity in body cells on a moment to moment basis and are involved in critical functions like blood pressure regulation, insulin sensitivity, and immune system and anti-inflammatory responses. They are also involved in literally hundreds of other functions, many of which have yet to be fully identified in research. If you have a problem producing PGs or experience an imbalance between the different kinds of PGs, overall health can be significantly affected. The series three PGs are formed from EPA, which reduces the production of the bad PGs from arachidonic acid. EFA deficiency can lead to high blood pressure, hormonal dysfunction, impaired immune function, coagulation problems, inflammatory changes, dry itchy skin, peripheral edema, and many other conditions. A number of nonsteroidal anti-inflammatory agents, including salicylates, indomethacin, diclofenac, mefenamic acid, piroxicam, sulindac, phenylbutazone, and ibuprofen, have been found to variably influence the formation of PGs803–807 and thus to influence protein synthesis and Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 69 2.11.2007 12:16pm Compositor Name: VAmoudavally Exercise and Protein Metabolism 69 catabolism. For example, indomethacin, a cyclooxygenase inhibitor, blocks the conversion of arachidonic acid, via the cyclooxygenase pathway, into PGs, endoperoxides, and thromboxanes. Aspirin also functions in this way. As we have seen, recent discoveries have demonstrated that hormones and growth factors exert their effects in part through eicosanoids. Thus there are significant implications in the manipulation of these compounds for exercise-induced muscle hypertrophy and for athletic performance. The future will demonstrate these compounds to be critical not only in intracellular (molecular) events but also in the effects they produce that are far from the source of origin. Once more studies are done and more information is available, we should be able to manipulate our diet and supplements to change these compounds and maximize their anabolic potential. HORMONAL EFFECTS OF AMINO ACIDS There are several studies that document the hormonal changes secondary to dietary protein and amino acid intake. Increased blood amino acid levels, secondary to a high protein meal, has been shown to cause insulin and GH levels to rise.808 Increasing these hormones, as well as increasing amino acid levels, but at the same time decreasing muscle catabolism leads to an enhanced anabolic response. Studies have also shown that the hormone response to exercise is modified by BCAA ingestion.809 A study looked at the effects of active amino acids and dipeptides as anabolic agents on surgically induced wound healing in lower extremity SM in diabetic and normal rats.810 Their data suggest that amino acids, primarily methionine, and dipeptides methionine–glutamine and tryptophan–isoleucine have a profound anabolic effect on muscle wound healing and on protein synthesis. They found that the amino acid and two dipeptides could override or block the increase in glucocorticoid levels found in diabetic patients. Another study on rats demonstrated that dietary protein appeared to stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, as well as increasing insulin and free T3 levels.811 This apparent direct influence of dietary protein on muscle has also been found in man. RNA turnover is also regulated by amino acids either directly,812–815 or via cellular hydration816 (see above). Early studies have shown that the anabolic effects of intense training are enhanced by a diet high in protein.817 It is also well known that a high protein diet is essential for the anabolic effects of anabolic steroids.818 When intensity of effort is maximal and stimulates an adaptive response, protein needs increase in order to provide increased muscle mass. In another earlier study, it was been shown that protein taken after training may increase both insulin and GH, and thus have anabolic effects.819 The authors of this study examined the effect of carbohydrate and=or protein supplements on the hormonal state of the body after weight-training exercise. Nine experienced male weight lifters were given water (control) or an isocaloric carbohydrate (CHO; 1.5 g=kg body weight), protein (PRO; 1.38 g=kg body weight), or carbohydrate–protein (CHO=PRO; 1.06 g carbohydrate=kg body weight and 0.41 g protein=kg) supplement immediately and 2 h after a standardized weight-training workout. CHO=PRO stimulated higher insulin concentrations than PRO and control. CHO=PRO led to an increase in GH 6 h postexercise that was greater than PRO and control. The results of this study suggest that nutritive supplements after weight-training exercise can produce a hormonal environment during recovery that may be favorable to muscle growth secondary to insulin and GH elevations. Increased amino acid availability has been shown to directly influence protein and glycogen synthesis especially within a few hours after physical exercise.820 The rate of protein synthesis, protein catabolism, and amino acid transport is normally increased after exercise and depends on amino acid availability.821 If there is an increase in the availability of amino acids during this Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C002 Final Proof page 70 2.11.2007 12:16pm Compositor Name: VAmoudavally 70 Amino Acids and Proteins for the Athlete: The Anabolic Edge postexercise time interval or window, then the catabolic processes are more than offset by the increased anabolic processes resulting in an overall increase in cellular contractile protein. Thus it is vital to increase the absorption of amino acids as quickly as possible after exercise. Major operative trauma has been shown to result in increases in arterial levels of glucagon, cortisol, and norepinephrine, a relative lack of insulin, increased muscle proteolysis, increased efflux of amino acids (taurine, serine, glycine, valine, methionine, isoleucine, leucine, phenylalanine, lysine, arginine, and total amino acid nitrogen) and lactate from muscle, and negative nitrogen balance.822 A study has shown that the signs of muscle protein catabolism elicited by administration of stress hormones can be attenuated by simultaneous administration of a conventional amino acid solution, although the solution used did not contain glutamine.823 In this study healthy male volunteers were administered a stress hormone infusion including epinephrine, cortisol, and glucagon either alone or combined with a balanced glutamine-free amino acid solution over a period of 6 h after which a decrease was observed in SM protein synthesis as measured by the size distribution and concentration of ribosomes.824 The decrease was prevented by an infusion of the balanced amino acid solution. Following the triple-hormone infusion, a decrease was noted in the content of the total free amino acids in both muscle and plasma. After including amino acids in the infusion solution, the significant decrease in muscle glutamine caused by the triple hormones was not seen. Plasma cortisol, insulin, and glucose increased in response to the triple-hormone infusion alone or in combination with amino acids. There has been much interest recently in the metabolism of the individual amino acids, especially glutamine. Glutamine is the most abundant amino acid in blood. Its levels in muscle and blood decrease markedly following injury and sepsis, and it is consumed rapidly by replicating cells, such as fibroblasts, lymphocytes, and intestinal endothelial cells. Glutamine and alanine transport two-thirds of the circulating amino acid nitrogen and in the postabsorptive and injured state comprise more than 50% of the amino acids released by muscle.825 In the stressed state the glutamine released by muscle is taken up by the intestinal tract where some is converted to alanine, which is then converted by the liver to glucose. It is likely that a good portion of the alanine converted by hepatic gluconeogenesis is supplied by the intestine. It has been found that gluconeogenesis from the small intestine can at times also directly supply significant amounts of systemic glucose. For example, it has been found that after 72 h of fasting, gluconeogenesis in the small intestine, mainly using glutamine and glycerol, provides up to onethird of the total endogenous glucose production.826 Glutamine is also metabolized to ammonia by the gut. The ammonia is then transported by the portal vein directly to the liver for disposal via the urea cycle. In the stressed state it appears that glutamine can replace glucose as a fuel.825 Similar observations have been made during glucocorticoid administration.827 It is possible that the use of glutamine may decrease protein catabolism in the intestine and in SM under certain conditions. This section is only meant to give an overview of the effects of amino acids on hormonally mediated anabolic and anticatabolic effects. More detailed discussions on the direct and indirect effects of amino acids on protein synthesis, the endogenous hormonal levels, and subsequently, on the anabolic and body composition effects of exercise can be found under the individual amino acids. REFERENCES 1. Borer, K.T., The effects of exercise on growth, Sports Med., 20(6), 375–397, 1995. 2. Tipton, K.D., Ferrando, A.A., Williams, B.D., and Wolfe, R.R., Muscle protein metabolism in female swimmers after a combination of resistance and endurance exercise, J. Appl. Physiol., 81(5), 2034–2038, 1996. 3. 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Physiol., 261(3 Pt 2), F488–F494, 1991. 798. Mene, P., Taranta, A., Pugliese, F., Cinotti, G.A., and D’Agostino, A. Thromboxane A2 regulates protein synthesis of cultured human mesangial cells [see comments], J. Lab. Clin. Med., 120(1), 48–56, 1992. 799. Stiegler, H., Rett, K., Wicklmayr, M., and Mehnert, H., Metabolic effects of prostaglandin E1 on human skeletal muscle with special regard to the amino acid metabolism, Vasa, 28(Suppl.), 14–18, 1989. 800. Liu, S., Baracos, V.E., Quinney, H.A., and Clandinin, M.T., Dietary omega-3 and polyunsaturated fatty acids modify fatty acyl composition and insulin binding in skeletal-muscle sarcolemma, Biochem. J., 299 (Pt 3), 831–837, 1994. 801. Kruger, M.C., Eicosapentaenoic acid and docosahexaenoic acid supplementation increases calcium balance, Nutr. Res., 15, 211–219, 1995. 802. Graham, J., Franks, S., and Bonney, R.C., In vivo and in vitro effects of gamma-linolenic acid and eicosapentaenoic acid on prostaglandin production and arachidonic acid uptake by human endometrium, Prostaglandins Leukot. Essent. Fatty Acids, 50(6), 321–329, 1994. 803. MacDonald, I.D., Graff, G., Anderson, L.A., and Dunford, H.B., Optical spectra and kinetics of reactions of prostaglandin H synthase: Effects of the substrates 13-hydroperoxyoctadeca-9,11-dienoic acid, arachidonic acid, N,N,N0 ,N0 -tetramethyl-p-phenylenediamine, and phenol and of the nonsteroidal antiinflammatory drugs aspirin, indomethacin, phenylbutazone, and bromfenac, Arch. Biochem. Biophys., 272(1), 194–202, 1989. 804. Simon, L.S.and Mills, J.A., Nonsteroidal anti-inflammatory drugs, N. Engl. J. Med., 302, 1179–1185, 1980. 805. Kobayashi, K., Arakawa, T., Satoh, H., et al., Effect of indomethacin, tiaprofenic acid and dicrofenac (sic) on rat gastric mucosal damage and content of prostacyclin and prostaglandin E2, Prostaglandins, 30, 609–618, 1985. 806. Oliw, E., Lunden, I., and Anggard, E., In vivo inhibition of prostaglandin synthesis in rabbit kidney by non-steroidal anti-inflammatory drugs, Acta Pharmacol. Toxicol., 42, 179–184, 1978. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 106 43803_C002 Final Proof page 106 2.11.2007 12:17pm Compositor Name: VAmoudavally Amino Acids and Proteins for the Athlete: The Anabolic Edge 807. Furst, D.E., Are there differences among nonsteroidal antiinflammatory drugs? Comparing acetylated salicylates, nonacetylated salicylates, and nonacetylated nonsteroidal antiinflammatory drugs, Arthritis Rheum., 37, 1–9, 1994. 808. Sukkar, M.J., Hunter, W.M., and Passmore, R., Changes in plasma levels of insulin and growth hormone levels after a protein meal, Lancet, 2, 1020–1022, 1967. 809. Carli, G., Bonifazi, M., Lodi, L., et al., Changes in the exercise-induced hormone response to branched chain amino acid administration, Eur. J. Appl. Physiol., 64, 272–277, 1992. 810. Pitkow, H.S., Brekke, M., Harte, G., Labbad, Z.G., and Bitar, M.S., William J. Stickel Bronze Award. Amino acids, dipeptides, and leg skeletal muscle wound healing in diabetes, J. Am. Podiatric Med. Assoc., 84(11), 564–573, 1994. 811. Jepson, M.M., Bates, P.C., and Millward, D.J., The role of insulin and thyroid hormones in the regulation of muscle growth and protein turnover in response to dietary protein in the rat, Br. J. Nutr., 59(3), 397–415, 1988. 812. Lardeux, B.R. and Mortimore, G.E., Amino acid and hormonal control of macromolecular turnover in perfused rat liver. Evidence for selective autophagy, J. Biol. Chem., 262(30), 14514–14519, 1987. 813. Mortimore, G.E., Lardeux, B.R., and Heydrick, S.J., Mechanism and control of protein and RNA degradation in the rat hepatocyte: Two modes of autophagic sequestration, Rev. Sobre Biol. Cell., 20, 79–96, 1989. 814. Balavoine, S., Feldmann, G., and Lardeux, B., Rates of RNA degradation in isolated rat hepatocytes. Effects of amino acids and inhibitors of lysosomal function, Eur. J. Biochem., 189, 617–623, 1990. 815. Balavoine, S., Feldmann, G., and Lardeux, B., Regulation of RNA degradation in cultured rat hepatocytes: Effects of specific amino acids and insulin, J. Cell. Physiol., 156(1), 56–62, 1993. 816. Stoll, B., Gerok, W., Lang, F., and Haussinger, D., Liver cell volume and protein synthesis, Biochem. J., 287(Pt 1), 217–222, 1992. 817. Kreider, R.B., Dietary supplements and the promotion of muscle growth with resistance exercise, Sports Med., 27(2), 97–110, 1999. 818. Freed, D.L.J., Banks, A.J., Longson, D., and Burley, D.M., Anabolic steroids in athelics: Crossover double-blind trial on weightlifters, Br. Med. J., 2(5969), 471–473, 1975. 819. Chandler, R.M., Byrne, H.K., Patterson, J.G., and Ivy, J.L., Dietary supplements affect the anabolic hormones after weight-training exercise, J. Appl. Physiol., 76(2), 839–845, 1994. 820. Manninen, A.H., Hyperinsulinaemia, hyperaminoacidaemia and post-exercise muscle anabolism: The search for the optimal recovery drink, Br. J. Sports Med., 40(11), 900–905, 2006. 821. Biolo, G., Maggi, S.P., Williams, B.D., Tipton, K.D., and Wolfe, R.R., Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans, Am. J. Physiol., 268(3 Pt 1), E514–E520, 1995. 822. Finley, R.J., Inculet, R.I., Pace, R., et al., Major operative trauma increases peripheral amino acid release during the steady-state infusion of total parenteral nutrition in man, Surgery, 99(4), 491–500, 1986. 823. Hammarqvist, F., von der Decken, A., Vinnars, E., and Wernerman, J., Stress hormone and amino acid infusion in healthy volunteers: Short-term effects on protein synthesis and amino acid metabolism in skeletal muscle, Metabolism: Clin. Exper., 43(9), 1158–1163, 1994. 824. Wernerman, J., Botta, D., Hammarqvist, F., et al., Stress hormones given to healthy volunteers alter the concentration and configuration of ribosomes in skeletal muscle, reflecting changes in protein synthesis, Clin. Sci., 77(6), 611–616, 1989. 825. Souba, W.W., Smith, R.J., and Wilmore, D.W., Glutamine metabolism by the intestinal tract, J. Parenter. Enteral Nutr., 9, 608–617, 1985. 826. Mithieux, G., Bady, I., Gautier, A., Croset. M., Rajas, F., and Zitoun, C., Induction of control genes in intestinal gluconeogenesis is sequential during fasting and maximal in diabetes, Am. J. Physiol. Endocrinol. Metab., 286, E370–E375, 2004. 827. Souba, W.W. and Wilmore, D.W., Gut-liver interaction during accelerated gluconeogenesis, Arch. Surg., 120, 66–70, 1985. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 107 29.10.2007 7:50pm Compositor Name: JGanesan 3 Energy Metabolism INTRODUCTION The production of a steady supply of energy is crucial in maintaining life. Energy is needed to form and repair cellular materials, to maintain the integrity of the body cells and tissues, and for movement. As such, control systems have evolved to maintain energy homeostasis in times of excess and decreased energy availability, involving the preservation of important sources of energy that is used when needed. Mammals derive energy from the carbon skeleton of macronutrients like proteins, carbohydrates, and lipids. The ultimate fate of any nutrient—if not used structurally in the body, for example, as skeletal muscle—as readily available substrates in tissues and circulation (this reservoir of glucose, amino acids, glycerol, ketones, and fatty acids is very small in comparison to total body macronutrient availability), or immediately for energy, is to be transformed into glucose and be deposited as glycogen, or into lipids and deposited mainly as triglycerides in fat cells. Energy is released when carbon–carbon and carbon–hydrogen bonds of the macronutrients that come from our diets, and that are part of our bodies, are broken and either used directly or stored in other chemical bonds such as the high-energy phosphate bonds of adenosine triphosphate (ATP). When the high-energy phosphate body is broken, energy is released and made available to do work. Energy production in the human body revolves around the rebuilding of ATP, and to a lesser extent other nucleoside triphosphates such as guanosine triphosphate (GTP), cytidine triphosphate (CTP), thymidine triphosphate (TTP), and uridine triphosphate (UTP). ATP generation is normally accomplished mostly under aerobic conditions by shuttling all three micronutrients, in different proportions, through the citric acid cycle and oxidative phosphorylation, ending up with ATP, CO2, and water. The mechanisms regulating energy production are complex and involve interactions on many levels, including control of protein synthesis and breakdown, under resting and exercising conditions. For example, increased energy demands—reflected by several changes including increased levels of adenosine monophosphate (AMP) and decreased levels of ATP and creatine phosphate— lead to 50 -AMP-activated protein kinase (AMPK) activation, inhibiting mTOR which in turn decreases protein synthesis and increases protein breakdown.1–3 Another example is the need for energy during exercise leading to increases in AMPK activity and reduced 4E-BP1 phosphorylation, which in turn leads to decreases in protein synthesis.4 Since all cells in the body require energy for maintenance, repair, and growth, and to maintain function, procuring this energy from food is the foundation for life. All three of the macronutrients are sources of energy. Although the focus in this chapter is on the utilization of proteins and amino acids in energy production, some basic information on energy metabolism is needed in order to understand the relevant protein and amino acid dynamics. The central nervous system (CNS) has been recognized as being critically important to sensing energy status and needs, and for mediating fuel utilization and peripheral metabolism.5–9 A number of studies have linked the pathway of fatty acid synthesis in the CNS to the system that regulates food intake and peripheral energy expenditure.10,11 Although many of the major players and intermediates of this regulatory system have been identified, the mechanism(s) by which this system connects to the neuropeptide=neural circuitry that alters behavioral and energy balance remains to be elucidated. 107 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 108 29.10.2007 7:50pm Compositor Name: JGanesan 108 Amino Acids and Proteins for the Athlete: The Anabolic Edge ENERGY SENSING In order to determine the need for energy—whether due to stress, exercise, or other conditions—it is necessary for the body to sense this need and then put the appropriate mechanisms into play, both acutely and chronically. Exercise results in a vast increase in energy needed, and in order to fulfill this need a number of mechanisms, meant to increase ATP availability, are put into play. Variations in cellular energy status likely act as signals that alter gene expression and energy metabolic processes. While the mechanisms involved in the sensing and transducing processes have not been worked out, the recent identification of several signaling molecules gives us a basis for understanding these processes.12 Of the various signaling molecules, AMPK may be one of the main signaling systems invoking these changes since it can both regulate cell metabolism acutely and induce transcription of genes encoding various proteins. AMPK is a multisubstrate serine=threonine protein kinase, which is ubiquitously expressed and functions as an intracellular fuel sensor activated by depletion of high-energy phosphate compounds.13 AMPK responds to cellular energy deprivation by increasing ATP production and decreasing energy-consuming processes.14 AMP concentration, which is felt to be exercise-energy dependent, increases when there is a high energy turnover, with initially only a minimal decrease in ATP levels. AMP activates AMPK, which in turn has profound actions on metabolism of glucose, lipids, and proteins as well as on the expression of a large number of genes.15 However, more research is still needed before we can be sure of the effects of AMPK in regulation of muscle metabolism and gene expression during exercise. METABOLIC PATHWAYS PRODUCING ATP Energy, mainly in the form of ATP, can be produced by both aerobic and anaerobic pathways. Overall, the aerobic pathways produce most of the ATP while the anaerobic pathways are crucial for maintaining energy production under general or tissue hypoxic conditions such as intense exercise and ischemia. During strenuous exercise, regeneration of ATP, on which muscle contraction is dependent, is based on both aerobic and anaerobic processes at all times although the degree that each is involved depends mostly on oxygen availability. ANAEROBIC ENERGY PRODUCTION There are at least four anaerobic pathways. Three of these involve substrate-level phosphorylation from phosphocreatine, during anaerobic glycolysis, and in one of the steps in the tricarboxylic acid (TCA) cycle. The fourth is less well known and also involves the TCA cycle. The former two reactions occur in the cytoplasm while the latter occur in the mitochondria. CYTOPLASMIC ANAEROBIC SUBSTRATE-LEVEL PHOSPHORYLATION 1. Phosphagen mobilization is the simplest mechanism for generating ATP. The best-known example creatine phosphate (PCr), catalyzed by creatine phosphokinase (CPK), can readily transfer its phosphoryl group to ADP to form ATP PCr þ ADP þ Hþ ! ATP þ creatine Phosphagen mobilization also occurs between ATP, ADP, and AMP. This recoupling allows the formation of more ATP from shuffling high phosphate groups, for example, two ADP recouple to form one ATP and one AMP.16,17 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 109 29.10.2007 7:50pm Compositor Name: JGanesan 109 Energy Metabolism Phosphocreatine levels can be influenced by creatine levels in the diet, levels of amino acid precursors in the diet, and by supplementation with creatine, and to a lesser extent from the amino acids methionine, arginine, and glycine. 2. Anaerobic substrate-level glycolysis, the partial catabolism of glucose to lactate, is the second cytoplasmic anaerobic means of forming ATP. If glycogen is the substrate instead of glucose, an extra mole of ATP is generated. The overall reaction is Glucose þ 2Pi þ 2ADP () 2 lactate þ 2H2 O þ 2ATP And this can be broken down to the two specific ATP-producing steps, which are offset somewhat by the ATP-requiring step early on in glycolysis 1,3-BPG þ ADP ! 3-PG þ ATP PEP þ ADP ! pyruvate þ ATP There are several factors involved in anaerobic glycolysis that influence the amount of ATP generated. For example, the formation of lactate by lactate dehydrogenase generates NADþ which allows glycolysis to continue. If NADþ is not being regenerated by the lactate dehydrogenase reaction, there is an inhibition of glyceraldehyde-3-phosphate dehydrogenase which in turn blocks ATP generation by substrate-level phosphorylation. ANAEROBIC MITOCHONDRIAL PHOSPHORYLATION Anaerobic mitochondrial metabolism can generate ATP via two pathways. The first is by substratelevel phosphorylation during the conversion of succinyl-CoA to succinate. In this reaction succinylCoA synthetase drives the reaction in which succinyl-CoA þ GDP þ Pi forms succinate þ GTP þ CoA. GTP is then coupled with ADP to produce ATP. The GTP and ATP formed in this way are generally considered to contribute to the overall mitochondrial ATP generation, most of which is formed by oxidative phosphorylation. However, there is some speculation that the products of mitochondrial substrate-level phosphorylation may be qualitatively different.18 In the second pathway, ATP is formed via electron transport in complexes I and II driven by reduction of fumarate to succinate coupled to the oxidation of reduced ubiquinone that is generated via nicotinamide adenine dinucleotide (reduced form) NADH from TCA cycle reducing equivalents.19–23 Both pathways contribute to elevations of succinate and are operative to some extent in all vertebrates with the extent being dependent on adaptive responses. For example, it has been shown that in diving vertebrates there is an accumulation of succinate in the blood during breath-hold dives.24 This production is thought to result from amino acid catabolism leading to TCA cycle intermediates such as fumarate, malate, succinyl-CoA, and a-ketoglutarate (AKG), which in turn are fermented with succinate. Amino acids can contribute to both aerobic and anaerobic energy metabolisms in several ways, both directly and indirectly: directly by providing the carbon skeletons that contribute to TCA cycle anaplerosis, and indirectly by being used as substrates for gluconeogenesis. AEROBIC ENERGY PRODUCTION The TCA cycle, also called the Krebs cycle and the citric acid cycle, is a mitochondrial metabolic pathway that underlies most of the metabolic processes in the body and is essential for aerobic energy production. For example, this pathway is an important source of biosynthetic building blocks used in gluconeogenesis, amino acid biosynthesis, and fatty acid biosynthesis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 110 29.10.2007 7:50pm Compositor Name: JGanesan 110 Amino Acids and Proteins for the Athlete: The Anabolic Edge The basic energy process of the TCA cycle is the energy-producing oxidation of acetyl-CoA (an ester of coenzyme A) through a series of intra-mitochondrial reactions. The acetyl-CoA that enters the Krebs cycle comes from several sources including the conversion of pyruvate from glycolysis to acetyl-CoA by pyruvate dehydrogenase, and the oxidation of fatty acids, ketone bodies, and the carbon skeletons of some amino acids. The energy released is trapped primarily as the reduced highenergy electron carriers NADH and FADH2, which transfer their chemical energy via oxidative phosphorylation to the electron transport chain to produce ATP. The nine steps of the TCA cycle, in which an acetyl-CoA molecule (two carbons) enters the cycle and condenses it with oxaloacetate (carbons) to create citrate (six carbons), which is subsequently decarboxylated twice (forming two molecules of carbon dioxide, which along with the CO2 released by pyruvate dehydrogenase when acetyl-CoA is formed from pyruvate, is the source of CO2 released into the atmosphere when you breathe) through a series of steps until it reaches oxaloacetate again, leaving the net carbons the same with each turn of the cycle, can be summarized as follows: Acetyl-CoA þ 3NADþ þ FAD þ GDP þ Pi þ 2H2 O þ 1CoA-SH ! 2CoA-SH þ 3NADH þ 3Hþ þ FADH2 þ GTP þ 2CO2 þ 1H2 O While none of the steps in the pathway directly use oxygen, the activity of the TCA cycle is linked to oxygen consumption to regenerate NADþ. If the oxygen supply is low the TCA cycle cannot continue even though some steps may proceed forward anaerobically to provide a limited amount of energy in the form of GPT. As well as oxygen, some TCA cycle enzymes need specific cofactors to function. For example, thiamine deficiency may lead to decreased activity of pyruvate dehydrogenase and AKG dehydrogenase, and a decrease in the ability of the TCA cycle to meet metabolic demands. TCA CYCLE The TCA cycle is more than just a catabolic pathway generating ATP. It is also a metabolic hub where the intermediates are used to produce other biological compounds. For example, a transamination reaction interconverts glutamate and AKG. Glutamate is a precursor for the synthesis of other amino acids and purine nucleotides. Succinyl-CoA is a precursor for porphyrins. Oxaloacetate can be transaminated to form aspartate. Aspartate itself is a precursor for other amino acids and pyrimidine nucleotides. Oxaloacetate and malate are substrates for gluconeogenesis and they in turn can be produced by the increase in other TCAI. Cytoplasmic and mitochondrial pathways are intertwined by the transfer of TCA cycle intermediates back and forth. For example, citrate can be exported out of the mitochondria into the cytostol where it is broken down by ATPcitrate lyase to yield oxaloacetate and acetyl-CoA. Under certain circumstances, the acetyl-CoA produced can be used as a precursor for fatty acids. The oxaloacetate produced in this reaction is reduced to malate which can either be transported back into the matrix of the mitochondria where it is reoxidized into oxaloacetate, or the malate can be oxidatively decarboxylated to pyruvate by malic enzyme which can then be transported back to the mitochondria. All these reactions and interchanges serve to optimize metabolic processes involved in anabolic and catabolic functions, as well as contributing to anaplerotic and cataplerotic pathways. The level of TCA cycle intermediates (TCAI) and the flux rate of the TCA cycle are both important determinants in TCA cycle function and energy production. While the amount of TCAI is relatively small and constant, varying in size by three or four times, the turnover and throughput of the cycle can vary dramatically depending on demand. In order to sustain flux through the TCA cycle and thus regenerate ATP, it is important that TCAI be maintained within certain ranges, which requires that leakage of TCAI and their use in various metabolic pathways, termed as ‘‘cataplerosis,’’ must be balanced by anaplerosis, supplying TCAI to the cycle from various sources. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 111 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 111 The main anaplerotic sources are pyruvate, certain amino acids, and three- to five-carbon derivatives of fatty acid metabolism. Amino acids are involved not only in supplying anaplerotic substrates directly into the TCA cycle, but also involved in supplying oxaloacetate and malate either by supplying glucose for glycolysis via gluconeogenesis or by supplying pyruvate and phosphoenolpyruvate, which in turn are carboxylated to form oxaloacetate and malate. ROLE OF PROTEIN AND AMINO ACIDS IN ENERGY METABOLISM Amino acids have three possible fates in the body. They can be used for (and influence) protein synthesis, to produce pyruvate and TCAI (and thus be used for gluconeogenesis, glyconeogenesis, and fatty acid synthesis), or their carbon skeletons can be used for energy production. All three processes are ongoing to some extent—the degree that each is active depends on the nutritional environment. For example, in times of dietary surplus, the potentially toxic nitrogen of amino acids is eliminated via transaminations, deamination, and urea formation; the carbon skeletons, while used to some extent for energy production, are mostly conserved as carbohydrate, via gluconeogenesis, or as fatty acid via fatty acid synthesis pathways. However, even though amino acids have traditionally been relegated to a minor role in energy production, with fats and carbohydrates considered the major substrates, they nonetheless under some circumstances have a substantial and crucially important role in energy metabolism. In extreme cases by providing substrates for direct energy production; and indirectly by providing carbon skeletons for the formation of TCAI, glucose, glycogen, acetyl units, and fatty acids, amino acids can provide up to half or more of energy requirements. As we have seen, the history of protein requirements for athletes and energy contribution of protein has had its ups and downs from the mid-1800s when the popular opinion was that protein was the primary fuel for working muscles25 to today when we are starting to reexamine the role of proteins in energy metabolism and finding that while it may not be the primary fuel it is more important than held in current popular opinion. In 1866 a paper based on urinary nitrogen excretion measures (in order for protein to provide energy its nitrogen must be removed and subsequently excreted primarily in the urine) suggested that protein was not an important fuel, and contributed only about 6% of the fuel used during a 1956 m climb in the Swiss Alps.26 This paper and others led to the perception that exercise does not increase one’s need for dietary protein. This view has persisted to the present. Recently, however, there has been some evidence to show that protein contributes more than is generally believed. The data in the 1866 study likely underestimated the actual protein use for several methodological reasons. For example, though the subjects consumed a protein-free diet before the climb, postclimb excretion measures were not made, and other routes of nitrogen excretion may have been substantial. Nevertheless, based largely on these data, this belief has persisted throughout most of the 20th century. This is somewhat surprising because Cathcart27 in an extensive review of the literature prior to 1925 concluded ‘‘the accumulated evidence seems to me to point in no unmistakable fashion to the opposite conclusion that muscle activity does increase, if only in small degree, the metabolism of protein.’’ Based on results from a number of separate experimental approaches, the conclusions of several more recent investigators support Cathcart’s conclusion.28,29 Under normal conditions it is generally accepted today that the contribution made by proteins to the energy value of most well-balanced diets is usually between 10% and 15% of the total, and seldom exceeds 20%. For example, at rest and with a diet that contains significant amounts of carbohydrates, fats, and proteins—carbohydrates and fats are the preferred substrates for energy production. At that time the oxidation of amino acids accounts for less than 10% of the total energy needs. However, under certain conditions, for example, very low carbohydrate diets, because of the role that protein has in gluconeogenesis and TCA cycle anaplerosis, proteins make a much larger contribution to energy metabolism. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 112 29.10.2007 7:50pm Compositor Name: JGanesan 112 Amino Acids and Proteins for the Athlete: The Anabolic Edge An example would be when energy consumption in the form of carbohydrates and fats is low. In this case, whether due to a lack of fat and carbohydrates availability even in diets containing adequate energy in the form of protein, or due to a hypocaloric diet, the energy needs of the body take priority and dietary and tissue proteins will be utilized at the expense of the building or repair processes of the body. However, the amount of protein involved in energy metabolism can be substantial and in some cases overshadow the roles of both fats and carbohydrates, at least as far as the ultimate source of energy. That is because the body will adjust macronutrient oxidation in response to changes in diet composition. Energy is obtained from the catabolism of fats, carbohydrates, and proteins. In the average North American diet about 15% of the total energy is obtained from proteins. However, this amount varies according to the macronutrient composition of the diet. Although we do not fully understand how macronutrient composition of diets affects energy balance it likely does so in a number of ways, including changes in nutrient intake; absorption and partitioning; thermic effect; physical activity; under basal and exercise conditions; and secondary to the daily energy intake, other components of energy expenditure, neuroendocrine and gastrointestinal factors, and energy partitioning. In people on high-protein, low-carbohydrate diets, and especially in bodybuilders and elite endurance and power athletes who may be on very high-protein isocaloric and hypocaloric diets, the contribution can be as high as 50%, and in selective cases even more. In addition, protein oxidation rises in various other situations such as trauma and sepsis. The contribution that protein makes to energy metabolism, including both the ways of complete oxidation of the carbon skeletons, directly or indirectly by the various transformations (gluconeogenesis and glycolytic-TCA cycle interactions), is underappreciated and more studies need to be done to determine the amount of protein carbon that actually contributes to energy metabolism both at rest and during exercise. As it stands even in studies where the contribution of protein is obvious,30 that contribution is often glossed over. FATE OF DIETARY PROTEIN Unlike the fats and carbohydrates that can be stored in the form of triglycerides and glycogen, there is no storage form of protein or amino acids. All the protein and amino acids (except for a very small but important amount of free amino acids that make up the plasma and intracellular amino acid pool) serve either a structural or metabolic function. Recently, however, there is some evidence to show that protein contributes more to energy metabolism than is generally believed. Excess amino acids from dietary protein are normally deaminated or transaminated by the liver and the nonnitrogenous portion of the molecule is transformed into glucose and used directly or converted into fat or glycogen. The unneeded nitrogen is converted to urea and excreted in the urine. Endogenous protein goes through anabolic and catabolic phases, usually in time with absorptive and postabsorptive states, and under various stressors including exercise. Part of this protein is also used for energy metabolism. In general, the more protein that is ingested and the larger the body muscle mass, the more deamination and transamination of amino acids occurs to fulfill energy needs. Each kilocalorie (kcal) needed for basal metabolism leads to the excretion of 1–1.3 mg of urinary nitrogen. Because of the increased use of amino acids for energy production, nitrogen excretion increases during exercise and heavy work, although this increase is not always in proportion to the amount of energy generated. Protein requirements and the use of amino acids for energy production are raised if other energy sources are not ingested in adequate amounts—as is sometimes seen in persons and athletes on either low-fat or low-carbohydrate diets to decrease their weight and body-fat levels. Amino acids ingested without other energy sources makes for a less-than-ideal diet since amino acids are not efficiently incorporated into protein. This inefficiency is partly because of the energy Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 113 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 113 lost during amino acid metabolism and the energy cost of protein synthesis—incorporation of each amino acid molecule into peptides requires three high-energy phosphate bonds. Consequently, excess of dietary energy over basal needs, whether as fats, carbohydrates, or proteins, improves the efficiency of nitrogen utilization. PROTEIN CONTRIBUTION TO ENERGY METABOLISM In the body, amino acids may be used for primarily protein synthesis or energy production, depending upon: 1. 2. 3. 4. 5. Protein quality Caloric level of the diet Stage of development, including growth, pregnancy, and lactation Prior nutritional status Stress factors such as exercise, fever, injury, and immobilization Protein oxidation is a term used to describe the release of energy from the carbon skeleton of amino acids after deamination. Four ways in which proteins provide energy include: 1. 2. 3. 4. Direct oxidation of the carbon skeletons Gluconeogeneis via alanine, glutamine, and lactate Stored as fat or glycogen Anapleurotic contributions that cycle through the mitochondria and cytoplasm and result in the oxidation of resulting two-carbon units in the TCA cycle AMINO ACID CATABOLISM The catabolism of amino acids yields energy, about 4 kcal=g of protein, notwithstanding the higher energy costs of processing dietary protein. In order for amino acids to be used for energy, the amino group (NH2) must first be removed by a process known as deamination where the amino group is removed, or by transamination where the amino group is transferred to another structure to form a new another amino acid (see below). The deaminated carbon skeletons—a-keto acid residues—that are not used for forming new amino acids to enter the glycolytic and TCA cycle intertwining between mitochondrial and cytoplasm pathways involving two-, three-, and four-carbon intermediates (see below) are either converted to glucose and glycogen (glucogenic, gycogenic pathways) or metabolized in fat pathways (ketogenic pathway) either to be fully oxidized via the TCA cycle, or used for fatty acid formation. For more information see the section on the TCA cycle. When energy consumption in the form of carbohydrates and fats is low, the energy needs of the body take priority and dietary and tissue proteins will be utilized at the expense of the building or repair processes of the body. OXIDATION OF AMINO ACIDS Once the needed proteins are synthesized and the amino acid pools replenished, additional amino acids are degraded and used for energy or stored mainly as fat and to a lesser extent as glycogen. The primary site for degradation of most amino acids is the liver. The liver is unique because of its capacity to degrade amino acids and to synthesize urea for elimination of the amino nitrogen. There are a number of mechanisms brought into play to both conserve nitrogen when fed a low-protein diet, and to oxidize the surplus when presented with an immediate excess. For example, Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 114 29.10.2007 7:50pm Compositor Name: JGanesan 114 Amino Acids and Proteins for the Athlete: The Anabolic Edge the nitrogen catabolic enzymes and those involved in the first step in the catabolism of the essential amino acids are up- and downregulated in response to an increase or decrease in dietary protein. The first step in the degradation of amino acids begins with deamination—the removal of the a-amino group from the amino acid to produce an oxoacid, which may be a simple metabolic intermediate itself (e.g., pyruvate) or be converted to a simple metabolic intermediate via a specific metabolic pathway. The intermediates are either oxidized via the TCA cycle or converted to glucose via gluconeogenesis. For instance, deaminated alanine is pyruvic acid. This can be converted into glucose or glycogen. Or it can be converted into acetyl-CoA, which can then be polymerized into fatty acids. Also, two molecules of acetyl-CoA can condense to form acetoacetic acid, which is one of the ketone bodies. The larger the body muscle mass, the more transamination of amino acids occurs to fulfill energy needs. Each kilocalorie needed for basal metabolism leads to the excretion of 1–1.3 mg of urinary nitrogen. For the same reason, nitrogen excretion increases during exercise and heavy work. The human body, through metabolic and hormonal controls, has evolved to meet its continuous metabolic and energy needs even though eating, and thus provision of essential nutrients, is intermittent. Nutrients in the absorptive state (the time during and right after a meal usually lasting about 4–8 h after a meal depending on the composition of the meal and the state of the organism) coming from the food ingested, are filtered through the liver (with the liver modifying or simply conveying glucose, amino acids, and fatty acids to the systemic circulation and thus to the rest of the body) and are used preferentially for energy production and energy storage. In the postabsorptive period (after a meal has been completely digested and the resulting nutrients absorbed into the body, again usually 4–8 h after a meal during which the transition from the postprandial to the fasting state occurs, and even under fasting conditions as long as the fasting state is not extensive), the body tends to keep a constant energy output by mobilizing internal substrates (glycogen, cellular and body fat, cellular protein) that can be used as energy sources. Postabsorptive energy sources include both circulating and stored macronutrients. Those that are readily available include circulating glucose, fatty acids, and triglycerides; liver and muscle glycogen; muscle triglycerides; BCAAs (used by skeletal muscle); and the amino acids alanine and glutamine, which are released from skeletal muscle and used for gluconeogenesis—in the case of glutamine directly as fuel by the immune system and gastrointestinal (GI) tract.31,32 Circulating glucose is replenished mainly through hepatic glycogenolysis (breakdown of glycogen) and gluconeogenic (formation of glucose from other compounds such as lactate, glycerol, alanine, and glutamine) processes mainly in the liver, but also involving skeletal muscle, kidney, and GI tract. In a normal 70 kg man these postabsorptive energy sources provide up to 1200 kcal (800 kcal from carbohydrate sources). These sources are exhausted in less than 24 h if no other food is consumed. In an individual who is dieting to lose weight or in an athlete who is limiting both caloric and fat intake in order to maximize lean body mass and minimize body fat, these sources may amount to less than 500 kcal since liver and muscle glycogen levels as well as circulating triglycerides and fatty acids are often limited, and, in the case of the athlete, would easily be exhausted during a training session. In cases where the available energy is used up while training, other energy sources are called upon to supply the energy needed both during and after training until nutrients are able to be absorbed from a posttraining meal. Fatty acids and ketones formed by the catabolism of body fat (triglycerides) can be used by most tissues in the body. However, in the short term certain tissues such as the brain rely mostly on glucose and to some extent lactate for energy until ketones begin to be used. Thus glucose or substances that can be converted to glucose are vital to survival. The lack of glucose in the short term results in the formation of glucose through gluconeogenesis, a process by which glucose is formed from other substrates, mainly lactate, pyruvate, glycerol, and amino Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 115 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 115 acids.33–35 Amino acids, major substrates for glucose production, come from skeletal muscle leading to a loss of both contractile and noncontractile protein. Short-term extreme adaptations to energy needs beyond the immediate available energy are varied. Certain tissues, such as the heart, kidney, and skeletal muscle, change their primary fuel substrate from glucose to fatty acids and ketone bodies rather readily. Others like the brain take longer and use glucose (from gluconeogenesis) and ketones as their main energy sources.36 Although fatty acids cannot be directly converted to glucose, the energy produced from the oxidation of free fatty acids (FFA) can be used to fuel the conversion of certain other substrates into glucose (gluconeogenesis being an energy-requiring process), thus sparing somewhat the use of structural and contractile protein for glucose formation. For example, lactate and pyruvate are formed from anaerobic glycolysis and released systemically. In the liver they are converted to glucose, which is then released for other tissues to use. This shuttle is called the Cori cycle.37,38 The energy garnered through fatty acid oxidation is also used for transforming glycerol into glucose. With ongoing energy deprivation or low carbohydrate intake other adaptations appear. The brain, which ordinarily obtains energy only by glucose oxidation, acquires the ability to use keto acids for its fuel requirements, and this further contributes to protein conservation. The brain can metabolize a variety of substrates besides glucose, either directly or through the astrocyte–neuron nutrient shuttle, including lactate (considered by some as a preferred substrate even over glucose and produced by red blood cells [RBC], muscle, and astrocytes, and by others as being an insignificant source of energy), TCAI, fatty acids, ketones, glycerol, and amino acids, especially alanine, glutamine, and glutamate. The general consensus is that fatty acids cannot cross the blood–brain barrier, and they are not involved in energy metabolism in the CNS. Although there is very little evidence so far that fatty acids contribute directly to energy metabolism, at least as far as b-oxidation, there is a possibility that in fact that does occur. There is evidence that unsaturated and even saturated long-chain fatty acids can cross the blood–brain barrier.39–48 There is also evidence that circulating long-chain fatty acids are involved in the physiological regulation of feeding, and act as signals of nutrient surplus in the hypothalamus, secondary to changes in the cellular pool of malonyl-CoA, CPT-1, and fatty acyl-CoAs.49–51 Astrocytes and neurons are linked as far as energy metabolism and function. An example is the astrocyte–neuron nutrient shuttle. Astrocytes have also been postulated to help control blood flow to the brain.52–54 GLUCONEOGENESIS As we have seen, certain processes such as glycogenolysis, gluconeogenesis, increasing fatty acid oxidation, utilization of ketones, proteolysis, and others insure that the body has a continuous supply of available energy and substrates for its metabolic needs. Endogenous glucose production depends on glycogenolysis (glucose from glycogen) and gluconeogenesis (glucose from amino acids, lactate, and glycerol). The rise in blood glucose and the subsequent rise in insulin that occurs after a meal promote glycogen storage in liver and muscle and fat deposition in adipose tissue, skeletal muscle, and liver. After a meal both glucose and insulin levels fall and liver glycogen through the process of glycogenolysis becomes the primary source of available glucose. Maintenance of plasma glucose concentrations within a narrow range despite wide fluctuations in the demand (e.g., vigorous exercise) and supply (e.g., large carbohydrate meals) of glucose results from coordination of factors that regulate glucose release into, and removal from, the circulation.55 The degree of contribution to glucose production from glycogenolysis versus gluconeogenesis varies with the metabolic environment, macronutrient intake, and body composition.56 For example, due to limited endogenous carbohydrate stores, dietary restriction of carbohydrates results in an Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 116 29.10.2007 7:50pm Compositor Name: JGanesan 116 Amino Acids and Proteins for the Athlete: The Anabolic Edge increase in gluconeogenesis to both supply glucose directly and to increase hepatic glycogen levels thus supporting glycogenolysis.57 Muscle glycogen levels cannot contribute to blood glucose levels directly although they can contribute to glycolysis providing anaerobic ATP for skeletal muscle contractions, and contribute to the formation of lactate, glycerol, alanine, and glutamine substrates for hepatic gluconeogenesis. It is often believed that gluconeogenesis from amino acids is a process that only occurs under physical stress and starvation in order to provide glucose, and that the need to provide glucose is a driving factor in the increased gluconeogenesis taking place at these times. It is also generally believed that gluconeogenesis from amino acids is a minor process in the fed state. In fact, conversion of the glucogenic moieties of the degraded amino acids to glucose occurs even in the fed state. A detailed quantitative analysis of the energy exchanges associated with the degradation of amino acids in humans demonstrates that gluconeogenesis and export of glucose is essential, because complete oxidation of the amino acid mixture by the liver would provide much more ATP than needed by the liver, and the oxygen consumption greater than that available to the liver.58 Gluconeogenesis from amino acids must thus be regarded as a normal process associated with amino acid degradation, occurring at higher rates under conditions of normal food intake than during fasting. AMINO ACIDS AND GLUCONEOGENESIS Lactate, pyruvate, glycerol, and certain amino acids are used by the liver, kidney, and intestinal tract to form glucose. Under normal conditions, amino acids, especially alanine and glutamine, serve as major substrates for gluconeogenesis.59 It has recently been shown that even under normal conditions a major fate of dietary amino acids is for hepatic gluconeogenesis. Interestingly enough, dietary carbohydrate content only minimally affects hepatic gluconeogenesis from dietary amino acids, with only a slight increase seen during severe carbohydrate restriction.60 In addition, an increase in FFAs has been shown to increase gluconeogenesis but only at high plasma FFA concentrations.61 In one study the contribution under various nutritional regimens of several amino acids and lactate to gluconeogenesis was estimated by measuring the glucose formation from 14C-labeled substrates.62 Isolated rat hepatocytes were incubated for 60 min in a Krebs–Ringer bicarbonate buffer pH 7.4 containing lactate, pyruvate, and all the amino acids at concentrations similar to their physiological levels found in rat plasma, with one precursor labeled in each flask. In all conditions, lactate was the major glucose precursor, providing over 60% of the glucose formed. Glutamine and alanine were the major amino acid precursors of glucose, contributing 9.8% and 10.6% of the glucose formed, respectively, in hepatocytes isolated from starved rats. Serine, glycine, and threonine also contributed to gluconeogenesis in the starved liver cells at 2.6%, 2.1%, and 3.8%, respectively, of the glucose formed. In this study it was shown that the availability of dietary carbohydrates varies the gluconeogenic response. The rate of glucose formation from the isolated hepatocytes of the starved rats and those fed either high protein or high fat was higher than from rats fed a nonpurified diet. The low-insulin and the high-glucagon output that characterizes the postabsorptive phase and fasting (the lower the glucose level the lower the insulin and higher the glucagon output) stimulates both glycogenolysis and gluconeogenesis. Initially approximately 70%–75% of the hepatic glucose output is derived from glycogenolysis and the remainder from gluconeogenesis. Starvation is associated initially with an increased release of the gluconeogenic amino acids from muscles and an increase in gluconeogenesis.63 In one study looking at amino acid balance across forearm muscles in postabsorptive (overnight fasted) subjects, fasting significantly reduced basal insulin and increased glucagon, and increased muscle release of the principal glycogenic amino acids (alanine, glutamine, glycine, threonine, serine, methionine, tyrosine, and lysine).64 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 117 29.10.2007 7:50pm Compositor Name: JGanesan 117 Energy Metabolism In this study alanine release increased 59.4%. The increase in release for all amino acids averaged 69.4% and was statistically significant for threonine, serine, glycine, alanine, a-aminobutyrate, methionine, tyrosine, and lysine. In the same study seven subjects were also fasted for 60 h. In these subjects there was a reduction of amino acid release as the fasting continued. Since these changes reproduce those observed after a few days of total fasting, it has been suggested that it is the carbohydrate restriction itself, and the subsequent decrease in insulin and increase in glucagon, which is responsible for the metabolic and hormonal adaptations of brief fasting.65 A reduction in the release of substrate amino acids from skeletal muscle largely explains the decrease in gluconeogenesis characterizing prolonged starvation. A similar response, an initial increase in gluconeogenesis followed in time by a decrease, is seen in trauma such as burn injury66 and in prolonged exercise.67 During prolonged mild exercise that increases the hepatic glucose output twofold, the relative contribution of gluconeogenesis to the overall hepatic output increases from 25% to 45%, indicating a threefold rise in the absolute rate of gluconeogenesis. After 12–13 h of fasting, hepatic gluconeogenesis replaces glycogenolysis as the main source of glucose.68 In one study on six subjects the contribution of gluconeogenesis to glucose production was 41%, 71%, and 92% at 12, 20, and 40 h, respectively.69 There is also an increase in gluconeogenesis with type II diabetes70 and obesity. A recent study found that the fractional and absolute contribution of gluconeogenesis to glucose production is higher in obesity and is associated with increased rates of protein turnover and insulin resistance of glucose, lipid, and protein metabolism.56 The study also found a lower contribution to glucose production from hepatic glycogenolysis. Insulin resistance, by mechanisms that involve suppression of PI3K=Akt signaling leading to activation of caspase-3 and the ubiquitin-proteasome proteolytic pathway, leads to an increase in muscle protein degradation,71 and a subsequent increase in gluconeogenesis. HORMONAL CONTROL OF GLUCONEOGENESIS Among the hormones responsible for a rise in gluconeogenesis one must take into account adrenaline, glucagon, glucocorticoids, growth hormone, and insulin. These hormones can act either directly on the gluconeogenic enzymes or on the mobilization of precursors necessary for gluconeogenesis (the three-carbon precursors, including lactate, alanine, and glycerol). On a moment-to-moment basis, however, these processes are controlled mainly by the glucocorticoids, insulin, and glucagon, whose secretions are reciprocally influenced by the plasma glucose concentration. The glucocorticoids increase the activity of the glucose–alanine cycle. Insulin decreases the supply of gluconeogenic substrates and inhibits the glucose–alanine cycle. Thus, the exerciseinduced hypoinsulinemia is a promoting factor in gluconeogenesis. In the resting postabsorptive state, release of glucose from the liver (usually equally via glycogenolysis and gluconeogenesis) is the key regulated process. Glycogenolysis depends on the relative activities of glycogen synthase and phosphorylase, the latter being the more important. The activities of fructose-1,6-diphosphatase, phosphoenolpyruvate carboxylkinase, and pyruvate dehydrogenase, whose main precursors are lactate, alanine, and glutamine, regulate gluconeogenesis. In the immediate postprandial state, due to the availability of substrates coming from the GI tract, suppression of liver glucose output and stimulation of skeletal muscle glucose uptake are the most important factors. Glucose disposal by insulin-sensitive tissues is regulated initially at the transport step and then mainly by glycogen synthase, phosphofructokinase, and pyruvate dehydrogenase. Hormonally induced changes in intracellular fructose-2,6-bisphosphate concentrations play a key role in muscle glycolytic flux and both glycolytic and gluconeogenic flux in the liver. Under stressful conditions (e.g., hypoglycemia, trauma, vigorous exercise), increased secretion of other hormones such as adrenaline, cortisol, and growth hormone and increased activity of the Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 118 29.10.2007 7:50pm Compositor Name: JGanesan 118 Amino Acids and Proteins for the Athlete: The Anabolic Edge sympathetic nervous system come into play. Their actions to increase hepatic glucose output and to suppress tissue glucose uptake are partly mediated by increases in tissue fatty acid oxidation. In diabetes, the most common disorder of glucose homeostasis, fasting hyperglycemia results primarily from excessive release of glucose by the liver due to increased gluconeogenesis; postprandial hyperglycemia results from both impaired suppression of hepatic glucose release and impaired skeletal muscle glucose uptake. These abnormalities are usually due to the combination of impaired insulin secretion and tissue resistance to insulin. As noted earlier, under normal conditions the amount of readily available energy coming from carbohydrate sources is only about 800 kcal—and includes the 120 g or so of glycogen from the body’s skeletal muscle, 70 g of glycogen in the liver, and 10 g of glucose in the blood. Adipose tissue on the other hand provides an almost limitless supply of available energy, about 100,000 calories from the average person’s 15 kg of fat. FFAs from adipose tissue and intramuscular stores and muscle glycogen provide energy both at rest and during low-intensity exercise. At these levels, substrate availability does not limit activity since there is so much potential energy available from body fat. FFA mobilization from adipose tissue (catecholamines stimulate adipose lipase which breaks each triglyceride molecule down to form one glycerol and three fatty acid molecules) starts with any kind of activity. FFAs are metabolized in muscle cells to acetyl-CoA that subsequently enters the TCA cycle forming ATP as it is oxidatively metabolized. While FFA mobilization may lag at first, it soon results in high FFA blood levels of up to six times the normal level. These levels of FFA are so high that they cannot all be utilized by muscle. Perhaps one of the reasons for the overproduction of FFA is to release glycerol, a substrate for gluconeogenesis. EFFECTS OF AMINO ACIDS ON HEPATIC GLUCOSE METABOLISM Amino acids have direct (substrate-mediated) and indirect (hormone-mediated) effects on hepatic glucose metabolism. The hormonal effects include the simultaneous increase in endogenous levels of insulin and glucagon.72,73 This simultaneous increase leads to an increase in endogenous glucose production mainly by gluconeogenesis since higher insulin concentrations are needed to suppress gluconeogenesis as compared to glycogenolysis.74 A recent study found that conditions creating postprandial amino acid elevation stimulate secretion of insulin and glucagon, but do not affect glycemia despite markedly increased gluconeogenesis.75 The rise in endogenous glucose production was counterbalanced by an insulinstimulated increase in glucose disposal, which thereby served to maintain fasting plasma glucose concentrations. USE OF AMINO ACIDS FOR ENERGY—CATABOLIC EFFECTS OF EXERCISE Typical responses to acute exercise are suppressed protein synthesis and elevated protein degradation. Comparison of these responses in muscles containing various types of fibers indicated that the rate of protein synthesis was suppressed and the rate of protein degradation was elevated mainly in muscles less active during exercise. Thus, the less active muscles, but not those that fulfill the main task during exercise performance, are used as a reservoir for mobilization of protein resources. The exercise-induced catabolism is not extended to contractile proteins. During exercise the catabolic response is extended to the smooth muscle of the GI tract, lymphoid tissue, liver, and kidney. Both the antianabolic and catabolic effects have to be considered as tools for mobilization of protein resources during a stressful situation. As a result, an increased pool of available free amino acids is created. Due to the suppressed protein synthesis, the free amino acids pool is used for supplying the necessary protein synthesis by ‘‘building materials’’ only to a minor extent. The amino acids are mainly used for additional energy supply to contracting muscles. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 119 29.10.2007 7:50pm Compositor Name: JGanesan 119 Energy Metabolism There are at least three pathways connecting free amino acids with energy processes. One of these consists in the oxidation of BCAAs. The main site of this pathway is the contracting muscle. An increased oxidation of leucine during exercise was established in human as well as in animal studies. The metabolism of several amino acids leads to the formation of metabolites of the TCA cycle, which also has a beneficial effect on muscle metabolism by increasing the capacity of the TCA cycle for oxidizing the acetyl-CoA units generated from pyruvate and FFAs. AMINO ACID METABOLISM IN MUSCLE There are three principal sources of amino acids for energy metabolism: dietary protein, plasma and tissue free amino acid pools, and endogenous tissue protein. All three sources are in equilibrium. Dietary protein is a relatively minor source of amino acids during normal exercise since ingesting a large protein meal prior to exercise is rarely done. The plasma pool of free amino acids is much smaller than the free amino acid pool of human skeletal muscle largely because skeletal muscle makes up about 40% of body weight and contains about 75% of the whole-body free amino acids. The free amino acid pools, however, are much smaller than the amount of amino acids available from endogenous protein breakdown. It has been estimated that the intramuscular amino acid pool contains less than 1% of the metabolically active amino acids. It has also been estimated that the amount of leucine oxidized during a prolonged exercise bout is approximately 25 times greater than the free leucine concentration in muscle, liver, and plasma. Therefore, the free amino acid pool is only a minor source of amino acids during exercise, whereas the most important source is endogenous protein breakdown. PATHWAYS OF AMINO ACID METABOLISM IN MUSCLE Many of the amino acids can be oxidized by skeletal muscle including alanine, arginine, aspartate, cysteine, glutamate, glycine, phenylalanine, threonine, tyrosine, and the three BCAAs. Not all of these amino acids, however, have the same metabolic potential in muscle and it appears that the BCAAs are the dominant amino acids oxidized by skeletal muscle. The catabolism of amino acids involves the removal of the a-amino group by transamination or oxidative deamination, followed by conversion of the carbon skeleton to metabolites that are common to the pathways of carbohydrate and fat metabolism. It should be noted that the avenues available for amino acid catabolism and conversion are extensive, and the final products, their fates, and the pathways available to them are all quite complex. SKELETAL MUSCLE CATABOLISM Significant gaps remain in our knowledge of the pathways of amino acid catabolism in humans. Sufficient information, however, does exist to allow a broad picture of the overall process of amino acid oxidation to be developed along with approximate quantitative assessments of the role played by liver, muscle, kidney, and small intestine. For example, one study found that amino acids are the major fuel of liver, that is, their oxidative conversion to glucose accounts for about one-half of the daily oxygen consumption of the liver, and no other fuel contributes nearly so importantly.76 The daily supply of amino acids provided in the diet cannot be totally oxidized to CO2 in the liver because such a process would provide far more ATP than the liver could utilize and not enough ATP would be generated in other tissues. Instead, most amino acids are oxidatively converted to glucose. This results in an overall ATP production during amino acid oxidation very nearly equal to the ATP required to convert amino acid carbon to glucose. ATP is thus produced in the muscle cells both by anaerobic glycolysis and oxidation of amino acids, while ATP is used up in liver cells to produce glucose from the carbon skeletons. Thus the glucose–alanine cycle in an indirect way transfers ATP from the liver to the peripheral tissues. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 120 29.10.2007 7:50pm Compositor Name: JGanesan 120 Amino Acids and Proteins for the Athlete: The Anabolic Edge It is commonly thought that only ATP, pyruvate carboxylase (PC), and glycolysis (the conversion of glucose into pyruvic acid) provide cellular energy under anaerobic conditions. This is not the case, however, since we have seen amino acid catabolism coupled with the TCA cycle can also provide a source of anaerobic energy production. In a study on anoxic heart muscle, glutamate and aspartate catabolized at a higher rate as compared with oxygenation.77 The data obtained from this study suggest that the constant influx of intermediates into the cycle from amino acids is supported by coupled transamination of glutamate and aspartate. This leads to the formation of ATP and GTP in the TCA cycle during blocking of aerobic energy production. Succinate, a component of the TCA cycle, is thought to result from amino acid catabolism. In another study on cat heart papillary muscle, supplying hypoxic, contracting muscles with aspartate resulted in maintenance of muscular function to some extent and led to augmented release of succinate and lactate.78 The data from this study indicate that anaerobic succinate formation is correlated to the energy-requiring processes of the myocardium. Maintenance of myocardial function by the supply of amino acids may be related to their conversion into succinate. In some studies it has been shown that in the human heart glutamate may be used as an anaerobic fuel through conversion to succinate coupled with GTP formation.79 Since amino acids can be used as fuel in both aerobic and anaerobic states, muscle catabolism occurs under both conditions to provide the needed substrates. The use of parenteral or enteral amino acid mixtures and combinations of specific amino acids can attenuate this catabolic and counterproductive response to exercise. OXIDATION OF AMINO ACIDS The contribution made by proteins to the energy value of most well-balanced diets is usually between 10% and 15% of the total and seldom exceeds 20%. In some athletes in power sports and in bodybuilders who may be on very high-protein diets, the contribution can be as high as 50%, and in selective cases even more. Once the needed proteins are synthesized and the amino acid pools replenished, additional amino acids are degraded and used for energy or stored mainly as fat and to a lesser extent as glycogen. The primary site for degradation of most amino acids is the liver. The liver is unique because of its capacity to degrade amino acids and to synthesize urea for elimination of the amino nitrogen. The first step in the degradation of amino acids begins with deamination—the removal of the a-amino group from the amino acid to produce an oxoacid, which may be a simple metabolic intermediate itself (e.g., pyruvate) or be converted to a simple metabolic intermediate via a specific metabolic pathway. The intermediates are either oxidized via the TCA cycle or converted to glucose via gluconeogenesis. For instance, deaminated alanine is pyruvic acid. This can be converted into glucose or glycogen. Or it can be converted into acetyl-CoA, which can then be polymerized into fatty acids. Also, two molecules of acetyl-CoA can condense to form acetoacetic acid, which is one of the ketone bodies. Deamination occurs mainly by transamination, the transfer of the amino group to some other acceptor compound. Deamination generally occurs by the following transamination pathway: a-Ketoglutaric acid þ amino acid ! glutamic acid þ a-keto acid Glutamic acid þ NADþ þ H2 O ! NADH þ Hþ þ NH3 þ a-ketoglutaric acid Note from this schema that the amino group from the amino acid is transferred to a-ketoglutaric acid, which then becomes glutamic acid. The glutamic acid can then transfer the amino group to still other substances or can release it in the form of ammonia (NH3). In the process of losing the amino group, the glutamic acid once again becomes a-ketoglutaric acid, so that the cycle can be repeated again and again. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 121 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 121 To initiate this process, the excess amino acids in the cells, especially in the liver, induce the activation of large quantities of aminotransferases, the enzymes responsible for initiating most deamination. Hepatic tissue contains high concentrations of the degradative enzymes, including the aminotransferases (with the exception of the branched-chain aminotransferase) which remove the a-amino groups during the first step in amino acid degradation. Branched-chain aminotransferase results in the release of BCAAs into circulation and is exclusively a mitochondrial enzyme in skeletal muscles.80 For example, degradation of the BCAAs is initiated by the reversible transamination of the BCAA to the a-keto acid with transfer of the a-amino group to AKG, forming glutamate. This step appears to be nearly at equilibrium with little physiological control. The second and rate-limiting step is decarboxylation of the branched-chain keto acids by branched-chain keto acid dehydrogenase (BCKAD). BCKAD activity is highly regulated by phosphorylation and dephosphorylation to the inactive and active forms, respectively. This step is stimulated by increases in the concentration of the leucine keto acid, a-ketoisocaproate (KIC). During periods of increased energy needs, such as starvation, trauma, and exercise, increased levels of BCAAs stimulate BCKAD, and the BCAAs are degraded to energy in skeletal muscle. During absorption periods when muscles are using glucose as a primary fuel, muscle transaminates the BCAA and releases the keto acids into circulation for complete oxidation by the liver or kidney. In the liver, these intermediates formed either in the liver or from other tissues are either oxidized via the TCA cycle or converted to glucose via gluconeogenesis. A deaminated amino acid can enter the TCA cycle at different levels including acetyl-CoA, pyruvate, oxaloacetate, AKG, succinyl-CoA, or fumarate.81 Both isoleucine and valine form succinyl-CoA, whereas leucine forms acetyl-CoA. Of these, only the carbons of acetyl-CoA from leucine can be oxidized directly in the TCA cycle; the other intermediates must first be converted to pyruvate via phosphoenolpyruvate before their carbons are available for oxidation in the TCA cycle as acetyl-CoA. Within skeletal muscle, BCAA degradation involves transfer of the a-amino group to ketoglutarate to form glutamate. Glutamate serves as an important intermediate in nitrogen metabolism. While glutamate is formed de novo in skeletal muscle, there is no net release. Once the amino group is transferred to glutamate, there are three primary fates. The amino group can be transferred either to pyruvate in synthesis of alanine or onto oxaloacetate for the synthesis of aspartate, or the glutamate can release the amino group in the form of ammonia and reform ketogluterate, at the same time releasing two hydrogen atoms which are oxidized to form ATP. Alanine is released from muscle into circulation and is ultimately removed by the liver for gluconeogenesis. Aspartate is an important component of the purine nucleotide cycle which is central to maintaining the pool of ATP in muscle. The purine nucleotide cycle serves to regenerate inosine monophosphate (IMP) and also produces fumarate and free ammonia (NH3). The ammonia can be combined with glutamate via glutamine synthetase to form glutamine (glutamate þ ammonia þ ATP ! glutamine þ ADP).82 Glutamine is ultimately released from muscle, with the majority of glutamine being used by the gut and the immune system as a primary energy source. Together, alanine and glutamine represent 60%–80% of the amino acids released from skeletal muscle while they account for only 18% of the amino acids in muscle protein. During exercise, glutamine synthesis and glutamine levels in muscle decline due to inhibition of glutamine synthetase. The net effect of oxidation of amino acids to glucose in the liver is to make nearly two-thirds of the total energy available from the oxidation of amino acids available to peripheral tissues, without necessitating that peripheral tissues synthesize the complex array of enzymes needed to support direct amino acid oxidation. As a balanced mixture of amino acids is oxidized in the liver, nearly all carbon from glucogenic amino acids flows into the mitochondrial aspartate pool and is actively transported out of the mitochondria via the aspartate–glutamate antiport linked to proton entry. In the cytoplasm the Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 122 29.10.2007 7:50pm Compositor Name: JGanesan 122 Amino Acids and Proteins for the Athlete: The Anabolic Edge aspartate is converted to fumarate utilizing urea cycle enzymes; the fumarate flows via oxaloacetate to phosphoenolpyruvate and on to glucose. Thus carbon flow through the urea cycle is normally interlinked with gluconeogenic carbon flow because these metabolic pathways share a common step. Liver mitochondria experience a severe nonvolatile acid load during amino acid oxidation. It is suggested that this acid load is alleviated mainly by the respiratory chain proton pump in a form of uncoupled respiration. GLUCOGENIC AND KETOGENIC AMINO ACIDS All tissues have some capability for synthesis of the nonessential amino acids, amino acid remodeling, and conversion of non-amino acid carbon skeletons into amino acids and other derivatives that contain nitrogen. However, the liver is the major site of nitrogen metabolism in the body. In times of dietary surplus, the potentially toxic nitrogen of amino acids is eliminated via transaminations, deamination, and urea formation; the carbon skeletons are generally conserved as carbohydrate, via gluconeogenesis, or as fatty acid via fatty acid synthesis pathways. In this respect amino acids fall into three categories: glucogenic, ketogenic, or glucogenic and ketogenic. Glucogenic amino acids are those that give rise to a net production of pyruvate or TCA cycle intermediates, such as AKG or oxaloacetate, all of which are precursors to glucose via gluconeogenesis. All amino acids except lysine and leucine are at least partly glucogenic. Lysine and leucine are the only amino acids that are solely ketogenic, giving rise only to acetyl-CoA or acetoacetyl-CoA, neither of which can bring about net glucose production. See Figure 3.1 to see the entry of amino acids into the pathway of gluconeogenesis. Entry of precursors into the pathways of gluconeogenesis Tryptophan Threonine Alanine Cysteine Serine Glycine Aspartate Asparagine Aspartate Phenylalanine Tyrosine Valine Isoleucine Threonine Methionine Arginine Glutamine Glutamate Histidine Proline Lactate Glycerol Pyruvate Glycerol-P Oxaloacetate Glucose Fumarate Citric acid cycle Succinyl-CoA α-Ketogluterate Rate limiting steps Oxaloacetate → PEP FDP → F-6-P G-6-P → Glucose Leucine and lysine pathway into TCA cycle is via acetyl-CoA FIGURE 3.1 Entry of precursors into the pathways of gluconeogenesis. Abbreviations: PEP, phosphoenolpyruvate; FDP, fructose-1,6-phosphate; F-6-P, fructose-6-phosphate; G-6-P, glucose-6-phosphate. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 123 29.10.2007 7:50pm Compositor Name: JGanesan 123 Energy Metabolism TABLE 3.1 Glucogenic and Ketogenic Amino Acids Glucogenic Alanine Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Histidine Hydroxyproline Isoleucine Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine Ketogenic Both Leucine Lysine Isoleucine Phenylalanine Threonine Tryptophan Tyrosine A small group of amino acids comprised of isoleucine, phenylalanine, threonine, tryptophan, and tyrosine give rise to both glucose and fatty acid precursors and are thus characterized as being glucogenic and ketogenic (see Table 3.1). Finally, it should be recognized that amino acids have a third possible fate. During times of stress and starvation there is an increase in the use of the reduced carbon skeleton, through the transformation to acetyl-CoA either directly or indirectly via the TCA cycle, for energy production, with the result that it is oxidized to CO2 and H2O. GLUCOGENIC AMINO ACIDS Metabolized to AKG, Pyruvate, Oxaloacetate, Fumarate, or Succinyl-CoA Alanine Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Histidine Hydroxyproline Isoleucine Methionine Phenylalanine Proline Serine Threonine Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 124 29.10.2007 7:50pm Compositor Name: JGanesan 124 Amino Acids and Proteins for the Athlete: The Anabolic Edge Tryptophan Tyrosine Valine KETOGENIC AMINO ACIDS Metabolized to Acetyl-CoA or Acetoacetate Leucine and lysine are the only amino acids that are solely ketogenic giving rise only to acetyl-CoA or acetoacetyl-CoA, neither of which can bring about net glucose production. Five amino acids (isoleucene, phenylalanine, threonine, tryptophan, and tyrosine) are both glucogenic and ketogenic. Isoleucine Leucine Lysine Phenylalanine Threonine Tryptophan Tyrosine ALANINE AND GLUTAMINE Intense exercise has dramatic effects on amino acid metabolism. After an exhausting load a significant rise and concurrent drop of serum isoleucine, threonine, ornithine, leucine, serine, glycine, asparagine, and glutamine occur with a rise in serum alanine.83 The rise of alanine suggests the existence of a glucose–alanine cycle (see below). The drop of branched amino acids is probably due to their enhanced entry into muscles. The drop of glutamine reflects increased use by the gut and immune system, even though the release of glutamine by muscle cells increases secondary to proteolysis. Similarly, under stress, glucose consumption increases and the efflux of lactate, alanine, glutamine, and total amino acid nitrogen (i.e., net muscle protein catabolism) increases. In a series of studies Brown et al. have shown that catabolic hormones alone fail to reproduce the stress-induced efflux of amino acids.84 In this study the net balance of amino acids was determined in five healthy volunteers prior to and following a 2 h infusion of the catabolic hormones epinephrine, cortisol, and glucagon into the femoral artery. This hormonal simulation of stress in normal volunteers increased glutamine efflux from the leg to an extent similar to that of burn patients. Alanine efflux, however, was not affected by the hormonal infusion. Because alanine efflux constituted a major proportion of the total peripheral amino acid catabolism in the burn patients, there was significantly less total amino acid nitrogen loss from the healthy volunteers receiving the stress hormones. Thus catabolic hormones alone fail to reproduce the stress-induced pattern and quantity of amino acid efflux from human skeletal muscle. This discrepancy is largely due to unresponsiveness of alanine to hormonally induced muscle protein catabolism. In states of stress alanine becomes increasingly important to sustain increases in gluconeogenesis. Alanine, via deamination to pyruvate, is a principal precursor for hepatic gluconeogenesis and is normally produced secondary to insulin resistance—important since insulin is known to inhibit muscle protein breakdown especially in severely stressed states. In another study Brown et al. have shown that alanine efflux may depend on pyruvate availability during stress and the rate of glycolysis within peripheral tissues may be a major factor in regulating the quantity of alanine efflux.85 Several studies have hypothesized that alanine decreases plasma ketone body levels by increasing availability of oxaloacetate, thus allowing acetyl groups to enter the TCA cycle and releasing Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 125 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 125 CoA. The significant elevation in plasma-free carnitine concentration found after alanine ingestion is consistent with the hypothesis that alanine increases the oxidation of acetyl-CoA by providing oxaloacetate for the TCA cycle.86 Glutamine also becomes increasingly important during stress both due to its role as a major precursor for ammoniagenesis by the kidney and its avid consumption by visceral organs, fibroblasts, and lymphoid tissue.87 The authors of this study suggest that ‘‘any attempt to minimize muscle protein catabolism should be accompanied by amino acid supplementation, especially of glutamine and alanine.’’ Thus the provision of exogenous alanine may decrease insulin resistance, and the provision of exogenous alanine and glutamine may decrease the need to obtain these amino acids from endogenous protein and therefore decrease muscle protein breakdown. INTERORGAN EXCHANGE OF AMINO ACIDS Many of the alterations in the hormonal ensemble during exercise favor (or even constitute the main cause of) protein and amino acid mobilization. As a result of exercise, there is an interorgan exchange of amino acids particularly the BCAAs, alanine, and glutamine. The main features of this interorgan exchange include: . . . Movement of the BCAA (leucine, valine, and isoleucine) from the splanchnic bed (liver and gut) to skeletal muscles Movement of alanine from muscle to the liver Movement of glutamine from muscle to the gut The interorgan exchange of these amino acids has several functions including: . . . . . Maintaining amino acid precursors for protein synthesis Assisting in the elimination of nitrogen wastes Providing substrates for gluconeogenesis Providing glutamine for gut and immune system function Maintaining the purine nucleotide cycle Early studies by Felig et al. provided evidence about amino acid movement among tissues.88 By examining arterial–venous differences in substrate concentrations across tissues, they observed that skeletal muscle had a net uptake of BCAAs with a subsequent release of alanine greater than the amount present in muscle protein. The released alanine was removed by the splanchnic bed. On the basis of these observations, they proposed the existence of a glucose–alanine cycle for maintenance of blood glucose and the shuttling of nitrogen and gluconeogenic substrate from muscle to liver. Another study by Ahlborg et al. using six untrained adult males examined the arterial–venous differences across the splanchnic bed and across the leg during 4 h of cycling at 30% of VO2 max.89 They observed decreases in plasma glucose and insulin, and increases in glucagon and FFAs. Plasma glucose declined from a preexercise level of 90 mg=dl but stabilized at 60 mg=dl as release of glucose by the liver continued throughout the exercise period. Glucose remained an important fuel throughout the exercise, but there was a significant increase in use of FFAs. There was also a significant increase in amino acid flux, including a fourfold increase in the release of BCAAs from the splanchnic area and a corresponding increase in the uptake by exercising muscles. In return, muscles released alanine, which was removed by the liver for gluconeogenesis. This study demonstrated a change in amino acid flux during exercise with movement of BCAAs from visceral tissues to skeletal muscles and the return of alanine as a precursor for hepatic synthesis of glucose. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 126 29.10.2007 7:50pm Compositor Name: JGanesan 126 Amino Acids and Proteins for the Athlete: The Anabolic Edge Lemon and Mullin further established the relationship of amino acid metabolism to glucose homeostasis.90 In this study they demonstrated a dramatic increase in sweat urea nitrogen in men previously depleted of glycogen stores and who had exercised for 1 h at 61% VO2 max, but no change in urinary urea nitrogen. The authors found that amino acid catabolism provided 10.4% of total energy for the exercise in carbohydrate-depleted individuals while in the carbohydrate-loaded group protein provided 4.4% of the energy needs. These results suggest that production of nitrogen wastes during exercise is related to carbohydrate status and that sweating may be an important mechanism for elimination of nitrogen wastes. Other studies also support the relationship of increased amino acid degradation with maintenance of hepatic glucose production.67,91 Exercise and periods of food restriction or fasting share many similarities in amino acid metabolism. In both there is a net release of amino acids from protein due to suppression of protein synthesis, and an increase in protein breakdown. This change in protein turnover results in an increase in tissue levels of free amino acids and net release of amino acids from most tissues. Tissues with high rates of protein turnover are most affected, with the liver and GI tract incurring the largest losses during short-term fasts. Skeletal muscle, which has lower rates of protein turnover, responds somewhat less dramatically, but because of its total mass, muscle is the predominate source of FFAs during prolonged periods of food restriction. Amino acids have been shown to play important roles in energy homeostasis during periods of high energy need or low energy intake. While the qualitative role is becoming clear, the quantitative level of this role remains uncertain. Most of the studies examining this role have been designed using leucine and alanine as metabolic tracers. These studies establish changes in protein turnover, increases in amino acid flux, and increase in leucine oxidation. However, questions remain about the potential to generalize from these findings to other amino acids or to dietary protein requirements. Endurance exercise causes a reduction in muscle protein synthesis and net protein breakdown—but is there a real loss of amino acids? Are there increases in urea production during exercise? Are the effects unique to leucine or do they relate to other amino acids? These questions remain to be fully answered. Using multiple isotope tracers, Wolfe et al.92 examined amino acid metabolism during aerobic exercise. They found inhibition of protein synthesis, increased leucine oxidation, and increased flux of alanine. These findings are consistent with most of the earlier reports. However, using direct isotopic measures, they failed to find any increase in urea synthesis. Further, using lysine, a second indispensable amino acid, they demonstrated that the inhibition of protein synthesis was less than half that estimated by leucine (17% vs. 48%) and that there was no increase in lysine oxidation. PROTEIN METABOLISM AND AMMONIA METABOLISM OF AMMONIA Ammonia is formed in a number of reactions in the body during the degradation of amines, purines, and pyrimidines,93–95 and the metabolism of amino acids. It seems, however, that the quantitatively important source of ammonia during exercise is the metabolism of amino acids.96 Skeletal muscle produces a large quantity of ammonia during prolonged submaximal (i.e., endurance) exercise, the source of which is the subject of some controversy. Ammonia production in skeletal muscle involves the purine nucleotide cycle and the amino acids glutamate, glutamine, and alanine and probably also includes the BCAAs as well as aspartate.97 Several authors have proposed that the degradation of amino acids, specifically, the BCAAs leucine, valine, and isoleucine, may be a source of ammonia during this type of activity. The ammonia response can be suppressed by increasing the carbohydrate availability and this may be mediated by decreasing the oxidation of the BCAAs.98 MacLean et al. performed a study in which the purpose was to examine the effects of oral BCAA supplementation on amino acid and ammonia metabolism in exercising humans.99 Five men exercised the knee extensors of one leg for 60 min with and without (control) an oral supplement of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 127 29.10.2007 7:50pm Compositor Name: JGanesan 127 Energy Metabolism BCAA. The subjects consumed 77 mg=kg of the supplement administered in two equal doses of 38.5 mg=kg at 45 and 20 min before the onset of exercise. According to the authors of the study, the 500 mg capsules of the commercially available BCAA supplement were reported to contain only the three BCAA in the following proportions: 220, 150, and 130 mg of L-leucine, L-valine, and Lisoleucine, respectively. This was confirmed by dilution of the capsules in water and with analysis by high-performance liquid chromatography. The researchers concluded that their study clearly shows that BCAA supplementation results in significantly greater muscle ammonia production during exercise. Also, BCAA supplementation imposes a substantial ammonia load on muscle, as indicated by the consistently larger total alanine and glutamine releases observed during exercise. The elevated BCAA levels also suppress the degree of net protein degradation that normally occurs during exercise of this magnitude and duration. Also of importance in this was the finding that the degree of contractile protein degradation was not increased during exercise for either trial. The authors noted that previous studies have reported that muscle protein catabolism occurs primarily in noncontractile protein while contractile protein catabolism is spared or even decreased. MacLean et al. found no significant elevations in the release of 3-methylhistidine (a marker of contractile protein catabolism, 3-methylhistidine is a musclespecific amino acid that is produced and lost in the urine only when muscle protein is broken down) from muscle for either trial. Since it is produced in degradation reactions, ammonia is usually considered an end product of metabolism which must be converted to urea, a nontoxic compound, for excretion. In most mammals, including man, urea is produced in the liver by a cyclical series of reactions, known as the urea cycle. The urea cycle, because of the intricacy of amino acid metabolism, and in particular the contribution of different tissues, is more complex than is generally thought. In addition to its role in the urea cycle, ammonia can play a number of other roles in metabolism including: . . . . Acting as a substrate for the glutamate dehydrogenase reaction in the formation of nonessential amino acids As a substrate for glutamine synthetase in the formation of glutamine in muscle Acting as an important regulator of the rate of glycolysis in muscle, brain, and possibly other tissues Regulation of acid=base balance by the kidney Ammonia, generated from glutamine within the tubule cells, acts as a third renal system to help the body cope with large acid loads. Once formed, ammonia diffuses into the tubule to combine with the protons as follows: NH3 þ Hþ ! NH 4þ The hydrolysis of glutamine to glutamate in the distal tubules of the kidney, through the activity of the glutaminase enzyme, is the primary pathway for renal ammonia production, although additional ammonia can also be produced by the further conversion of glutamate to AKG, through the activity of the enzyme glutamate dehydrogenase. Furthermore, oxidation of glutamine carbons by the kidney gives rise to bicarbonate (HCO3) ion production for release into the bloodstream, to further buffer hydrogen ion production. These processes allow the kidneys to effectively excrete excess protons, and protect the body from acidosis. Consequently, during acidosis, major changes in renal glutamine metabolism occur to support ammoniagenesis. Renal uptake of glutamine has been shown to be accelerated by activation of the Naþ-dependent membrane transport system. The rate of glutamine hydrolysis and the maximal activity of the glutaminase enzyme were also increased, such that renal glutamine consumption was increased six- to tenfold, making the kidney the major organ of glutamine utilization during acidosis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 128 29.10.2007 7:50pm Compositor Name: JGanesan 128 Amino Acids and Proteins for the Athlete: The Anabolic Edge The glutamine is produced in muscle (although brain and liver may contribute small amounts) and transported to the kidney where the two nitrogen atoms are released as ammonia. Since the glutamine release by muscle may be at the expense of alanine, the excretion of the nitrogen from amino acid metabolism in muscle is effectively switched from urea, which is produced from the metabolism of alanine in the liver, to ammonia, which is produced from glutamine metabolism in the kidney. In this way, the end product of nitrogen metabolism is put to good use by the body in achieving control of acidosis. UREA FORMATION BY THE LIVER The ammonia released during deamination is removed from the blood almost entirely by conversion into urea; two molecules of ammonia and one molecule of carbon dioxide combine in accordance with the following net reaction: 2NH3 þ CO2 ! H2 NCONH2 þ H2 O Practically all urea formed in the human body is synthesized in the liver. In pathological states such as serious liver dysfunction ammonia can accumulate in the blood and become toxic. For example, high levels of ammonia can disrupt the functioning of the CNS sometimes leading to a state called hepatic coma. The series of reactions that form urea is known as the urea cycle or the Krebs–Henseleit cycle. In short, the stages in the formation of urea involve ornithine, citrulline, and arginine. Ornithine plus CO2 and NH3 form citrulline, which combines with another molecule of NH3 to form arginine. Arginine plus water produces urea and reforms ornithine through the action of arinase. Once formed, urea diffuses from the liver cells into the body fluids and is excreted by the kidneys. The diagram below of the urea cycle offers more details. NH2 Carbamoyl | C O phosphate | OPO3−2 2ADP + Pi CPSI NH2 | (CH2)3 | CH COO− | NH3+ Ornithine | HCO3− 2ATP NH4+ + HCO3− || NH4+ OTC H3N+ Arginase | | | | | | | H2O NH2+ COO− || | C NH2 CH CH2 COO− Argininosuccinate | lyase | NH NH Arginine | | C NH (CH2)3 CH COO− (CH2)3 | | + CH COO− Argininosuccinate NH3 COO− | | NH3+ CH || Fumarate CH | COO− || NH2+ COO− | CH Aspartate | CH2 | COO− | Argininosuccinate synthetase AMP + PPi Pi | (From http:==isu.indstate.edu=mwking) NH2 | C O | NH2 Urea || ATP | | | | | NH2 O COO− || | C NH (CH2)3 CH NH3+ Citrulline Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 129 29.10.2007 7:50pm Compositor Name: JGanesan 129 Energy Metabolism HIGH LEVELS OF PROTEIN INTAKE AND AMMONIA With increasing protein intake there is an increased oxidation of amino acids and subsequently an increased production of ammonia. Animals can adapt to a high-protein diet by upregulating urea synthesis and excretion, and can also by upregulating amino acid metabolizing enzymes such as alanine and aspartate aminotransferases, glutamate dehydrogenase, and argininosuccinate synthetase,100 and increase mitochondrial glutamine hydrolysis in hepatocytes.101 In addition, the metabolism of increased dietary protein can be modulated at the gastrointestinal level, for example, with decreased gastric emptying in order to decrease the rate of absorption and thus allowing the liver to better deal with the amino acid load.102–104 But even with all these mechanisms in play it is possible that high dietary protein can lead to hyperammonemia, perhaps secondary to the limited capacity of liver to form urea, and the toxic effects. Unfortunately there is very little documentation on the effects of the chronic intake of large amounts of dietary protein on normal people. One phenomenon known as ‘‘rabbit starvation’’ seems to occur in diets that are very high in protein and very low in fat and carbohydrates such as the consumption of very lean rabbit meat.105 I believe that the basis behind the toxicity associated with eating just lean meat is hyperammonemia, which occurs secondary to the large amount of protein needed in an attempt to derive all of one’s energy from just protein. This is accentuated by the fact that the amount of useable energy derived from protein is less than that derived from carbohydrates and fats. Using amino acids as a primary energy source is less efficient than using either carbohydrates or fats. That is because the deamination and formation of urea, as well as the oxidation of amino acids and conversion into glucose, require energy and this decreases the net amount of energy obtainable from amino acids. As such, more protein needs to be metabolized to meet the energy requirements of the body and this potentially leads to even higher levels of systemic ammonia. It has been shown that as the protein content of meals was increased, the rate of urea synthesis maxes out and beyond this point ammonia levels increase. In one study the maximum rate corresponded to a protein intake of just over 3 g=kg of body weight or approximately 40% of dietary energy.106 However, anecdotally, higher levels than this have not caused any adverse effects, for example, the Inuit’s ancestral diet consisted of fat and protein with protein levels approximating 50% of the energy intake. It is possible that with time an upregulation of hepatic enzymes would make it possible to subsist without adverse effects on a diet in which protein makes up more than 50% of total energy intake. It has been demonstrated that when protein intake is either decreased or increased urinary nitrogen output is adjusted to match protein intake.107 In one study measurements were made, at 6 h intervals, of urinary nitrogen output and of the activity of some hepatic enzymes in the rat during adaptation from one level of dietary protein to another. The enzymes measured were arginase, argininosuccinate lyase, argininosuccinate synthetase, glutamate dehydrogenase, and alanine and aspartate aminotransferases.108 When the dietary protein content was reduced from 135 to 45 g casein=kg, the urinary nitrogen output and the activities of the hepatic enzymes reached their new steady-state levels in 30 h. The reverse adaptation, from 45 to 135 g casein=kg, was also complete in 30 h. The rate of change of enzyme activity and the final activity as percentage of initial activity were very similar for all six enzymes, suggesting a common control mechanism. There was no simple relationship between the activity of the urea cycle enzymes and the amount of nitrogen excreted. When an equal amount of gelatin was substituted for casein, the nitrogen output was doubled but there was no change in the activity of the liver enzymes. The authors concluded that the results suggest that the activity of the urea-cycle enzymes depends in part on the amount of nitrogen available for excretion after the demands for synthesis have been met. The enzymes, however, appear to be present in excess so that an increased nitrogen load was not necessarily accompanied by an increase in enzyme activity. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 130 29.10.2007 7:50pm Compositor Name: JGanesan 130 Amino Acids and Proteins for the Athlete: The Anabolic Edge LOW-CARBOHYDRATE, HIGH-PROTEIN DIETS AND ENERGY METABOLISM The macronutrient mix of low-carbohydrate diets takes the body along different energy metabolic pathways than the more traditional higher carbohydrate diets. Following a low-carbohydrate diet shifts the body energy metabolism to the use of more fat and protein, both as immediate substrates and for the formation of ketones (in the case of both fats and amino acids) and glucose (in the case of amino acids). This shift changes the dynamics of energy metabolism to the extent where dietary carbohydrates no longer make up an important energy substrate. Instead fat oxidation is increased, glycogen spared, and amino acids are used to provide carbon skeletons for gluconeogenesis which in turn is used to supply baseline glucose and glycogen levels, which in turn are used for glycolysis as needed. Two pivotal enzymes in energy metabolism are pyruvate dehydrogenase (PDH), which decarboxylates pyruvate to form the two-carbon unit acetyl-CoA, and PC that carboxylates pyruvate to form oxaloacetate. Both enzymes are intimately involved in carbohydrate oxidation and help to control the degree of involvement of fatty acids and amino acids in energy metabolism. With low-dietary carbohydrates, PDH is downregulated so that less pyruvate is converted to acetyl-CoA, and subsequently the formation of acetyl-CoA from fatty acid oxidation increases proportionally. At the same time, because glucose cannot be formed from fat (except for the case of odd-chain fatty acids where the remaining three-carbon units can enter the TCA cycle and be used for gluconeogenesis) more amino acid carbon skeletons are involved in the formation of glucose, pyruvate, and TCAIs. METABOLIC ADVANTAGE OF A HIGH-PROTEIN, LOW-CARBOHYDRATE DIET The metabolic advantage in low-carbohydrate diets is that it is possible to achieve a greater weight loss and improved body composition compared to isocaloric diets containing various different macronutrient contents such as high carbohydrates, whether simple or complex carbohydrates are used, and low fat diets. DIETARY CALORIES FROM MACRONUTRIENTS It is widely held that a calorie is a calorie and by this it is usually meant that two isocaloric diets lead to the same weight loss. A calorie is a measure of heat energy and when talking about food it represents the total amount of energy stored in food. Used in this way all calories are equal, whether from fat, protein, or carbohydrates. However, the idea that a dietary calorie is a calorie as far as how it influences useable and storable energy, body weight, and composition under all conditions is simplistic at best. Many factors influence how much energy is actually derived from dietary macronutrient intake. Calories can be ‘‘wasted’’ in many ways. Decreased absorption from the GI tract and increased excretion are two obvious ways. An increase in thermogenesis (thermic effect of feeding) and energy expenditure will also waste calories. In the case of thermogenesis, or the heat generated in processing food, the thermic effects of nutrients is approximately 2%–3% for lipids, 6%–8% for carbohydrates, and 25%–30% for proteins.109 The energy costs of protein metabolism in themselves are almost enough to explain the metabolic advantage of low-carbohydrate, high-protein diets. But there is more involved. For example, it has been shown that increasing dietary protein increases fat oxidation.110 Also substituting other foods for high-carbohydrate foods also has effects that are conducive to weight loss. For example, one study found that compared to an isocaloric, equal-weight bagel-based breakfast, an egg-breakfast induced greater satiety and significantly reduced short-term food intake.111 The calorie cost in the use of the various macronutrients for energy also differs, with protein being the least efficient. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 131 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism 131 Through the interaction of both cytoplasmic and mitochondrial pathways it is possible to store both carbohydrates and protein as body fat. In the case of protein it involves both the glucogenic and ketogenic amino acids. The ketogenic influence on body fat is obvious since ketones are readily metabolized to two-carbon units and can be directly used for lipogenesis. The glucogenic amino acids can enter the TCA cycle as intermediates, and either through a short or long pathway end up as two-carbon units that can be exported to the cytoplasm for lipogenesis. Low-carbohydrate and higher-protein diets do more than increase weight loss. It has also been shown that low-carbohydrate, high-protein diets favorably affect body mass and composition112 and that these changes are independent of energy intake.113 But this is nothing new. Prior research also found that a low-carbohydrate diet results in a significant fat loss and an increased retention of muscle mass, either alone or in comparison to a high-carbohydrate diet. For example, in 1971 a group of researchers looked at the effects of three diets that had the same calorie and protein levels, but varying fat and carbohydrate levels.114 They found that as the carbohydrates in the diets went down, there was an increased weight and fat loss. In other words, the men who were on the lower-carbohydrate diets lost the most weight and body fat. In 1998, another study, this time involving obese teenagers, came up with similar results.115 After 8 weeks on a low-carbohydrate diet the teens not only lost significant amounts of weight and body fat, but they even managed to increase their lean body mass. In the study, a 6-week carbohydrate-restricted diet resulted in a favorable response in body composition (decreased fat mass and increased lean body mass) in normal weight men. The results of this study indicate that a low-carbohydrate diet mobilizes and burns up body fat more than a highcarbohydrate diet, while at the same time preserving muscle mass. Insulin, by varying the amount of fat and carbohydrate storage, can also make the body more efficient in the use of dietary calories. For example, decreased insulin levels, increased insulin sensitivity, and even lack of an insulin receptor in fat tissue lead to increases in energy expenditure and helps to protect against obesity even in obesiogenic environments.116 Also calories from different macronutrient mixes can affect appetite, satiety, compliance, shortand long-term compensatory responses, and changes in the oxidation of other substrates, and thus make a difference as far as weight loss and body composition. For example, a recent study found that fat mass status and the macronutrient composition of an acute dietary intake influence substrate oxidation rates.110 This study found that the intake of a highprotein, lower-carbohydrate single meal improved postprandial lipid oxidation in obese women and produced an increased thermic response. These responses were due to elevated insulin levels that occur with higher-carbohydrate meals as well as the increased energy needs associated with the higher-protein meals. The enhanced weight loss on protein-enriched diets as compared to balanced diets has been often assigned to a greater food-derived thermogenesis, an effect generally attributed to the metabolic costs of peptide bond synthesis and breakdown, urogenesis, and gluconeogenesis.117 LOW-CARBOHYDRATE CONTROVERSY There has been a lot of controversy about both the effectiveness and safety of lower-carbohydrate diets. New studies from leading institutions including Duke and Harvard universities have shown that low-carbohydrate diets are safe, healthy, lead to more permanent fat and weight loss, and have shown improvements in the dieters’ blood lipid and cholesterol levels. Several studies have shown that low-carbohydrate diets are more effective for weight and fat loss than the high-carbohydrate diets. The results of a study published in July 2002 showed that the long-term use of a low-carbohydrate diet resulted in increased weight and fat loss, and a dramatic improvement in the lipid profile (decreased cholesterol, triglycerides, and LDL, and increased HDL levels).118 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 132 29.10.2007 7:50pm Compositor Name: JGanesan 132 Amino Acids and Proteins for the Athlete: The Anabolic Edge Two studies published in 2003 in the New England Journal of Medicine found that people on the high-protein, high-fat, low-carbohydrate diet lose twice as much weight over 6 months as those on the standard low-fat diet recommended by most major health organizations.119,120 In both studies, the low-carbohydrate dieters generally had better levels of ‘‘good’’ cholesterol and triglycerides, or fats in the blood. There was no difference in ‘‘bad’’ cholesterol or blood pressure. The 132 men and women in the study conducted by the Veterans Affairs Department (Foster et al.) started out weighing an average of 286 lb (129.7 kg). After 6 months, those on the low-carbohydrate diet had lost an average of 12.8 lb (5.7 kg); those on the low-fat diet had lost 4.2 lb (1.8 kg). The other study involved 63 participants who weighed an average of 217 lb (98.4 kg) at the start. After 6 months, the low-carbohydrate group lost 15.4 lb (7 kg), the group on the standard diet 7 lb (3.2 kg). In a follow-up to this study, the authors found that after 1 year there were several favorable metabolic responses to the low-carbohydrate diet.121 Another scientific study published in the same year compared the effects of a low-carbohydrate diet with a low-fat control diet on weight loss and commonly studied cardiovascular risk factors.122 In this study, healthy obese women on the low-carbohydrate diet lost 8.5 kg, more than twice the amount of weight lost by women on the control diet, over a 6-month period. Loss of fat mass was also significantly greater in the low-carbohydrate group. In a follow-up study the authors concluded that short-term weight loss is greater in obese women on a low-carbohydrate diet than in those on a low-fat diet even when reported food intake is similar.123 The authors did not find an explanation for these results since there were no measurable changes between the dieters. Another study published in 2004 found that not only was weight loss greater but serum triglyceride levels decreased more, and high-density lipoprotein cholesterol level increased more with the low-carbohydrate diet than with the low-fat diet.124 In the latest study researchers from the Harvard School of Public Health, after analyzing data collected over 20 years from more than 82,000 women participating in the nurses’ health study, concluded that low-carbohydrate diets do not seem be linked to a higher risk of heart disease in women.125 CONCLUSIONS AND RECOMMENDATIONS Both endurance and power athletes need more protein than the recommended dietary allowances (RDAs) in order to maximize body composition and performance. How much more is still under debate, as is the upper tolerable level. From the available research, and from my clinical and personal experience, athletes will benefit by taking in a minimum of twice the present RDA for protein, and for those wishing to gain significant muscle mass, up to four times the RDAs. The contribution that protein makes to energy metabolism depends on several variables, including the state of the organism, the energy, and macronutrient content of the diet, and level of physical activity. For those following a low-carbohydrate diet or on an energy-restricted diet, it is important to keep dietary protein levels relatively high as this will increase weight loss as well as helping minimize the loss of muscle mass and increasing the loss of body fat. In my view the contribution that protein makes to energy metabolism, including both the complete oxidation of the carbon skeletons, directly or indirectly by the various transformations (gluconeogenesis and glycolytic-TCA cycle interactions) is underappreciated. Obviously more research needs to be done in humans in several areas including determining: . . . . Amount of protein carbon that actually contributes to energy metabolism both at rest and during exercise Adaptive responses to acute and chronic increased dietary protein intake Response to protein intake when taken alone and with other macro- and micronutrients Response to dose and timing of protein intake during the day, and in and around training Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C003 Final Proof page 133 29.10.2007 7:50pm Compositor Name: JGanesan Energy Metabolism . . . . . . . . 133 Tolerable upper intake levels both for proteins as a whole and for individual amino acids If and how the high intake of protein from whole foods presents any risks, including investigating rabbit starvation where the primary food eaten is lean meat If and how the use of protein (both high and low quality) and amino acid supplements presents any risks, including the effects of fast (e.g., individual amino acids, hydrolysates, and whey protein) and slow proteins (e.g., casein) on protein dynamics (oxidation, gluconeogenesis, ammonia excretion, etc.) in the body Effects of exercise on substrate utilization and the roles of various energy depots (liver glycogen, muscle glycogen, adipose triacylglycerol, intramuscular triacylglycerol) in exercise and recovery Effects of high-protein diets, with or without restrictions on the other two macronutrients, on health, body composition, and athletic performance Alternating a low-carbohydrate, high-protein phase, with a higher-carbohydrate phase (phase shift diet) will result in increased performance for both strength and endurance athletes secondary to an improvement in protein and energy metabolism and also to the increased availability of both intramyocellular lipids and glycogen Effects of specific amino acids on energy metabolism, for example, their anaplerotic usefulness and subsequently their effects on both aerobic and anaerobic energy systems, including their contribution to and their effects on carbohydrate and fat metabolism Mechanisms by which acute and chronic physical activity alter substrate utilization and subsequently affect athletic performance and body composition REFERENCES 1. 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Krieger, J.W., Sitren, H.S., Daniels, M.J., and Langkamp-Henken, B., Effects of variation in protein and carbohydrate intake on body mass and composition during energy restriction: A meta-regression, Am. J. Clin. Nutr., 83, 260–274, 2006. 114. Young, C.M., Scanlan, S.S., Im, H.S., and Lutwak, L., Effect on body composition and other parameters in obese young men of carbohydrate level of reduction diet, Am. J. Clin. Nutr., 24, 290–296, 1971. 115. Willi, S.M., Oexmann, M.J., Wright, N.M., Collop, N.A., and Key, L.L. Jr., The effects of a high-protein, low-fat, ketogenic diet on adolescents with morbid obesity: Body composition, blood chemistries, and sleep abnormalities, Pediatrics., 101, 61–67, 1998. 116. Manninen, A.H., Metabolic advantage of low-carbohydrate diets: A calorie is still not a calorie, Am. J. Clin. Nutr., 83(6), 1442–1443, 2006. 117. Volpi, E., Lucidi, P., Cruciani, G., Monacchia, F., Reboldi, G., Brunetti, P., Bolli, G.B., and De Feo, P., Contribution of amino acids and insulin to protein anabolism during meal absorption, Diabetes, 45, 1245–1252, 1996. 118. Westman, E.C., Yancy, W.S., Edman, J.S., Tomlin, K.F., and Perkins, C.E., Effect of 6-month adherence to a very low carbohydrate diet program, Am. J. Med., 113(1), 30–36, 2002. 119. Foster, G.D., Wyatt, H.R., Hill, J.O., McGuckin, B.G., Brill, C., Mohammed, B.S., Szapary, P.O., Rader, D.J., Edman, J.S., and Klein, S., A randomized trial of a low-carbohydrate diet for obesity, N. Engl. J. Med., 348(21), 2082–2090, 2003. 120. Samaha, F.F., Iqbal, N., Seshadri, P., Chicano, K.L., Daily, D.A., McGrory, J., Williams, T., Williams, M., Gracely, E.J., and Stern, L., A low-carbohydrate as compared with a low-fat diet in severe obesity, N. Engl. J. Med., 348(21), 2074–2081, 2003. 121. Stern, L., Iqbal, N., Seshadri, P., Chicano, K.L., Daily, D.A., McGrory, J., Williams, M., Gracely, E.J., and Samaha, F.F., The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: One-year follow-up of a randomized trial, Ann. Intern. Med., 140(10), 778–785, 2004. 122. Brehm, B.J., Seeley, R.J., Daniels, S.R., and D’Alessio, D.A., A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women, J. Clin. Endocrinol. Metab., 88, 1617–1623, 2003. 123. Brehm, B.J., Spang, S.E., Lattin, B.L., Seeley, R.J., Daniels, S.R., and D’Alessio, D.A., The role of energy expenditure in the differential weight loss in obese women on low-fat and low-carbohydrate diets, J. Clin. Endocrinol. Metab., 90(3), 1475–1482, 2005. 124. Yancy, W.S. Jr., Olsen, M.K., Guyton, J.R., Bakst, R.P., and Westman, E.C., A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: A randomized, controlled trial, Ann. Intern. Med., 140(10), 769–777, 2004. 125. Halton, T.L., Willett, W.C., Liu, S., Manson, J.E., Albert, C.M., Rexrode, K., and Hu, F.B., Low-carbohydrate-diet score and the risk of coronary heart disease in women, N. Engl. J. Med., 355, 1991–2002, 2006. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 139 7.11.2007 5:32pm Compositor Name: BMani Protein 4 Dietary and Amino Acids EFFECTS OF PROTEIN ON DIETARY INTAKE AND APPETITE Studies have been rather inconclusive regarding the effect of protein on dietary intake and appetite. Until recently the effect of dietary protein on the level of free essential amino acids in brain and on serotonin levels has been the focus of these studies.1–7 In a review of these studies8 the authors concluded that analysis of evidence of associations among dietary protein content, brain amino acid and serotonin concentrations, and protein self-selection by rats suggests that: 1. Protein intake is not regulated precisely, although rats will select between low- and highprotein diets to obtain an adequate, but not excessive, amount of protein. 2. Associations between brain-serotonin concentration and protein intake are weak, although consumption of single meals of protein-deficient diets will elevate brain-serotonin concentration. 3. Nature of signals that drive rats to avoid diets containing inadequate or excessive amounts of protein remains obscure. 4. Whole brain amino acid and serotonin concentrations are quite stable over the usual range of protein intakes, owing to competition among amino acids for uptake across the blood– brain barrier and effective metabolic regulation of blood amino acid concentrations. 5. Protein intake and preference are not in themselves regulated, but what appears to be regulation of intake and preference is a reflection of the responses of systems for control of plasma amino acid concentrations. 6. Relative stability of the average protein intake of groups of self-selecting rats (which gives the appearance of regulation) results from averaging the variable behavioral responses— learned aversions and preferences—of rats to the variety of sensory cues arising from diets that differ in protein content. Studies in humans have shown that high-protein versions of the same food systems show more sensory-specific satiety and decrease hunger more than lower-protein versions.9 CLASSIFICATION OF PROTEINS Depending on the composition, proteins may be divided into two main categories: (1) simple and (2) conjugated. These two main categories can be further subdivided. The following text lists the various categories of proteins along with their distinguishing characteristics. SIMPLE PROTEINS Simple proteins consist of only amino acids or their derivatives and include: 139 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 140 43803_C004 Final Proof page 140 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge Albuminoids (scleroproteins): These proteins are insoluble in water, highly resistant to enzymatic digestion, and some become gelatinous upon boiling in water or dilute acids or bases. Examples include collagen, elastin, and keratin. They are found mainly in supporting tissues and are sometimes referred to as fibrous protein. Albumins: These proteins are readily soluble in water and coagulate upon heating. They are present in egg, milk, and serum. Globulins: These proteins have low solubility in water but their solubility increases with the addition of neutral salts. They coagulate upon heating. They are abundant in nature. Examples are serum globulins, muscle globulins, and numerous plant globulins. Glutelins: These proteins are insoluble in water but soluble in dilute acids or bases. They are abundant in cereal grains. Examples are wheat glutenin, rice oryzenin, and barley hordenin. Prolamines: These proteins are insoluble in water, absolute alcohol or neutral solvents, but soluble in 80% ethanol. On hydrolysis they give large quantities of proline and ammonia. Examples are zein in corn and gliadin in wheat. Wheat, because of its gluten content, occupies a unique position in food. Gluten is a mixture of two proteins, gliadin and glutenin; these two, when mixed with water, give the characteristic stickiness which enables the molecules, present in wheat flour, to be bound together by moderate heat with the production of dough from which bread is baked. Rye has a small content of gluten and so (with difficulty) can be made into a loaf. Oats, barley, maize, millets, and rice cannot be made into bread. The grains may be eaten after boiling or their flour made into porridge, bannocks, tortillas, etc. Protamines and histones: These are basic polypeptides, soluble in water, but not coagulated by heat. Large amounts are found in male fish roes and also in cellular nucleoproteins. Protamines have a practical use in the commercial production of delayed-action insulins. CONJUGATED PROTEINS Conjugated proteins are joined to various nonprotein substances and include: Chromoproteins: Combination of a protein and a pigmented (colored) substance. A common example is hemoglobin-hematin and protein. Lecithoproteins: Combination of protein and lecithin. They are found in fiber of clotted blood and vitellin of egg. Lipoproteins: Water-soluble combination of fat and protein. They act as a vehicle for the transport of fat in the blood. They all contain triglycerides, cholesterol, and phospholipids in varying proportions. For example, low-density lipoprotein (LDL; ‘‘bad cholesterol’’) is the major carrier of cholesterol in the bloodstream. Metalloproteins: Proteins that are complexed with metals. One example is transferrin, a metalloprotein that can bind with copper, iron, and zinc. Various enzymes contain minerals. Mucoproteins or glycoproteins: Contain carbohydrates such as mannose and galactose. Examples are mucin from the mucus secretion, and some hormones such as the human chorionic gonadotropin found in the urine of pregnant females. Nucleoproteins: Combination of simple proteins and nucleic acids. Present in germs of seeds and glandular tissue. Phosphoproteins: Compounds containing protein and phosphorus in a form other than phospholipid or nucleic acid. Examples are casein in milk and ovovitellin in eggs. Possibly a third category, derived proteins, may be added to the two discussed earlier. Essentially derived proteins are the products of digestion. They are fragments of various sizes. From largest to smallest, in terms of the number of amino acids, derived proteins are proteoses, peptones, polypeptides, and peptides. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 141 7.11.2007 5:32pm Compositor Name: BMani 141 Proteins may also be classified according to their structure, an important property for their ability to function biologically. Some proteins are round to ellipsoidal and are called globular proteins. These are found in the tissue fluids of animals and plants in which they readily disperse, either in true solutions or colloidal suspension. Enzymes, protein hormones, hemoglobin, and globulins, including caseinogen in milk, albumin in egg white, and albumins and globulins of blood, are all globular proteins. These proteins, while not easily digestible, are high-quality proteins and contain a good proportion of the essential amino acids. Other proteins form long chains bound together in a parallel fashion, and are called fibrous proteins. These consist of long coiled or folded chains of amino acids bound together by peptide linkages. They are the proteins of connective tissue and elastic tissue including collagen, elastin, and keratin, and are found in the protective and supportive tissues of animals such as skin, hair, and tendons. Many of the fibrous proteins are not very digestible, and if they are, they are usually poorquality proteins. While animal proteins can be divided into fibrous and globular, plant proteins are not so easily classified but broadly speaking most are glutelins or prolamines. FUNCTIONS OF PROTEINS Each distinctive protein performs a specific function in the body. In general, proteins may be classified as performing four basic functions. GROWTH The formation of new tissues requires the synthesis of protein. Such conditions occur during periods of growth—from infancy to adulthood, in pregnancy and lactation (each liter of human milk contains about 12 g of protein), and during adaptation to exercise. Wound healing requires the synthesis of new proteins, as do burns, fractures, and hemorrhage. MAINTENANCE Protein in all humans, regardless of age, is continually being degraded and resynthesized—a process called protein turnover. Blood cells must be replaced every 120 days, while cells which line the intestine are renewed every 1.5 days. Moreover, as discussed in detail later on, protein is lost from the body in the perspiration, hair, fingernails, skin, urine, and feces. This constant turnover and loss of protein requires that the body has a pool of amino acids upon which it can draw to replace losses. This pool is replenished by dietary protein. REGULATORY Proteins in the cells and body fluids provide regulatory functions. Hemoglobin, an iron-bearing protein carries oxygen to the tissues. Water balance and osmotic pressure are regulated by the proteins of the blood plasma. Furthermore, the proteins act as buffers controlling the acid–base balance of the body, especially in the intracellular fluids. Many of the hormones which regulate body processes are proteins; for example, insulin, glucagon, GH, and the digestive-tract hormones. Enzymes, the catalysts for almost all chemical reactions of the body, are proteins. Antibodies, which protect the body from infectious diseases, are proteins. The blood-clotting mechanism of the body is dependent upon proteins. Certain amino acids derived from dietary protein or synthesized in the body also perform important regulatory functions. For example, arginine participates in the formation of the final metabolic product of nitrogen metabolism—urea—in the liver and glycine participates in the synthesis of purines, porphyrins, creatine, and glyoxylic acid. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 142 43803_C004 Final Proof page 142 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge ENERGY The catabolism of amino acids yields energy, about 4 kcal=g of protein. In order for amino acids to be used for energy, the amino group (NH2) must first be removed by a process known as deamination (see later). The carbon skeleton remaining following deamination—a-keto acid residues—may be either converted to glucose (glycogenic pathway) or metabolized in fat pathways (ketogenic pathway). When energy consumption in the form of carbohydrates and fats is low, the energy needs of the body take priority and dietary and tissue proteins will be utilized at the expense of the building or repair processes of the body. In the body, amino acids may primarily be used for protein synthesis or energy production, depending upon: 1. 2. 3. 4. 5. Protein quality Caloric level of the diet Stage of development, including growth, pregnancy, and lactation Prior nutritional status Stress factors such as fever, injury, and immobilization COAGULATION AND DENATURATION Many water-soluble proteins coagulate when subjected to heat at about 1008C or above, as occurs in the normal process of cooking. A familiar example would be the change of the white of an egg on boiling. Once a protein has undergone this change, its specific properties (e.g., enzymic, hormonal, or immunological) are permanently destroyed. Proteins also undergo denaturation in which they become less soluble in water. This occurs when they are exposed to a variety of agents such as moderate heat, ultraviolet light, or alcohol and mild acids or alkalis. Proteins are most easily denatured at their isoelectric point, that is, the particular pH at which the electric charges on their NH2 and COOH groups precisely balance; this varies from one protein to another. The exact nature of the denaturation process is obscure although it involves some disorganization of the specific arrangement of the component amino acids. To a certain extent, it is reversible once normal conditions are restored, but most enzymes and allergens lose their specific properties once denatured. PROTEIN DIGESTION, ABSORPTION, AND METABOLISM Protein digestion is the mechanical, chemical, and enzymatic breakdown of the protein in food into smaller units. Digestion involves several stages including the mechanical extraction of the protein from the food, denaturation of the protein, and hydrolysis of the peptide bonds. Protein is mechanically extracted from the food in the process of mastication and by the action of the stomach. The low pH of the stomach plays a role in denaturation of the extracted protein, thus making it more accessible to the proteolytic enzymes of the gut. The initial increase in gastric volume and delay in gastric emptying that occurs with macronutrient intake10 helps to ensure effective extraction and denaturation of the protein prior to its entry in the duodenum. In the small intestine, enzymes from the pancreas and the small intestine split dietary protein into peptides (small groups of bonded amino acids) and individual amino acids. Protein must be digested to amino acids or di- and tripeptides before absorption can occur, although at times larger peptides can be absorbed. In the first few days of life and in certain disease states, polypeptides and undigested protein may be absorbed.11 Measurements of the net absorption of amino acids after a meal containing 15 g of milk protein show that it is 70%–80% complete in 3 h. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 143 7.11.2007 5:32pm Compositor Name: BMani 143 There is some evidence that hydrolyzed-protein fragments (i.e., peptides) cross the small intestine and reach peripheral tissue via the systemic circulation.12 Dietary peptides can have specific actions locally, on the GI tract, or at more distant sites having an influence on physiological processes. These bioactive peptides can alter cellular metabolism and may act as vasoregulators, growth factors, releasing hormones, or neurotransmitters. The concept of dietary bioactive peptides offers an explanation for varying effects of diet on physiologic responses. Current experimental evidence indicates that diets that possess the capability of producing luminal peptides are superior to diets lacking this capacity. More research needs to be done to determine the anabolic and ergogenic effects of dietary bioactive peptides before they can be used effectively as nutritional supplements. For the early stages of the digestion of protein, four types of enzymes are important, pepsins (secreted by serous cells in the gastric gland of the stomach), trypsins, elastases, and the chymotrypsins (all secreted by the acinar cells of the pancreas). The product of the action of these proteolytic enzymes is a series of peptides of various sizes. These are degraded further by the action of several peptidases (exopeptidases) that remove terminal amino acids, including carboxypeptidases A and B which hydrolyze amino acids sequentially from the carboxyl end of peptides. Aminopeptidases, which are secreted by the absorptive cells of the small intestine, hydrolyze amino acids sequentially from the amino end of peptides. In addition, dipeptidases, which are structurally associated with the brush border of the absorptive cells, hydrolyze dipeptides into their component amino acids. The extent to which these act on substrates in the lumen, within the membrane of the cells or within the cell itself, is not known. Protein digestion in the intestine results in the hydrolysis of both ingested and endogenous proteins, in the form of digestive enzymes, other secreted proteins, and desquamated epithelial cells. Between 50 and 70 g of endogenous protein is digested daily almost equal to the average daily amount ingested. The final stages of the hydrolysis of peptides to dipeptides and amino acids, and their absorption, occur in the jejunum and ileum. The transport of amino acids and di- and tripeptides from the lumen into the cell is an active process (more on this topic will be covered later when discussing protein and amino acid supplements). In general, proteins from animals are more completely and rapidly absorbed than vegetable proteins, possibly because of the cellulose covering in plant cells and the increased fiber associated with vegetable proteins. Until the early 1950s, the absorption products of protein digestion were widely thought to be free amino acids for which there were several discernable transport mechanisms. There is now good evidence that small peptides (mainly di- and tripeptides), but to a much lesser extent, and up to several amino acid residues are absorbed from the gut lumen at least as rapidly as free amino acids.13–17 The mammalian Hþ-peptide symporter PEPT1 expressed in intestinal epithelial cells mediates the absorption of small peptides following the digestion of dietary proteins.18,19 However, only a small proportion of these peptides taken up by the enterocytes appear in the systemic circulation. Many conditions, including dietary protein levels as well as certain free amino acids and peptides, are able to change PEPT1 expression in the small intestine and its maximal transport activity.20–27 In rats fed increasing quantities of dietary protein for 3 days, the abundance of PEPT1 in the brush border membrane increased almost proportionally to the protein intake followed by a concomitant increase in peptide transport activity.28 There is also a growing scientific interest in intestinal peptide transport in pharmacology, with PEPT1 as a prime target for efficient oral delivery of drugs.29,30 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 144 43803_C004 Final Proof page 144 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge REQUIREMENT FOR DIETARY PROTEIN—AMINO ACID NEEDS The requirement for dietary protein consists of two components: 1. Nutritionally essential amino acids (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) under all conditions, and for conditionally essential amino acids (arginine, cysteine, glutamine, glycine, histidine, proline, taurine, and tyrosine) under specific physiological and pathological conditions. 2. Nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine, and serine) and other physiologically important nitrogen-containing compounds such as nucleic acids, creatine, and porphyrins. With respect to the first component, it is usually accepted that the nutritive values of various foodprotein sources are to a large extent determined by the concentration and availability of the individual indispensable amino acids. Hence, the efficiency with which a given source of food protein is utilized in support of an adequate state of nutritional health depends both on the physiological requirements for the indispensable amino acids and total nitrogen and on the concentration of specific amino acids in the source of interest. QUALITY OF PROTEINS All proteins are made up of varying numbers of amino acids attached together in a specific sequence and having a specific architecture. The sequence of the amino acid and form of the protein differentiates one protein from another, and give the protein special physiological and biological properties. The quantity and quality of protein in the diet is important in determining the effects on protein metabolism. Increasing protein intake leads to increased amino acid levels systemically, which in turn leads to increased protein synthesis, decreased endogenous protein breakdown, and increased amino acid oxidation.31 The quality of dietary protein is also important in determining changes in protein metabolism. Certain proteins are considered to be biologically more effective. An example would be the difference between soy and milk proteins. Consumption of a soy-protein meal, as compared to casein, results in lower protein synthesis and higher oxidation of amino acids, as seen by the greater urea production than consumption of a casein-protein meal.32 Another study found that the ingestion of soy protein resulted in a lower whole-body retention of dietary nitrogen than did milk protein.33 In a recent review, milk-protein supplementation was found to be more effective than soyprotein supplementation in stimulating amino acid uptake and net protein deposition in skeletal muscle after resistance exercise.34 In another review, milk proteins were found to be more effective in stimulating amino acid uptake and net protein deposition in skeletal muscle after resistance exercise than hydrolyzed soy proteins.35 The findings revealed that even when balanced quantities of total protein and energy are consumed, milk proteins are more effective in stimulating amino acid uptake and net protein deposition in skeletal muscle after resistance exercise than are hydrolyzed soy proteins. Importantly, the finding of increased amino acid uptake was independent of the differences in amino acid composition of the two proteins. The authors proposed that the improved net protein deposition with milk-protein consumption is also not due to differences in amino acid composition, but is due to a different pattern of amino acid delivery associated with milk versus hydrolyzed soy proteins. These findings suggest that, in healthy human subjects, casein has a higher biological value (BV) than soy protein. It has been shown that proteins found in whole foods may impact on protein metabolism differently because of the presence of other macronutrients. In a recent study, milk ingestion was found to stimulate net uptake of phenylalanine and threonine representing net muscle protein Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 145 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 145 synthesis following resistance exercise.34 These results of the study suggest that whole milk may have increased utilization of available amino acids for protein synthesis. This is likely because carbohydrates and fats alter the rate of absorption of the amino acids, and the hormonal response, and subsequently the fate of the amino acids absorbed. For example, the addition of sucrose to milk proteins increased whole-body nitrogen retention, but primarily in splanchnic tissues, whereas addition of fat to milk proteins resulted in greater dietary nitrogen retention in peripheral tissues.36 The protein source seems also to be important for weight loss and body composition. A number of studies in the last decade, in both animals37,38 and humans,39–41 have suggested that supplementation with soy protein was effective and better than other proteins for weight loss. However, recently that notion has changed. A recent study has found that soy does not enhance weight loss.42 And another recent study evaluating the weight-loss efficacy, body-composition changes, and other effects of either three soy shakes or three casein shakes daily as part of a 16-week, energy-restricted diet for obese women found no difference between the two proteins.43 SLOW AND FAST DIETARY PROTEINS We all know that there are differences in carbohydrates—high glycemic, low glycemic, simple sugars, starches, etc. And we know that different carbohydrates are absorbed in the gut and appear in the blood at different rates depending on various factors—for example, simple sugars are absorbed quickly and more complex ones, depending on how quickly they can be broken down, are absorbed more slowly. This makes up the basis for the glycemic index of not only foods but also whole meals since the presence of protein and fat with the carbohydrates usually slows down the absorption over the whole digestive process. Fast and slow carbohydrates have different metabolic effects on the hormones and on various metabolic processes. Now we also have slow (e.g., whey and soy) and fast (e.g., casein) dietary proteins. The speed of absorption of dietary amino acids by the gut varies according to the type of ingested dietary protein and the presence of other macronutrients. The speed of absorption can affect postprandial (after meals) protein synthesis, breakdown, and deposition.44,45 It has been shown that the postprandial amino acid levels differ a lot depending on the mode of administration of a dietary protein; a single protein meal results in an acute but transient peak of amino acids whereas the same amount of the same protein given in a continuous manner, which mimics a slow absorption, induces a smaller but prolonged increase. Since amino acids are potent modulators of protein synthesis, breakdown, and oxidation, different patterns of postprandial amino acidemia (the level of amino acids in the blood) might well result in different postprandial protein kinetics and gain. Therefore, the speed of absorption by the gut of amino acids derived from dietary proteins will have different effects on whole-body protein synthesis, breakdown, and oxidation, which in turn control protein deposition. For example, one study looked at both casein- and whey-protein absorption and the subsequent metabolic effects.46 In this study two labeled milk proteins, casein and whey protein, of different physicochemical properties were ingested as one single meal by healthy adults, and postprandial whole-body leucine kinetics was assessed. Whey protein induced a dramatic but short increase of plasma amino acids. Casein induced a prolonged plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole-body protein breakdown was inhibited by 34% after casein ingestion but not after whey-protein ingestion. Postprandial protein synthesis was stimulated by 68% with the whey-protein meal and to a lesser extent (þ31%) with the casein meal. Under the conditions of this study, that is a single protein meal with no energy added, two dietary proteins were shown to have different metabolic fates and uses. After whey-protein ingestion, the plasma appearance of dietary amino acids is fast, high, and transient. This amino acid pattern is associated with an increased protein synthesis and oxidation and no change in protein breakdown. By contrast, the plasma appearance of dietary amino acids after a casein meal is slower, Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 146 43803_C004 Final Proof page 146 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge lower, and prolonged with a different whole-body metabolic response: protein synthesis slightly increases, oxidation is moderately stimulated, but protein breakdown is markedly inhibited. This study demonstrates that dietary amino acid absorption is faster with whey protein than with casein. It is very likely that a slower gastric emptying was mostly responsible for the slower appearance of amino acids into the plasma. Indeed, casein clots in the stomach whereas whey protein is rapidly emptied from the stomach into the duodenum. The results of the study demonstrate that amino acids derived from casein are indeed slowly released from the gut and that slow and fast proteins differently modulate postprandial changes of whole-body protein synthesis, breakdown, oxidation, and deposition. After whey-protein ingestion, large amounts of dietary amino acids flood the small body pool in a short time, resulting in a dramatic increase in amino acid concentrations. This is probably responsible for the stimulation of protein synthesis. This dramatic stimulation of protein synthesis and absence of protein breakdown inhibition is quite different from the pattern observed with classic feeding studies and with the use of only one protein source. In conclusion, the study demonstrated that the speed of amino acid absorption after protein ingestion has a major impact on the postprandial metabolic response to a single protein meal. The slowly absorbed casein promotes postprandial protein deposition by an inhibition of protein breakdown without excessive increase in amino acid concentration. By contrast, a fast dietary protein stimulates protein synthesis but also oxidation. This impact of amino acid absorption speed on protein metabolism is true when proteins are given alone, but as for carbohydrate, this might be blunted in more complex meals that could affect gastric emptying (lipids) and=or insulin response (carbohydrate). In light of the fact that both hyperaminoacidemia47–49 and resistance exercise50–54 independently stimulate muscle protein synthesis, a recent study (by Wilkinson et al. 2007) looked at how different proteins differ in their ability to support muscle protein accretion.55 The study investigated the effect of oral ingestion of either fluid nonfat milk or an isonitrogenous and isoenergetic macronutrient-matched soy-protein beverage on whole-body and muscleprotein turnover after an acute bout of resistance exercise in trained men. The authors hypothesized that the ingestion of milk protein would stimulate muscle anabolism to a greater degree than would the ingestion of soy protein, because of the differences in postprandial aminoacidemia, when compared whey against casein. In this study arterial–venous amino acid balance and muscle fractional synthesis rates were measured in young men who consumed fluid milk or a soy-protein beverage in a crossover design after a bout of resistance exercise. The primary finding of the current study was that intact dietary proteins, as against portions of intact proteins such as concentrates or isolates of whey, soy, or casein, can support an anabolic environment for muscle protein accretion. Two other studies carried out to date found that the ingestion of whole proteins after resistance exercise can support positive muscle protein balance.56,57 However, the Wilkinson study was the first to show that the source of intact dietary protein (i.e., milk compared with soy) is important for determining the degree of postexercise anabolism. The study55 also found a significantly greater uptake of amino acids across the leg and a greater rate of muscle protein synthesis in the 3 h after exercise with the milk-protein consumption as compared to soy-protein ingestion. Thus milk protein promoted a more sustained net positive protein balance after resistance exercise than did soy protein. The authors concluded that since the milk and soy proteins provided equal amounts of essential amino acids, and that the level of essential amino acids drive protein synthesis,58 it is likely that differences in the delivery of and patterns of change in amino acids are responsible for the observed differences in net amino acid balance and rates of muscle protein synthesis. Because of differences in digestion rates, milk proteins may provide a slower pattern of amino acid delivery to the muscle than soy protein. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 147 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 147 Ingestion of soy protein results in a rapid rise and fall in blood amino acid concentrations, whereas milk-protein ingestion produces a more moderate rise and a sustained elevation.59 Interestingly, these increases in anabolic processes were seen without any concurrent increases in whole-body protein oxidation. Part of the explanation for this lack of increase is that the test meals consumed by participants in this study had 30% of total energy from fat, which would likely have slowed digestion and, thus, the rate of appearance of amino acids into general circulation. Also, the dose of protein used (7.5 g indispensable amino acids) did not stimulate amino acid oxidation. Previous studies that examined the effect of ingestion of similar quantities of crystalline amino acids on muscle protein turnover have shown that increases in net protein balance with the ingestion of 40 g crystalline indispensable amino acids (8.3 g leucine)60 were similar in magnitude to that seen with the ingestion of only 6 g crystalline amino acids (2.2 g leucine).61 These data suggest that, when large quantities of amino acids are ingested, amino acids are likely to be directed to deamination and oxidation. The authors of this study55 proposed that the digestion rate and, therefore, the ensuing hyperaminoacidemia that differed between the milk and soy groups after exercise is what affected the net uptake of amino acids in the exercised leg. However, regardless of their conclusions, because there are variations between the proteins, it is still possible that the differences in amino acid composition between the two proteins had some effect on protein accretion. For example, the analysis of the proteins in this study found that the content of methionine in the soy protein (1.4%) was lower than that in the milk protein (2.6%). EFFECTS OF DIETARY PROTEIN ON PROTEIN METABOLISM Protein metabolism is affected by several factors including energy, protein and macronutrient intake, and exercise.62,63 An increase in dietary protein results in an increase in nitrogen retention64 and an increase in amino acid oxidation, as well as increases in the rate of protein turnover.65,66 As mentioned earlier, the speed of absorption can also modulate protein metabolism by modulating protein synthesis, protein catabolism, and amino acid oxidation. The speed of absorption is a function of the state of the GI system (previous dietary intake and presence or absence of any pathology), the type of dietary protein (free amino acids, hydrolysates, and whey protein are rapidly absorbed in comparison to say casein—see discussion under fast and slow dietary proteins),67 and the presence of other dietary macronutrients.68 Both high-protein diet and rapid absorption of amino acids tend to increase amino acid oxidation46 as well as gluconeogenesis.69 Both processes underlie the importance of dietary protein and amino acids to overall energy metabolism. PROTEIN QUALITY—AMINO ACID REQUIREMENTS Early studies on protein quality demonstrated that some proteins supported growth and even survival better than others, and that a certain quantity of dietary protein was needed. This early work on the effects of feeding proteins of known amino acid composition, or diets containing specified pure amino acids, on the rate of growth or nitrogen balance of an experimental animal resulted in proteins being classified as complete, partially incomplete, and incomplete and also led to the concept of essential or indispensable and nonessential or dispensable amino acids. It was also found that the amino acid requirements vary according to the species and age of an animal, and are determined in the last analysis by the amino acid composition of the tissue proteins formed during growth.70 Not all proteins are of equal nutritional value; this reflects their differing amino acid content. Although most proteins contain most of the 22 or so amino acids, these are present in widely differing proportions. Complete proteins contain all the essential amino acids in sufficient amounts Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 148 43803_C004 Final Proof page 148 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge to maintain life and support growth in that animal. Partially incomplete proteins can maintain life, but cannot support growth. Incomplete proteins cannot maintain life or support growth. Since plant proteins may be deficient in one or more essential amino acids and meat contains all the essential amino acids, the chances of developing a deficiency are greater for vegetarians than for meat eaters (see later). A lack of essential amino acids in the diet results in a variety of adverse effects that depend on the degree and length of deficiency. Dairy products, meat, fish, eggs, and poultry are examples of foods that contain complete proteins. Essential amino acids are those which cannot be synthesized by the body or that cannot be synthesized at a sufficient rate to supply the normal requirements for protein biosynthesis.71 These amino acids must therefore be present in the diet. The nonessential amino acids can be synthesized at a sufficient rate (provided, of course, that the supplies of amino nitrogen and carbon precursors are adequate). Thus an amino acid is nonessential if its carbon skeleton can be formed in the body, and an amino group can be transferred to it from some available donor compound, a process called transamination (see earlier discussion). For example, the body can make as much alanine as it needs quickly and easily because the carbon chain is pyruvate, a common metabolic product to which the amino group from another common nonessential amino acid can be added. The use of the word essential, however, does not mean that the other nonessential amino acids are not equally as essential for formation of the proteins but only that the others are not essential in the diet because they can be synthesized in the body. For protein synthesis to take place, all the amino acids required must be available. If the diet lacks one or more of these essential amino acids, the body’s ability to synthesize new protein is adversely affected.72 There are no RDAs for any of the individual amino acids. Instead, since protein is made up of amino acids, RDAs are expressed in terms of amino acid patterns in protein intake. Studies have shown that the currently accepted 1985 criteria for amino acid requirement pattern are not capable of maintaining body amino acid homeostasis or balance.73,74 Other diets, however, such as the Massachusetts Institute of Technology (MIT) requirement pattern (MIT diet), or the eggprotein pattern (egg diet) are capable of maintaining amino acid homeostasis. Thus, it is hypothesized that current international estimations of essential amino acid requirements are far too low and must be modified in light of present research.75 While the presence of essential amino acids is critical to protein synthesis, there is some evidence that lack of the nonessential amino acids can result in lower plasma levels of these amino acids,76 which may ultimately compromise protein synthesis in situations where there is rapid growth. During periods of active growth, or tissue repair, more nitrogen is ingested than excreted (positive nitrogen balance). Conversely, in malnutrition or starvation, less nitrogen is ingested than excreted (negative nitrogen balance). To determine whether an amino acid is essential, it is omitted from the diet while all the other amino acids are included. If the omission results in negative nitrogen balance, the amino acid is deemed essential. In the absence of this single amino acid, the body has been unable to synthesize certain proteins so that the nitrogen that would have been used in this synthesis is excreted. By this criterion, the following amino acids are essential in humans: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. However, not all the other amino acids can be considered nonessential at all times. In some cases of accelerated growth and other conditions, such as the nutritional support of the catabolic patient,77 certain amino acids may become essential because the body cannot synthesize enough amino acid for that particular situation or the amino acid becomes rate limiting for protein synthesis. These amino acids are often referred to as conditionally essential. For example, histidine is known to be required for normal growth in a child and is necessary for proper growth in the rat. It seems likely that histidine is also essential in human adults under certain conditions and under conditions of dietary deficiency may be a limiting factor for growth.78 Taurine may also be conditionally essential. The last 20 years have seen the status of taurine change from an end product of methionine and cysteine metabolism and substance conjugated to bile acids to that of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 149 7.11.2007 5:32pm Compositor Name: BMani 149 an important, and sometimes essential, nutrient. It is now added to most synthetic human-infant formulas and pediatric-parenteral solutions throughout the world.79 Proline and glutamine are other examples of amino acids that may be conditionally essential. In one study, the authors suggest that some nonessential amino acids such as proline and glutamate-glutamine may become limiting during lactation because of their unique contributions to milk-protein synthesis.80 Moreover, there are other special considerations for certain of the nonessential amino acids. Although humans can synthesize most of the nonessential amino acids from glucose and ammonia, tyrosine synthesis requires the availability of phenylalanine, and cysteine requires the availability of methionine. Both phenylalanine and methionine are essential amino acids, and if they are available in the diet at or below minimal requirement levels, tyrosine and cysteine can become essential amino acids in that the lack of precursor amino acids decreases the ability of the body to produce these amino acids and they then become rate limiting for protein synthesis (see later). For example, in some patients with liver disease, the hepatic conversion of phenylalanine to tyrosine, and methionine to cystine, is inadequate, and unless sufficient cystine=tyrosine is administered,81 repletion of lean tissue (net protein synthesis) will be substantially limited and body function will be impaired. However, while obtaining adequate amounts of the essential amino acids is important, the proportion of nonessential amino acid nitrogen has an influence on the essential amino acid requirements.82,83 However, while increasing the amount of dietary nonessential amino acids may increase protein synthesis by increasing the level of dietary protein, it may not influence the overall requirements for the essential amino acids.84 On the other hand, if the ratio of essential amino acids to the total nitrogen in a food is too high, essential amino acids will be used as sources of nitrogen for the nonessential amino acids. Not only completeness but also digestibility are issues of concern with respect to protein quality. It is well known that not all protein sources are utilized equally well and that the bioavailability of ingested protein varies according to the source of protein. A protein is digestible if a high proportion of its amino acids reach the body’s cells so that they can synthesize the proteins they require. This indicates that the nitrogen in the protein is utilized with little waste. High-quality proteins are both complete and highly digestible, meaning that smaller quantities need be ingested than would be the case for proteins of lower quality. Egg protein is usually the standard against which other sources are often compared since it is utilized for growth in animals with a 85%–90% efficiency (BV). Plant proteins, because they are more difficult to digest and have a lower essential amino acid content are utilized for growth only 50%–80% as well as egg protein. The quality or type of protein may also affect the absorption or utilization of other nutrients. In a study, the influence of different protein sources on zinc absorption was evaluated.85 The results of this study demonstrated that zinc absorption is inhibited by certain protein sources, such as bovine serum albumin and soybean-protein isolate, while other proteins have little or no effect. VEGETARIAN DIETS The word ‘‘vegetarian’’ is derived from the Latin vegetus, meaning whole, sound, fresh, lively. The original definition of vegetarian was ‘‘with or without eggs or dairy products’’ and that definition is still used today by the Vegetarian Society. However, most vegetarians in India exclude eggs from their diet, as did those in the classical Mediterranean lands such as Pythagoras. In fact prior to 1847, nonmeat eaters were generally known as ‘‘Pythagoreans’’ or adherents of the ‘‘Pythagorean System,’’ after the ancient Greek vegetarian Pythagoras. More and more people are becoming vegetarians. For some of them becoming a vegetarian is a statement to promote animal rights or to save the environment while for others it is simply a choice they make for various other reasons including health or just fitting in with alternative lifestyles. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 150 43803_C004 Final Proof page 150 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge In general, people have distorted ideas on just who a vegetarian is. The word vegetarian sometimes evokes visions of hippies with peace signs painted on various parts of their bodies carrying signs and demonstrating against the killing of animals. That caricature, like most, is untrue. Vegetarians are people in all walks of life who, for moral and other reasons, simply do not eat red meat and, to some extent, other animal foods. TYPES OF VEGETARIANS Vegetarians generally abstain from eating red meat and usually forgo eating poultry, fish, and other seafood. Depending on the type of vegetarian, the forbidden foods may also include any food of animal origin. While there are many different kinds of vegetarians, the following two basic types are the most common: 1. Lacto–ovo vegetarian: The most common form of vegetarianism avoids consuming animal flesh but allows dairy products and eggs with a diet of vegetables, fruits, nuts, seeds, legumes, and grains. Vegetarians may also avoid milk products but consume eggs (lacto vegetarian) or vice versa (ovo vegetarian). 2. Vegan: This type of vegetarian excludes all animal products including diary, eggs, and even honey. Their diet is derived exclusively from plants and is based on grains, vegetables, legumes, fruits, seeds, and nuts. Their entire protein intake is in the form of plant protein. Some vegans also refuse to eat yeast products. Also, the strict vegan avoids any products derived from animals such as leather, wool, fur, down, silk, ivory, and pearl. Additionally, cosmetics and household items that contain animal ingredients or that are tested on animals are avoided. Dietary vegans are people who follow a vegan diet, but do not necessarily try and exclude nonfood uses of animals. While these two types are the most common, there are many other types and classifications for vegetarian eating. For example, a fruitarian is even stricter and avoids any plant products except those parts of the plant that are cast off or dropped from the plant and that do not involve the destruction of the plant itself. For example, fruits can be picked without killing the plant, but not vegetables such as celery and carrots. There are also a number of confusing terms used for different kinds of vegetarians. Pescatarian is a vegetarian, usually a lacto–ovo, who also eats fish. A semi-vegetarian eats less meat than the average person does. A pseudo-vegetarian claims to be a vegetarian, but is not. For example, they may eat less meat than the average person may or they may eat dairy foods, eggs, chicken, and fish but no other animal flesh. This term is often used by true vegetarians to describe semi-vegetarians and pescatarians. In addition to these somewhat confusing terms, there are several others that are also in use that describe variable eating patterns. For example, a vegetable consumer means anyone who consumes vegetables, but may not necessarily be a vegetarian. An ‘‘herbivore’’ means eating grass and=or plants but again may not necessarily qualify as a vegetarian. A ‘‘plant-eater’’ mainly eats plants, but may not necessarily qualify as a vegetarian. As the term implies, nonmeat eater does not eat meat. Most commonly used definitions do not consider fish, fowl, or seafood to be ‘‘meat.’’ Animal fats and oils, bone meal, and skin are not considered meat. The term ‘‘kosher’’ is derived from a complex set of Jewish dietary laws but does not imply vegan or lacto–ovo vegetarian. NUTRITIONAL QUALITY OF VEGETARIAN DIETS While the nutritional quality of the foods eaten by lacto–ovo vegetarians is quite high and almost on par nutritionally with that eaten by those that eat red meat, the more restrictive the vegetarian diet, Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 151 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 151 the more difficult it is to get the nutrients you need. In cases of vegans, the diet must be very carefully planned so that the athlete can get his or her full quota of high-quality protein and other macro- and micronutrients. As the diet becomes much more restrictive (e.g., in fruitarians) it becomes impossible to meet the nutritional demands of high-level athletic activity. On the other hand, if appropriately planned, vegan diets, though restrictive, can provide adequate nutrition even for competitive athlete including those in the power sports such as bodybuilding, power-lifting, and Olympic lifting. One of the main concerns for vegetarian athletes is that they obtain enough high-quality protein in their diets to meet their increased protein needs. However, this can be accomplished if they choose wisely among the various plant proteins. Protein deficiencies may be more of a problem with vegans than with lacto–ovo vegetarians who eat eggs, milk, yogurt, cheese, and other dairy foods, since these foods are excellent sources of high-quality protein. The key for strict vegetarians, or vegans who consume no milk, eggs, or other animal proteins, is to eat a variety of grains that have complementary amino acids. Beans and rice are an example of mixing legumes (peas and beans) and grains. Also, tofu is an excellent addition to a vegetarian diet. Tofu is a high-quality plant protein that contains all essential amino acids and offers the bonus of phytochemicals that protect against heart disease and cancer. VEGETARIAN FOOD GUIDE—BASIC FOUR FOOD GROUPS It is not difficult for most people to become vegetarians. The vegetarian foods make up much of the U.S. Department of Agriculture and Department of Health and Human Services’ Food Guide Pyramid, which recommends 6–11 daily servings of bread, cereal, rice, and pasta. Daily intakes advised for other foods are three to five servings of vegetables; two to four servings of fruits; two to three servings of milk, yogurt, and cheese; and two to three servings of meat, poultry, fish, dry beans, eggs, and nuts. The guide advises using fats, oils, and sweets sparingly. Looking at the food pyramid it is easy to see that an ovo–lacto vegetarian and even a vegan (except for the dairy servings) can easily fulfill each category in the food pyramid. For vegans the four food groups of meat, dairy, grains, and fruits=vegetables are replaced by the four food groups of grains, legumes, vegetables, and fruits. Let us examine these groups in more detail: Whole grains: This group includes bread, rice, pasta, hot or cold cereal, corn, millet, barley, bulgur, buckwheat groats, and tortillas. Build a meal based on a dish of whole grains as these foods are rich in fiber and other complex carbohydrates, as well as protein, B vitamins, and zinc. Vegetables: Vegetables contain many nutrients such as vitamin C, b-carotene, riboflavin and other vitamins, iron, calcium, and fiber. Dark-green, leafy vegetables such as broccoli, collards, kale, mustard and turnip greens, chicory, or bok choy are especially good sources of these important nutrients. Dark-yellow and orange vegetables such as carrots, winter squash, sweet potatoes, and pumpkins provide extra b-carotene. Include generous portions of a variety of vegetables in your diet. Legumes: Beans, peas, and lentils are all good sources of fiber, protein, iron, calcium, zinc, and B vitamins. This group also includes chickpeas, beans, soy milk, tofu, tempeh, and textured vegetable protein. Fruit: Fruits are rich in fiber, vitamin C, and b-carotene. Be sure to include at least one serving each of fruits that are high in vitamin C—citrus fruits, melons, and strawberries are all good choices. Choose whole fruit over juices, which do not contain as much healthy fiber. Juices may be preferable for those who need to take in a lot of calories but who are getting enough fiber. For lacto–ovo vegetarians, the incorporation of egg or milk products can make a world of difference as far as accessibility to first-quality protein and fewer problems with most micronutrient intakes. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 152 43803_C004 Final Proof page 152 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge SUPPLYING REQUIRED NUTRIENTS Vegetarians who eat no meat, fish, poultry, or dairy foods face the greatest risk of nutritional deficiency. On the other hand, no matter which diet you choose it is important to plan that diet so that it not only matches your vegetarian needs but also provides you with the nutrients you need to remain healthy. Nutrients most likely to be lacking and some nonanimal sources of these nutrients are presented in Table 4.1. NUTRITIONAL CONSIDERATIONS OF VEGETARIANS Special Nutrient Needs of Vegetarians While choosing to be a vegetarian is a personal decision, it is important to know that the vegetarian way of eating can cause some macro- and micronutrient deficiencies. As such, it is vital to realize that the decision to become a vegetarian is also accompanied by a need to learn how to eat the right foods to ensure you are getting your healthy share of all the necessary nutrients. You have to know enough about creating a diet so you do not end up with any deficiencies. Just how careful you have to be depends on the type of vegetarian diet you have adopted. Being a vegetarian does not guarantee improved health. Depending on the type of vegetarian diet, you can make use of the various protein supplements and may need additional vitamins and minerals. For instance, if you abstain from dairy products you may need to supplement your diet with calcium, magnesium, and vitamin D. Several studies both on vegetarian men and women have shown that compared to nonvegetarians, they ingested less calcium, iron, zinc, and vitamin B12.86,87 Most subjects in these studies ate less than half the RDA for B12, a vitamin crucial for healthy red blood cells and nerve fibers. Since vitamin B12 is found only in animal products, such as red meat, fish, shellfish, eggs, and milk, strict vegetarians (or ‘‘vegans’’) must increase their consumption of foods, such as soy milk, that are fortified with this vitamin. French researchers caution that a strict vegan diet may lead to deficiency of important vitamins that are critical to eyesight. In a case report,88 a French patient lost most of his eyesight resulting from following a strict vegan diet. The patient did not supplement with vitamins, leading to B12 deficiency. This vitamin is important in maintaining the health of nerves, including the optic nerve that transmits signals from the eye to the brain. The researcher also found the patient had belownormal levels of vitamins B1, B12, A, C, D, E, and zinc and selenium. TABLE 4.1 Possible Nutrient Deficiencies and Nonanimal-Based Food Sources Nutrients Vitamin B12 Vitamin D Calcium Iron Zinc Food Sources Fortified soy milk and cereals Fortified margarine and sunshine Tofu, broccoli, seeds, nuts, kale, bok choy, legumes, greens, calcium-enriched grain products, and lime-processed tortillas Legumes, tofu, green leafy vegetables, dried fruit, whole grains, and iron-fortified cereals and breads, especially whole wheat Whole grains, whole-wheat bread, legumes, nuts, and tofu Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 153 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 153 VEGETARIAN ATHLETES Since vegetarian athletes need more of some of the critical nutrients, they have to be especially careful to make sure that their diets fulfill these needs. Thus, as vegetarian styles of eating become more popular among athletes, the risk of poorly planned diets leading to nutrient insufficiencies and deficiencies increases. Inadequate dietary intakes of iron and zinc have been observed in athletes who have eliminated meat from their diets. Marginal iron or zinc status may adversely affect, while frank iron or zinc deficiency definitely decreases exercise performance. For athletes, while it is possible to obtain all essential nutrients by eating a completely plantbased diet, it requires critical planning and execution.89 Athletes must also learn that it is not enough to just cut meats out of the diet. These foods have many essential nutrients that are more difficult to get elsewhere. Realistically, however, it may be difficult for the vegetarian athletes, especially vegans, to get the nutrients they need. Also because vegan diets are typically high in fiber, it may be difficult to take in enough food to satisfy energy requirements. Additionally, there are special considerations regarding age and gender of the vegetarian athlete. Amenorrhea may be more common among vegetarian than nonvegetarian female athletes.90 Efforts to maintain normal menstrual cycles might include increasing energy and fat intake, reducing fiber, and reducing strenuous training. When vegetarian female athletes are properly nourished, especially with adequate calories, their menstrual-cycle function should be normal compared with that of matched nonvegetarian women. Vegetarian diets have been shown to result in decreases in the levels of anabolic hormones even in lacto–ovo vegetarians.91 Male, endurance athletes on a lacto–ovo vegetarian diet exhibited lower total testosterone levels compared with those using a diet mixed with meat.92 Also, another study showed that in older men the consumption of a meat-containing diet contributed to greater gains in fat-free mass and skeletal muscle mass with resistance training than did a lacto–ovo vegetarian diet.93 A plant-based diet facilitates high-carbohydrate intake, which can be essential to support prolonged exercise. A well-planned vegetarian diet can provide athletes with adequate amounts of all known nutrients, although the potential for suboptimal iron, zinc, trace element, and protein intake exists if the diet is too restrictive. This concern, however, exists for all athletes, vegetarian or nonvegetarian, who have poor dietary habits. Athletes who consume diets rich in fruit, vegetables, and whole grains receive high amounts of antioxidant nutrients that help reduce the oxidative stress associated with heavy exertion. Whereas athletes are most often concerned with performance, vegetarian diets also seem to provide long-term health benefits and a reduction in risk of chronic disease. The bottom line is that if the decision to become a vegetarian is not a choice made because of moral or ethical principles, it may be more realistic and productive for athletes to include some meat in their diet. COMMON VITAMIN AND MINERAL DEFICIENCIES It is important to remember that vegetarians have special nutritional needs and concerns. Thus, vegetarians have to be careful that they receive their daily quota of the macronutrients, essential fatty acids, and vitamins and minerals. Because vegans do not eat dairy products or eggs, they are at a higher risk of having deficiencies for calcium, vitamin B12, and vitamin D. These nutrients, while abundant in dairy products, are less available in plant sources. Calcium is found in fortified foods, green leafy vegetables, beans, and tofu products. Vitamin D can be a problem for vegans with low sun exposure. However, exposure to the sun of limited parts of the body for 10–15 min=day is enough to supply the daily requirement of vitamin D. Because vitamin B12 is only found in animal products, vegans must obtain it from fortified foods or from nutritional supplements. The American Dietetic Association strongly recommends supplement use to ensure the proper quantity of B12. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 154 43803_C004 Final Proof page 154 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge SPECIFIC NUTRITIONAL NEEDS OF VEGETARIANS Lack of calories is common with vegans because of the bulk of food that must be consumed since plant foods are generally less dense calorically and a lot of fiber is consumed. Additionally, lacto– ovo vegetarians that allow eggs and dairy products in their diet and vegans, who do not allow any animal products in their diets, can face some nutrient deficiencies. Getting enough dietary protein is generally not a problem with lacto–ovo vegetarians because eggs and dairy products are nutritionally dense and contain first-quality protein. However, vegans, especially those who exercise vigorously or take part in sports, have to be extremely careful in planning their diets so as to supply enough energy since plant foods tend to be nutritionally less dense. Also, vegans have to mix proteins so that an overall amino acid balance is achieved. Considering the importance of dietary protein, especially for athletes, this topic deserves indepth attention. PROTEIN The quantity of protein in the diets of athletes, while important, is rarely a concern regardless of whether they are meat eaters or nonmeat eaters. Increased dietary intake can be accomplished easily by both choosing high-protein foods or by supplementing with protein powders. Nevertheless, the quality of protein can pose a problem. Well-processed soybean protein is equal in quality to animal protein. However, other legumes do not contain a full complement of the essential amino acids required for the efficient manufacture of protein by the human body. Previous vegetarian dietary guidelines recommended that a variety of plant-protein sources (such as grains and beans) be combined simultaneously at one meal to complement each other and provide a complete protein source. Current research supports the notion that by eating a variety of legumes, as well as all other food groups throughout the day, one can obtain the full array of essential amino acids required for efficient protein metabolism. Thus, by combining various plant proteins and making use of soybean protein, most vegetarians can easily get an adequate level of dietary protein. Vegetarian athletes, on the other hand, may find that combining complementary proteins at one meal may be beneficial. ENSURING ADEQUATE AND HIGH-QUALITY PROTEIN The mixture of proteins from grains, legumes, seeds, nuts, and vegetables provide a complement of amino acids so those deficits in one food are made up by another. At present it is felt that for most vegetarians not all types of plant foods need to be eaten at the same meal, since the amino acids are combined in the body’s protein pool. In general, proteins of animal origin contain adequate amounts of the essential amino acids and hence they are known as first-class proteins. On the other hand, many proteins of vegetable origin are relatively deficient in certain amino acids, notably lysine and the sulfur-containing amino acids. However, soy protein has been shown to be nutritionally equivalent in protein value to proteins of animal origin and, thus, can serve as the sole source of protein intake if desired. The essential amino acid lysine is consistently at a much lower concentration in all major plantfood protein groups than in animal foods. Since lysine is the limiting amino acid, the addition of limited amounts of lysine to cereal diets improves their protein quality. Studies in Peru and Guatemala have demonstrated that growing children benefited by this addition.94 In addition, the sulfur-containing amino acids are distinctly lower in legumes and fruits and threonine is lower in cereals compared with amounts found in proteins of animal origin. COMPLEMENTARY PROTEINS Although humans have used some 300 plant species for food and at least 150 species have been cultivated for commercial purposes, most of the world’s population depends on approximately 20 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 155 7.11.2007 5:32pm Compositor Name: BMani 155 different plant crops, which are generally divided into cereals, vegetables (including legumes), fruits, and nuts. The most important groups for human nutrition are cereal grains and food legumes, including oilseed legumes. Because of completeness and digestibility, some proteins are nutritionally better than others; they contain a more balanced range of the essential and conditionally essential amino acids. In general, proteins of animal origin contain adequate amounts of the essential amino acids and hence they are known as first-class proteins. On the other hand, many proteins of vegetable origin are relatively deficient in certain amino acids, notably lysine and the sulfur-containing amino acids. Mixtures of plant proteins can serve as a complete and well-balanced source of amino acids for meeting human physiological requirements.95 However, the combination of right foods is necessary to obtain the necessary levels of both the essential or indispensable and conditionally indispensable amino acids. There are important differences among and between food products of vegetable and animal origin including the concentrations of proteins and indispensable amino acids that they contain. The concentration of protein and the quality of the protein in some foods of vegetable origin may be too low to make them adequate, sole sources of proteins. In some of the poorer parts of the world, diets are based predominantly on a single plant (e.g., corn) and they frequently lead to malnutrition. Foods of plant origin contain many amino acids, but single food items usually do not contain all the essential amino acids in adequate amounts to support good health. Vegetarian diets are planned so that the amino acid content is adequate to support good nutritional health. Amino acids are the building blocks of protein, the nutrient that is supplied in greatest quantity by meat, fish, poultry, eggs, milk, and cheese. The mainstays of strict vegetarian diets are fruits and vegetables, whole grains, legumes, nuts, and seeds. Because these plant foods do not contain an optimal balance of essential amino acids when eaten separately, the vegetarian must combine foods that complement each other, thus providing the body with the right amino acid mix.95 The combining of right foods is necessary to obtain the necessary levels of both the essential or indispensable and conditionally indispensable amino acids. The addition of eggs or dairy products with meals or snacks greatly improves the overall protein quality of the vegetarian diet. Fortunately, the amino acid deficiencies in a protein can usually be improved by combining it with another so that the mixture of the two proteins will often have a higher food value than either one alone. For example, many cereals are low in lysine, but high in methionine and cysteine. On the other hand, soybeans, lima beans, and kidney beans are high in lysine but low in methionine and cysteine. When eaten together these types of proteins give a more favorable amino acid profile. Rice, the staple food for more than half of the world population, is inadequate in its amino acid content when it is the sole protein source, but when combined with another type of plant protein, such as legumes, a complete protein mixture is formed. A mixture of red beans or black-eyed peas and rice is an example of complementary protein foods as is a peanut butter sandwich (peanuts are legumes while the wheat of the bread is a grain). Other examples are corn tortillas with refried beans and tofu with fried rice. Another example would be the combination of soybean, which is low in sulfur-containing amino acids, with cottonseed, peanut and sesame flour, and cereal grains, which are deficient mainly in lysine. In general, oilseed proteins, in particular soy protein, can be used effectively in combination with most cereal grains to improve the overall quality of the total protein intake. A combination of soy protein, which is high in lysine, with a cereal that contains a relatively good concentration of sulfur-containing amino acids results in a nutritional complementation; the protein quality of the mixture is greater than that for either protein source alone. Common examples of complementary food proteins include beans and corn (as in tortillas); rice and black-eyed peas; whole wheat or bulgar, soybeans, and sesame seeds; and soybeans, peanuts, Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 156 43803_C004 Final Proof page 156 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge brown rice, and bulgar wheat. This kind of supplementation works when the deficient and complementary proteins are ingested together or within a few hours of each other. NUTRITIONAL RESPONSES TO COMBINING TWO DIETARY PROTEINS Various nutritional responses are observed when two dietary proteins are combined. These have been classified by Bressani et al.96 into one of four types. Type I is an example where no protein complementary effect is achieved. For example, this occurs with combinations of peanut and corn, where both the protein sources have a common and quantitatively similar lysine deficiency and are both also deficient in other amino acids. Type II is observed when combinations are made of two protein sources that have the same limiting amino acid, but in quantitatively different amounts. Corn and cottonseed flour, for example, are both limiting in lysine but cottonseed is relatively less inadequate than is corn. Type III demonstrates a true complementary effect because there is a synergistic effect on the overall nutritive value of the protein mixture; the protein quality of the best mix exceeds that of each component alone. This type of response occurs when one of the protein sources has a considerably higher concentration of the most-limiting amino acid in the other protein. An example of this response, based on studies in children, is observed when corn and soy flour are mixed so that 60% of the protein intake comes from corn and the remainder from soy protein. Type IV occurs when both protein sources have a common amino acid deficiency. The protein component giving the highest value is the one containing a higher concentration of the deficient amino acid. Combinations of some textured-soy proteins and beef protein follow this type of response. These nutritional relationships have been determined from rat bioassay studies. However, the more limited results available from human studies with soy and other legumes confirm the applicability of this general concept in human nutrition. This knowledge helps us to understand and evaluate how nutritionally effective combinations of plant protein foods can be achieved. Even when combinations of plant protein foods are used there is still the concern of timing of ingestion of complementary proteins. Is there a need to ingest different plant proteins at the same time, or within the same meal, to achieve maximum benefit and nutritional value from proteins with different, but complementary, amino acid patterns? This concern may also extend to the question of the need to ingest a significant amount of protein at each meal, or whether it is sufficient to consume protein in variable amounts at different meals and even different days as long as the average daily intake meets or exceeds the recommended or safe protein intakes. According to FAO=WHO=UNU,97 estimates of protein requirements refer to metabolic needs that persist over moderate periods. However, as discussed in the following chapters, the body does not store much protein outside of a meager free amino acid pool, and begins certain catabolic processes in the postabsorptive phase making the ingestion of regular amounts of protein critical for maximizing the anabolic effects of exercise. There is a limited database that we can consult to make a definitive conclusion on the timing of consumption of complementary proteins or of specific amino acid supplements for proteins that are deficient in one or more amino acids. An earlier work in rapidly growing rats suggested that delaying the supplementation of a protein with its limiting amino acid reduces the value of the supplement.98,99 Similarly, the frequency of feeding diets supplemented with lysine in growing pigs affects the overall efficiency of utilization of dietary protein.100 Studies in human adults showed that overall dietary protein utilization was similar whether the daily protein intake was distributed among two or three meals.101,102 The general consensus at present is that the various complementary proteins do not have to be consumed in the same meal in order to supply the necessary amino acids but can in fact be consumed over the course of the day.103,104 In general, especially under conditions where intakes of total protein are high, it may not be necessary to consume complementary proteins at the same time. Separation of the proteins among meals over the course of a day would still permit the nutritional benefits of complementation. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 157 7.11.2007 5:32pm Compositor Name: BMani 157 Dietary Protein and Amino Acids However, in athletes trying to maximize protein synthesis and muscular hypertrophy it is necessary to have a full complement of amino acids present for every meal in order to maximize the anabolic effects of exercise. There is a meal-related decrease in proteolysis and increase in protein synthesis. In one study whole-body leucine and a-ketoisocaproate (KIC) metabolism were estimated in mature dogs fed a complete meal, a meal devoid of BCAAs, and a meal devoid of all amino acids.105 Using a constant infusion of [4,5–3H]leucine and [1–14C]KIC, combined with dietary [5,5,5–2H3]leucine, the rate of whole-body proteolysis, protein synthesis, leucine oxidation, and interconversion of leucine and KIC were estimated along with the rate of leucine absorption. Ingestion of the complete meal resulted in a decrease in the rate of endogenous proteolysis, a small increase in the estimated rate of leucine entering protein, and a twofold increase in the rate of leucine oxidation. Ingestion of either the meal devoid of BCAAs or devoid of all amino acids resulted in a decrease in estimates of whole-body rates of proteolysis and protein synthesis, decreased leucine oxidation, and a decrease in the interconversion of leucine and KIC. The decrease in whole-body proteolysis was closely associated with the rise in plasma insulin concentrations following meal ingestion. Together these data suggest that the transition from tissue catabolism to anabolism is the result, at least in part, of decreased whole-body proteolysis. This mealrelated decrease in proteolysis is independent of the dietary amino acid composition or content. In contrast, the rate of protein synthesis was sustained only when the meal complete in all amino acids was provided, indicating an overriding control of protein synthesis by amino acid availability. NUTRITIONAL SUPPLEMENTS AND THE VEGETARIAN ATHLETE Because of the necessity to be more careful and consistent with their diets, nutritional supplements can be more of a necessity for vegetarian athletes than for nonvegetarian ones.106 It seems that vegetarians rely more on nutritional supplements to get the nutrients they need.107,108 The most useful nutritional supplements for vegetarians are those in which they may be marginally deficient. Also, like most athletes, they would benefit from increasing dietary protein with the use of high-quality soy protein for vegans and with one or more of whey, casein, egg, and soy protein in lacto–ovo vegetarians. There are also other nutritional supplements, such as creatine (see later) and some of the amino acids, which may be useful for enhancing performance. There is one caveat to using nutritional supplements. Vegetarians need to carefully examine the ingredients of the various nutritional supplements to make sure they do not contain any of the ingredients that are not on their food list. For example, many meal replacement powders use a combination of soy and milk proteins. Also there are many vitamin supplements that do not contain animal products. Considering that many vegetarian diets exclude dairy foods, it is important to ensure adequate intake of calcium, as well as iron, riboflavin, and vitamin D. To make up for possible calcium deficiencies in vegetarians, calcium supplements should be used by all vegan athletes, especially women and children. Extra calcium should also be taken by vegan women during pregnancy and when breast-feeding. A high-quality vitamin supplement should be included into a vegetarian’s diet. This would include vitamin D which may be needed if sunlight exposure is limited since sunlight activates a substance in the skin and converts it into vitamin D. Vegan diets should include a reliable source of vitamin B12 because this nutrient occurs only in animal foods. Vitamin B12 deficiency can result in irreversible nerve deterioration. If a diet is low in essential fatty acids, one teaspoon of flaxseed oil or two tablespoons canola=soybean oil may be added to the diet. Some supplements normally found in abundance in red meat may be correct for vegetarians to use since these supplements are synthesized and as such do not contain any animal products. An example of this would be creatine monohydrate although you will have to make sure that the creatine mixes do not contain any added nutrients that may be of animal origin. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 158 43803_C004 Final Proof page 158 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge CREATINE The estimated daily need for creatine in humans is about 2 g, whereas the daily intake from meat or fish is about l g in the average American diet. The body makes up the deficit by producing creatine in the liver, kidney, and pancreas, using as precursors glycine and arginine. When dietary supply is low, the body steps up its production of creatine, but may not completely compensate, especially among vegetarians, who have reduced body creatine pools. Creatine stores vary greatly among individuals, and apart from diet, the reasons are unclear. Athletes with low stores might be most apt to benefit from supplementation. Creatine supplementation of 20 g=day for at least 3 days has resulted in significant increases in total creatine for some individuals but not others, suggesting that there are ‘‘responders’’ and ‘‘nonresponders.’’ These increases in total concentration among responders are greatest in individuals who have the lowest initial total creatine, such as vegetarians. Research has shown that muscle and other creatine pools are significantly lower in vegetarians and that by using creatine supplements they as a group are more responsive to creatine supplementation and reach the ceiling levels that nonvegetarians attain with creatine supplementation.109–114 On average, muscle creatine levels increase an average of 20% after 6 days of supplementing at 20 g=day (rapid creatine loading). These higher levels can be maintained by ingesting as low as 2 g=day thereafter. A similar, but slower, 20% rise in muscle creatine levels occurs by ingesting 3 g=day for 1 month, the ‘‘no-load’’ method. FOOD PROCESSING The processing of food with the use of heat and chemicals can adversely affect amino acid availability.115 For example, lysine can be lost from mild heat treatment in the presence of reducing sugars such as glucose or lactose since these sugars react with free amino groups. This may also occur when the protein and sugar are stored together at low temperatures. Under severe heating conditions, either with or without the presence of either sugars or oxidized lipids, food proteins form additional chemical bonds and can become resistant to digestion so that availability of all amino acids is reduced. When protein is exposed to severe treatment with alkali, lysine and cysteine can be eliminated, with formation of lysinoalanine which may be toxic. The use of oxidizing agents such as sulfur dioxide (SO2) can result in a loss of methionine in the protein, through the formation of methionine sulfoxide. Heating can also have favorable effects. Heating soybean flour improves the utilization of protein by making the amino acid methionine more available, and heating raw soybeans destroys the inhibitor of the protein digestive enzyme trypsin. Cooking eggs destroys the trypsin inhibitor ovomucoid found in the white part of the egg. MEASURING PROTEIN QUALITY There are many factors that influence dietary protein quality including the amino acid composition and bioavailability of the protein. There are several ways to measure the protein quality of food. The four most quoted are the protein efficiency ratio (PER), biological value (BV), protein digestibility (PD), and the protein digestibility corrected amino acid score (PDCAAS). PROTEIN EFFICIENCY RATIO Up to this decade, the PER the best-known and most widely used procedure for evaluating protein quality, and was used in the United States as the basis for food labeling regulations and for the establishment of the protein RDA. In this procedure, immature rats are fed a measured amount of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 159 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 159 protein and weighed periodically as they grow. The PER is then calculated by dividing the weight gain (in grams) by the protein intake (also in grams). While simple and economical, the PER procedure is time consuming. Also, evidence from studies with rats indicates that the pattern of amino acids required for maintenance and tissue protein accretion is quite different in mature rats and in humans. Thus, the amino acid requirements of growing rats are not the same as for those of mature rats, much less mature human beings. Indeed, the intracellular muscle free amino acid pool of rats is probably less suitable for the investigation of amino acid metabolism, due to the great differences in its distribution in human and rat muscle.116 Nevertheless the PER is still used today although other methods are becoming more common. A study published over a decade ago compared the protein quality of different animal foods and of their mixtures with vegetable foods, mainly cereals, at the 30:70 for animal to vegetable protein proportion with experiments performed under the same conditions.117 The animal foods were eggs, beef, pork, barbecued lamb, chicken, ham, sausage, and milk powder. The vegetable foods used in the mixtures were rice, lime-treated corn flour, wheat flour, and cooked black beans. The protein concentrations in the raw and cooked materials were analyzed. The PER and digestibilities were determined in Fisher 344 weanling rats. Based on the corrected PER, the foods with the best protein quality were egg (3.24), sirloin beef (3.16), lamb (3.11), and chicken breast (3.07), which were significantly different from milk powder (2.88) and beef liver and beef round (2.81 and 2.70, respectively). Ham (2.63) and pork loin (2.57) had a similar protein quality to that of casein (2.50). The lowest protein quality was found in sausages (2.14). In most of the mixtures of animal and vegetable protein (30:70), the PER was similar to or higher than that of the animal food alone. Beans were the vegetable food that showed the lowest response to the addition of animal food. The conclusion of the study is that some 30:70 mixtures of animal to vegetable protein such as chicken, beef round, and pork with cereals could be utilized for regular meals because of their high PER and low cost. BIOLOGICAL VALUE To determine the relative utilization of a protein by the body, it is necessary to measure not only urinary, but also fecal losses of nitrogen when that protein is actually fed to human beings under test conditions. Even under these circumstances, small additional losses from sweat, sloughed skin, hair, and fingernails will be missed. This kind of experiment determines the BV of proteins, a measure used internationally. The BV test involves two nitrogen balance studies. In the first, no protein is ingested and fecal and urinary nitrogen excretions are measured. It is assumed that under these conditions nitrogen lost in the urine is the amount the body always necessarily loses by filtration into the urine each day, regardless of what protein is fed (endogenous nitrogen). The nitrogen lost in the feces (called metabolic nitrogen) is the amount the body invariably loses into the intestine each day, whether or not food protein is fed. In the second study, an amount of protein slightly below the requirement is ingested, and intake and losses are measured. The BV is then derived using the following formula: BV ¼ 100 food N ½ðfecal N metabolic NÞ ðurinary N endogenous NÞ food N ðfecal N metabolic NÞ The denominator of the BV equation expresses the amount of nitrogen absorbed: food N minus fecal N (excluding the N the body would lose in the feces anyway, even without food). The numerator expresses the amount of N retained from the N absorbed: absorbed N (as in the denominator) minus the N excreted in the urine (excluding the N the body would lose in the urine anyway, even without food). Thus, it can be more simply expressed as: BV ¼ (N retained=N absorbed) 100 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 160 43803_C004 Final Proof page 160 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge The BV method has the advantages of being based on experiments with human beings and of measuring actual nitrogen retention. Its disadvantages are that it is cumbersome, expensive, sometimes impractical, and is based on several assumptions that may not be valid. For example, the subjects used for testing may not be similar physiologically or in terms of their normal environment or typical food intake (e.g., dietary protein intake history) to those for whom the test protein may ultimately be used. Also, that protein is retained in the body does not necessarily mean that it is being well utilized. There is considerable exchange of protein among tissues (protein turnover) that is hidden from view when only nitrogen intake and output are measured. One tissue could be shorted, and the test of BV would not detect this. At present, the relationship between protein intakes and optimum organ function and health is poorly understood. Nevertheless, it would appear that the results obtained by the BV method would be more applicable to humans.118 As examples we can compare the BV of two proteins that are on the opposite end of the scale, vegetable proteins and hydrolyzed whey protein. Plant proteins have a low BV and therefore supplementation is a necessity for most plant-based vegetarians (as against lacto–ovo and lacto vegetarians) athletes who wish to increase lean body mass. A study measured the effects of a diet made up of plant protein in rats.119 The results demonstrate the insufficiency of vegetable sources of food with respect to proteosynthesis and the content of limiting amino acids (decisive for the synthesis of peptide chains) in the period of the organism development. On the other hand, whey protein has a higher BV and is much more suitable for athletes wishing to increase muscle mass. The BV of whey protein also increases once it has been predigested (i.e., its high molecular weight protein fractions have been broken down into more efficiently absorbed short chain peptides: di-, tri-, and oligopeptides). PROTEIN DIGESTIBILITY CORRECTED AMINO ACID SCORE The latest way to assess dietary protein quality is PDCAAS, recommended by the FAO=WHO Joint Expert Commission on Protein Evaluation at Rome in 1990. Several factors such as inadequacies of PER (the poorest test) and other animal assays, advancements made in standardizing methods for amino acid analysis and protein digestibility, availability of data on digestibility of protein and individual amino acids in a variety of foods, and reliability of human amino acid requirements and scoring patterns make the amino acid score—corrected for true digestibility of protein—the most suitable routine method for predicting protein quality of foods for humans.120 The PDCAAS method is a simple and scientifically sound approach for routine evaluation of protein quality of foods. The PDCAAS is directly applicable to humans, and incorporates factors for more real-life variables than either PER or BV. The amino acid pattern for humans 2–5 years is used as the basis for determination of PDCAAS, since this age group matches or exceeds amino acid requirement patterns of older children and adults. Corrections for digestibility of protein are also taken from human data. PDCAAS scores range from 1.0 to 0.0, with 1.0 being the upper limit of protein quality (able to support growth and health). Interestingly, using PDCAAS results in a score of 1.0 for isolated soy protein, identical to other proteins with scores of 1.0 (such as casein, lactalbumin, or ovalbumen).121 This finding, although it likely reflects the successful, long-term use of soy protein as the exclusive protein source for infants, who have even higher amino acid requirements than children or adults, does not give us the full story as to the quality of soy protein. Discrepancies between the PER and PDCAAS are related to the differences between young growing rats and humans. Growing rats have a larger requirement for sulfur-containing amino acids to generate the larger amounts of keratin for whole-body coats of hair, which humans do not possess. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 161 7.11.2007 5:32pm Compositor Name: BMani 161 Thus soy protein can be added to the list of complete proteins for humans, along with milk and egg proteins. Other vegetable proteins (pea, beans, peanuts, wheat, oats, etc.) still have less than adequate protein scores. Unfortunately the PDCAAS is far from perfect. First of all, our diets rarely contain just one type of protein and the PDCAAS is not able to assess the impact that a mixture of proteins will have on the quality of the combination of proteins that together, but not individually, make up a high-quality protein. This is especially evident when dealing with the combining of proteins done by vegetarians. For example, grain protein has a low PDCAAS because of the low lysine, but it has a high methionine content. On the other hand, most pulses have a low PDCAAS because of low methionine, but they have high lysine content. Because they compliment each other, the two proteins together make up a high-quality protein source. On the other hand, it contains more than enough methionine. White bean protein (and that of many other pulses) has a PDCAAS of 0.6–0.7, limited by methionine, and contains more than enough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of the combined constituent is 1.0, because each constituent’s protein is complemented by the other. There is also the fact that several proteins that are different in some ways, since they meet the criteria decided upon at the 1990 FAO=WHO meeting, all have identical scores of 1.0 since their actual scores are rounded off. As such, soy protein, with a score of 0.9 is considered much closer in quality than it should be to egg protein, with an actual score of 1.2. DIETARY PROTEIN REQUIREMENTS The term protein requirement means that amount of protein which must be consumed to provide the amino acids for the synthesis of those body proteins irreversibly catabolized in the course of the body’s metabolism. The dietary requirement for protein comprises two components: (1) a requirement for total nitrogen and (2) a requirement for the nutritionally indispensable amino acids. The nutritionally indispensable amino acids alone do not maintain body nitrogen balance; a source of ‘‘nonspecific’’ nitrogen from dispensable amino acids, such as from glycine and alanine or other nitrogen compounds, is also required.122 The recommended allowances for protein (nitrogen) are based upon experiments in which normal requirement is defined as the intake necessary to achieve zero balance between intake versus output. Most estimates assume normal digestion and absorption and normal metabolism. In some cases, estimates of daily turnover are used to determine the amount of nutrient required to maintain body stores. Thus the intake of nitrogen from protein must be sufficient to balance that excreted; this basic concept is called nitrogen balance. The minimum daily requirement, that is the minimum amount of dietary protein which will provide the needed amounts of amino acids to optimally maintain the body, is impossible to determine for each individual without expending a good deal of time and effort for every person. To eliminate the necessity of determining individual nutrient requirements, a system called the RDA has been devised. As research accumulates for each of the many essential nutrients, its associated RDA is revised. The RDAs (1941–1989) were established to try to cover the nutritional needs of all normal, healthy persons living in the United States without the necessity of determining individual nutrient requirements. Canada had a similar system called the recommended nutrient intake (RNI). Several countries have their own RDAs. In the past, RDAs were established for protein (there was no RDA for fat or carbohydrate), vitamins A, D, E, K, B1, B2, B3, B6, B12, folacin, C, calcium, magnesium, iron, iodine, phosphorus, Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 162 43803_C004 Final Proof page 162 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge selenium, and zinc. The RDAs were set to allow for the vast majority of all normal healthy persons in the United States, but do not cover people with illness or chronic disease. In order to cover some of the outliers, there is a margin of safety built into the RDAs so that the average, healthy person can consume one-third less than the RDA and still not run into any deficiencies. The Food and Nutrition Board of National Academy of Sciences set the values for the RDAs based on human and animal research. They meet every 5 years or so to review contemporary research on nutrients. As research accumulated for each of the many essential nutrients, its associated RDA was revised. The last revision for the RDAs was in 1989.123 In the last decade, some significant changes have taken place in the detail and scope of official dietary recommendations. In 1997, the Food and Nutrition Board of the National Academy of Sciences introduced dietary reference intakes (DRIs), changing the way nutritionists and nutrition scientists evaluate the diets of healthy people. There are four types of DRI reference values: the estimated average requirement (EAR), the RDA, the adequate intake (AI), and the tolerable upper intake level (UL). The primary goal of having new dietary reference values was to not only prevent nutrient deficiencies, but also reduce the risk of chronic diseases such as osteoporosis, cancer, and cardiovascular disease. Ten DRI reports have been completed since 1997 and several articles and hundreds of papers discussed them.124–126 Dietary Reference Intakes Definitions Recommended dietary allowance (RDA): The average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all (97%–98%) healthy individuals in a particular life stage and gender group. Adequate intake (AI): A recommended intake value based on observed or experimentally determined approximations or estimates of nutrient intake by a group (or groups) of healthy people, that are assumed to be adequate—used when an RDA cannot be determined. Tolerable upper intake level (UL): The highest level of daily nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals in the general population. As intake increase above the UL, the potential risk of adverse effects increases. Estimated average requirement (EAR): A daily nutrient intake value that is estimated to meet the requirement of half of the healthy individuals in a life stage and gender group—used to assess dietary adequacy and as the basis for the RDA. Acceptable macronutrient distribution range (AMDR): A range of intakes for a particular energy source that is associated with reduced risk of chronic diseases while providing adequate intakes of essential nutrients. Estimated energy requirement (EER): The average dietary energy intake that is predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity consistent with good health. In 2000, an expert nutrition review panel, consisting of both American and Canadian scientists selected by the National Academy of Sciences, was formed to establish the DRIs for macronutrients and energy. The panel’s mission was, based on the current scientific literature, to update, replace, and expand upon the old RDAs in the United States (NRC 1989) and RNIs in Canada (Health and Welfare Canada 1990). In 2002, the DRI report was released with new recommendations for healthy individuals for energy, dietary carbohydrates and fiber, dietary fats which include fatty acids and cholesterol, dietary protein, and the indispensable amino acids.127 Although the panel was instructed to consider increased physical activity as a factor affecting requirements, the DRIs for protein that applied to most healthy individuals were felt to be adequate for those that were physically active. After reviewing the scientific literature investigating the protein needs of athletes, the panel stated: ‘‘In view of the lack of compelling evidence to the Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 163 7.11.2007 5:32pm Compositor Name: BMani 163 contrary, no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise.’’128 The general criterion used to establish the EAR for protein in the adult, and subsequently the RDA, was to determine the point at which the body is able to maintain body protein content at its current level (i.e., maintenance of nitrogen balance). This daily value was 0.66 g=kg of body mass per day for adults (men and women 19 years of age and older), which resulted in an RDA of 0.80 g=kg. This is the same daily value as the 1989 RDA and slightly lower than the 1990 RNI of 0.86 g=kg (Health and Welfare Canada 1990).129 In the case of protein and amino acid requirements, the RDA is age and gender dependent and is set at twice the minimum value of the subject who required the most protein and=or amino acid in all the studies conducted. By greatly increasing the recommended intake figure over that experimentally determined it was hoped that the protein and amino acid needs of the majority or 95% of the American population would be met. The RDA for protein was originally quite high. For many years, it was set at 1 g=kg body weight for the average adult male. The average adult male was assumed to weigh 70 kg (about 155 pounds) so the RDA was 70 g=day. In the last three decades, however, the RDA for protein has been adjusted steadily downward until the recommendations of 1989. HOW WAS THE RDA ESTABLISHED? Ideally, sufficient research is conducted to show that (1) a given nutrient is needed by the human, (2) certain deficiency signs can be produced, (3) these signs can be avoided or reversed if the missing nutrient is administered, and (4) no further improvement is observed if the nutrient is administered at levels above that which reversed the deficiency symptoms. Next, studies are conducted on a variety of subjects to determine their minimal need. Since humans vary so much, it is not possible to measure the requirements over a broad range of human variability. To allow for this variability, a safety factor is added on to the determined minimum needs of the group of subjects studied. As more subjects are studied and more data accumulated, the added safety factor becomes smaller. For the purposes of determining protein needs, some shortcuts are taken. For example, since proper nitrogen balance studies are laborious and are performed over a period of several days, a shortcut is often taken and an estimate of nitrogen balance is made by collecting and measuring nitrogen in the urine, since the end products of protein metabolism leave the body mainly via this route, and other losses are also estimated. Estimations are made of the nitrogen lost in the feces and the small losses of protein from skin, hair, fingernails, perspiration, and other secretions. About 90% of the nitrogen in urine is urea and ammonia salts—the end products of protein metabolism. The remaining nitrogen is accounted for by creatinine (from creatine), uric acid (products of the metabolism of purines and pyrimidines), porphyrins, and other nitrogen-containing compounds. Urinary nitrogen excretion is related to the basal metabolic rate (BMR). The larger the muscle mass in the body, the more calories are needed to maintain the BMR. Also, the rate of transamination is greater as amino acids and carbohydrates are interconverted to fulfill energy needs in the muscle. Approximately, 1–1.3 mg of urinary nitrogen is excreted for each kilocalorie required for basal metabolism. Nitrogen excretion also increases during exercise and heavy work. Fecal and skin losses account for a significant amount of nitrogen loss from the body in normal conditions, and these may vary widely in disease states. Thus, measurement of urinary nitrogen loss alone may not provide a predictable assessment of daily nitrogen requirement when it is most needed. Fecal losses are due to the inefficiency of digestion and absorption of protein (93% efficiency). In addition, the intestinal tract secretes proteins in the lumen from saliva, gastric juice, bile, pancreatic enzymes, and enterocyte sloughing. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 164 43803_C004 Final Proof page 164 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge Taking all these losses into consideration and using nitrogen balance as a tool, the minimum daily dietary allowance for protein may be derived on the following basis: 1. Obligatory urinary nitrogen losses of young adults amount to about 37 mg=kg of body weight. 2. Fecal nitrogen losses average 12 mg=kg of body weight. 3. Amounts of nitrogen lost in perspiration, hair, fingernails, and sloughed skin are estimated at 3 mg=kg of body weight. 4. Minor routes of nitrogen loss such as saliva, menstruation, and seminal ejaculation are estimated at 2 mg=kg of body weight. 5. Total obligatory nitrogen lost—which must be replaced daily—amounts to 54 mg=kg, or in terms of protein lost this is 0.34 g=kg (0.054 3 6.25 1 g N ¼ 6.25 g of protein). 6. To account for individual variation the daily loss is increased by 30%, or 70 mg=kg. In terms of protein, this is 0.45 g=kg of body weight. 7. This protein loss is further increased by 30%, to 0.6 g=kg of body weight, to account for the loss of efficiency when consuming even a high-quality protein such as egg. 8. Final adjustment is to correct for the 75% efficiency of utilization of protein in the mixed diet of North Americans. Thus, the RDA for protein becomes 0.8 g=kg of body weight for normal healthy adult males and females, or 63 g of protein per day for a 174 lb (79 kg) man and 50 g=day for a 138 lb (63 kg) woman. The need for dietary protein is influenced by age, environmental temperature, energy intake, gender, micronutrient intake, infection, activity, previous diet, trauma, pregnancy, and lactation. The minimum daily requirement, that is the minimum amount of dietary protein which will provide the needed amounts of amino acids to optimally maintain the body, is impossible to determine for each individual without expending a good deal of time and effort for every person. At present, the normal amount of protein recommended for sedentary people is 0.8 g of protein per kilogram (0.36 g=lb) of body weight per day. This RDA for protein presumes that the dietary protein is coming from a mixed diet containing a reasonable amount of good-quality proteins. For the average person subsisting on mixtures of poor-quality proteins, this RDA may not be adequate. RECOMMENDED DAILY INTAKES FOR ATHLETES The RDAs make little provision for changes in nutrient requirements for those who exercise. Energy requirements increase with exercise as the lean (muscle) mass increases and as resting metabolic energy expenditure increases. Increased physical activity at all ages promotes the retention of lean muscle mass and requires increased protein and energy intake. In athletes, several factors can increase the amount of protein needed, including duration and intensity of exercise, degree of training, and current energy and protein intake of the diet. Athletes who train hard need more protein than the average individual. This holds true for both endurance and power sports. HISTORICAL OVERVIEW The history of protein requirements for athletes is both interesting and circular. In the mid-1800s the popular opinion was that protein was the primary fuel for working muscles.130 This was an incentive for athletes to consume large amounts of dietary protein. In 1866, a paper based on urinary nitrogen excretion measures (in order for protein to provide energy its nitrogen must be removed and subsequently excreted primarily in the urine), suggested Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 165 7.11.2007 5:32pm Compositor Name: BMani Dietary Protein and Amino Acids 165 that protein was not an important fuel, and contributed about 6% of the fuel used during a 1956 m climb in the Swiss Alps.131 This paper and others led to the perception that exercise does not increase one’s need for dietary protein. This view has persisted to the present. Recently, however, there is some evidence to show that protein contributes more than is generally believed at present. The data in the 1866 study likely underestimated the actual protein use for several methodological reasons. For example, the subjects consumed a protein-free diet before the climb, post-climb excretion measures were not made, and other routes of nitrogen excretion may have been substantial. However, based largely on these data, this belief has persisted throughout most of the 20th century. This is somewhat surprising because Cathcart,132 in an extensive review of the literature prior to 1925, concluded ‘‘the accumulated evidence seems to me to point in no unmistakable fashion to the opposite conclusion that muscle activity does increase, if only in small degree, the metabolism of protein.’’ Based on results from a number of separate experimental approaches, the conclusions of several more investigators support Cathcart’s conclusion.133 Several studies have shown that the current RDA for protein is not enough for many people, including athletes and the elderly. The current dietary protein recommendations for protein (0.8 g=kg) may be insufficient for athletes and those wishing to maximize lean body mass and strength. These athletes may well benefit from protein supplementation. With exercise and under certain conditions, the use of protein and amino acid supplements may have significant anabolic and anticatabolic effects. Athletes who train hard, because of the increased use of amino acids for energy metabolism and protein synthesis, need more protein than the average individual. Over a decade ago, Peter Lemon of the Applied Physiology Research Laboratory, Kent State University, addressed the issue of the protein requirements of athletes.134 Lemon remarked that current recommendations concerning dietary protein are based primarily on data obtained from sedentary subjects. However, both endurance and strength athletes, he says, will likely benefit from diets containing more protein than the current RDA of 0.8 g=kg=day, though the roles played by protein in excess of the RDA will likely be quite different between the two sets of athletes. For strength athletes, Lemon states that protein requirements will probably be in the range of 1.4–1.8 g=kg=day, whereas endurance athletes need about 1.2–1.4 g=kg=day. There is no indication that these intakes will cause any adverse side effects in healthy humans. On the other hand, there is essentially no valid scientific evidence that protein intakes exceeding about 1.8–2.0 g=kg=day will provide an additional advantage. EFFECTS OF EXERCISE ON DIETARY PROTEIN REQUIREMENTS High protein intake has been the mainstay of most athletes’ diets. Athletes in general and strength athletes and bodybuilders in particular consume large amounts of protein.135 One reason for their increased protein consumption is their increased caloric intake. Another is that most athletes deliberately increase their intake of protein-rich foods and often use protein supplements. As we have seen, many scientific and medical sources feel that protein supplementation and high-protein diets are unnecessary and that the RDA supplies more than adequate amounts of protein for the athlete.136 In fact, overloading on protein is felt to be detrimental because of the increased load to the kidneys of the metabolic breakdown products formed when the excess protein is used as an energy source. The results of a number of investigations involving both strength and endurance athletes indicate that, in fact, exercise does increase protein=amino acid needs.137–141 In a review, the overall consensus has been that all athletes need more protein than sedentary people, and that strength athletes need the most.142 Over a decade ago, a group of researchers at McMaster University in Hamilton, Ontario concluded that the current Canadian RNI for protein of 0.86 g=kg=day is inadequate for those Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 166 43803_C004 Final Proof page 166 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge engaged in endurance exercise.143 Moreover, their results indicated that male athletes may have an even higher protein requirement than females. A recent study found that endurance athletes need a protein intake of at least 1.2 g=kg to achieve a positive nitrogen balance.144 Butterfield145 performed a review of the literature and recommended high protein intakes (up to 2–3 g=kg) for physically active individuals. She found evidence for the existence of an intricate relationship between protein and energy utilization with exercise: When energy intake is in excess of need, the utilization of even a marginal intake of protein will be improved, giving the appearance that protein intake is adequate. When energy intake and output are balanced, the improvement in nitrogen retention accomplished by exercise seems to be fairly constant at protein intakes greater than 0.8 g=kg=d, but falls off rapidly at protein intakes below this. When energy balance is negative, the magnitude of the effect of exercise on protein retention may be decreased as the activity increases, and protein requirements may be higher than when energy balance is maintained. Somewhat in agreement with Butterfield’s conclusions are the results of another study by Piatti et al.146 They investigated the effects of two hypocaloric diets (800 kcal) on body weight reduction and composition, insulin sensitivity, and proteolysis in 25 normal obese women. The two diets had the following composition: 45% protein, 35% carbohydrate, and 20% fat (high-protein diet); and 60% carbohydrate, 20% protein, and 20% fat (high-carbohydrate diet). The results, according to the authors, suggest that (1) a hypocaloric diet providing a high percentage of natural protein can improve insulin sensitivity and (2) conversely, a hypocaloric high-carbohydrate diet decreases insulin sensitivity and is unable to spare muscle tissue. In another study it was shown that a protein intake as high as four times the recommended RDA (3.3 g=kg of bodyweight per day versus RDA of 0.8 g=kg=day) resulted in significantly increased protein synthesis even when compared to a protein intake that was almost twice the RDA.147 This observation that a protein intake of approximately four times the RDA, in combination with weight training, can promote greater muscle size gains than the same training with a diet containing what is considered by many to be more than adequate protein is in tune with what many bodybuilders and other weight-training athletes believe. The effects of two levels of protein intake (1.5 or 2.5 g=kg=day) on muscle performance and energy metabolism were studied in humans submitted to repeated daily sessions of prolonged exercise at moderate altitude.148 The study showed that the higher level of protein intake greatly minimized the exercise-induced decrease in serum BCAAs. Several studies and papers in the last decade have championed the need for protein above the RDA for athletes and those involved in physical exercise. The reasons are many and involve those athletes in strength and endurance sports, and those wanting to increase muscle mass. Many support the idea that the protein needs of athletes are substantially higher than those of sedentary subjects because of the oxidation of amino acids during exercise and gluconeogenesis, as well as the retention of nitrogen during periods of muscle building.149–151 Intense muscular activity increases both protein catabolism and protein utilization as an energy source.152,153 Thus, a highprotein diet may decrease the catabolic effects of exercise by several means—including the use of dietary protein as an energy substrate thus decreasing the catabolism of endogenous protein during exercise. Athletes have for years maintained that a high-protein diet is essential for maximizing lean body mass. And even though there have been attempts to discourage it, the popularity of high-protein diets has not waned. Athletes seem to feel intuitively that they need higher levels of protein than the average sedentary person. This intuitive feeling is backed up by their claims of the ergogenic effects of high-protein diets. Are these effects simply psychological? Not according to studies that have shown the anabolic effects of increased dietary protein intake. For example, in one study done in rats,154 dietary energy had no identifiable influence on muscle growth. In contrast, increased dietary protein appeared to Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 167 7.11.2007 5:32pm Compositor Name: BMani 167 stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, as well as increasing insulin and free T3 levels. Supplements that may work through a placebo effect but have no intrinsic effects, although perhaps popular for a while, eventually fall by the wayside, and are abandoned by the majority. High-protein diets are used because they work. The use of protein supplements is also popular because of their effectiveness above and beyond a whole-food, high-protein diet. Although there has been some concern about the effects of a high dietary protein intake on the kidney, there seems to be no basis for these concerns in healthy individuals.141,155 In fact, some animal studies have pointed to a beneficial effect of high-protein diets on kidney function.156 A recent review of some of the issues of high dietary protein intake in humans suggested that the maximum protein intake based on bodily needs, weight-control evidence, and protein toxicity avoidance would be approximately 25% of energy requirements at approximately 2–2.5 g=kg of weight corresponding to 176 g protein per day for an 80 kg individual on a 12,000 kJ=day or 3,000 calories=day diet.157 The authors also stated that this is well below the theoretical maximum safe intake range for an 80 kg person (285–365 g=day). There has also been some concern about the adverse effects of high-protein diets on the serum lipid profile and on blood pressure. However, it would seem that these concerns also have little basis in fact. In one study, a diet higher in lean animal protein, including beef, was found to result in more favorable HDL (‘‘good’’ cholesterol) and LDL (‘‘bad’’ cholesterol) levels.158 The study involved 10 moderately hypercholesterolemic subjects (6 women, 4 men). They were randomly allocated to isocaloric high- or low-protein diets for 4–5 weeks, after which they switched over to the other. Protein provided either 23% or 11% of energy intake; carbohydrate provided 65% or 53%; and fats accounted for 24%. During the high-protein diet, mean fasting plasma total cholesterol, LDL, and triglycerides were significantly lower, HDL was raised by 12%, and the ratio of LDL to HDL consistently decreased. Other studies by the same group of researchers found that increased levels of dietary proteins resulted in beneficial changes in blood lipids in both normolipemic and dyslipemic humans.159,160 In addition, recent studies have found that increasing dietary plant and animal protein lowers blood pressure in hypertensive persons.161,162 Intense muscular activity increases protein catabolism (breakdown) and protein use as an energy source. The less protein available, the less muscle you are going to be able to build. A high-protein diet protects the protein to be turned into muscle by, among other things, providing another energy source for use during exercise. The body will burn this protein instead of the protein inside the muscle cells. In fact, studies have shown that the anabolic effects of intense training are increased by a highprotein diet. When intensity of effort is at its maximum and stimulates an adaptive, muscleproducing response, protein needs accelerate to provide for that increased muscle mass. It is also well known that a high-protein diet is necessary for anabolic steroids to have full effect. Once a certain threshold of work intensity is crossed, dietary protein becomes essential in maximizing the anabolic effects of exercise. Exercise performed under that threshold, however, may have little anabolic effect and may not require increased protein. As a result, while serious athletes can benefit from increased protein, other athletes who do not undergo similar, rigorous training may not. Whether or not you need to supplement your diet with extra protein depends on your goals. For those of us who do not have to worry about gaining some fat along with the muscle (traditionally athletes in sports without weight classes or those in the heavyweight classes in sports that do, where mass is an advantage, e.g., in the shot put and in weight lifting), high-caloric diets will usually supply all the protein you need, provided you include plenty of meat, fish, eggs, and dairy products. With the increased caloric intake and including high-quality protein foods, you will get your extra protein at the dinner table without thinking about it. Most athletes, however, need the economy of maximizing lean body mass and minimizing body fat. These athletes, both competitive and recreational, are on a moderate or at times a low caloric Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 168 43803_C004 Final Proof page 168 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge intake. In order to increase their protein intake, they need to plan their diets carefully and in many cases use protein supplements since they can not calorically afford to eat food in the volume necessary to get enough protein. On the average, I recommend a minimum of 1 g of high-quality protein per pound of bodyweight (2.2 g=kg) every day for any person involved in competitive or intense recreational sports who wants to maximize lean body mass but does not wish to gain weight or have excessive muscle hypertrophy. This would apply to athletes who wish to stay in a certain competitive weight class or those involved in endurance events. However, athletes involved in strength events such as the Olympic field and sprint events; those in football or hockey; or weight lifters, power-lifters, and bodybuilders, may need even more quantity than my recommendation to maximize body composition and athletic performance. In those attempting to minimize body fat and thus maximize body composition, for example in sports with weight classes and in bodybuilding, it is possible that as they diet, protein may well make up over 50% of their daily caloric intake. This is because if you are trying to lose weight or body fat it is important to keep your dietary protein levels high. The body transforms and oxidizes more protein on a calorie deficient diet than it would in a diet that has adequate calories. The larger the body muscle mass, the more transamination of amino acids occurs to fulfill energy needs. Thus, for those wishing to lose weight but maintain or even increase lean body mass in specific skeletal muscles, I recommend at least 1.5 g of high-quality protein per pound of bodyweight. The reduction in calories needed to lose weight should be at the expense of the fats and carbohydrates, not protein. PROTEIN (NITROGEN) BALANCE Protein balance is a function of intake relative to output (utilization and loss; see Figure 4.1). Intake comes from dietary sources and the recycling of proteins, especially of cellular protein sloughed off in the intestinal lumen, while losses occur due to the use of the carbon skeletons of amino acids with the excretion of nitrogenous compounds, as well as protein losses in feces, urine, sweat, and the integumentary system.163 Nitrogen balance in adult Sweat, sebum, nails, hair, skin Proteins Nitrogenous compounds Secretion Carbon skeleton Urine Amino acids NH3 Diet Intestinal lumen Urea Colon Feces N intake = N losses FIGURE 4.1 Metabolic pathways determining nitrogen balance. Nitrogen balance is the result of N intake (from the diet) and N losses, which consists of N recovered in urine and feces and miscellaneous losses. (From Tomé, D. and Bos, C., Dietary protein and nitrogen utilization, J. Nutr., 130(7): 1868S–1873S, 2000.) Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 169 7.11.2007 5:32pm Compositor Name: BMani 169 Body proteins are in a constant state of flux, with both protein degradation and protein synthesis constantly going on. Normally, these two processes are equal with no net loss or net gain of protein taking place. Protein intake usually equals protein lost. However, if protein synthesis (anabolism) is greater than protein degradation (catabolism), the overall result is anabolic with a net increase in body protein. If protein degradation is greater than protein synthesis, the overall result is catabolic with a net decrease in body protein. Although the end result may be the same, protein balance and protein turnover are not synonymous. Protein turnover is a measure of the rate of protein metabolism, and involves both protein synthesis and catabolism. Protein catabolism is necessary to eliminate proteins that are no longer required by the cell, or that have become dysfunctional, and to provide substrates for energy production. Thus, degradation of proteins involves enzymes, hormones, and structural as well as contractile proteins. Protein and amino acids are lost from the body through the urine; feces; sweat; seminal, vaginal, buccal, and menstrual fluids; the desquamation of skin and mucosal areas; and hair and nail growth and loss. Nitrogen derived from amino acids is mainly lost in urine and feces and from sweat. Unlike the fats and carbohydrates that can be stored in the form of triglycerides and glycogen, there is no storage form of protein or amino acids. All the protein and amino acids (except for a very small amount of free amino acids that make up the plasma and intracellular amino acid pool) serve either a structural or metabolic function. Excess amino acids from protein are transaminated, and nonnitrogenous portion of the molecule is transformed into glucose and used directly or converted into fat or glycogen. The unneeded nitrogen is converted to urea and excreted in the urine. The larger the body muscle mass, the more transamination of amino acids occurs to fulfill energy needs. Each kilocalorie needed for basal metabolism leads to the excretion of 1–1.3 mg of urinary nitrogen. For the same reason, nitrogen excretion increases during exercise and heavy work. Urea accounts for over 80% of urinary nitrogen. The remaining nitrogen is excreted as creatinine, porphyrins, and other nitrogen-containing compounds. Thus total urine loss of nitrogen ¼ urinary urea nitrogen (mg=dL) 3 daily volume (dL)=0.8. Urinary nitrogen is related to the resting metabolic rate. Fecal and skin losses account for a large proportion of nitrogen loss from the body (about 40%) in normal circumstances. The magnitude of these losses, however, varies in disease states. Thus, measurement of urinary nitrogen often provides a useful index of daily nitrogen requirement. Minimal nitrogen loss (in grams per day) from a 70 kg person on a diet that is nitrogen free but energy adequate approximates 1.9–3.1 in urine, 0.7–2.5 in stool, and 0.3 from skin for a mean total loss of 4.4 g=day and a maximum total loss of 5.9 g=day. Equivalent protein loss can be calculated by multiplying nitrogen loss by 6.25 so that mean total loss by metabolism of protein is 4.4 3 6.25 or 27.5 g=day or about 0.4 g=kg body weight for a 70 kg person and maximum total loss by metabolism or protein is 5.9 3 6.25 or 36.9 g=day or about 0.5 g=kg body weight for a 70 kg person. The recommended protein allowance for adults varies from 0.6 to 0.9 g=kg to allow for a margin of safety. This protein allowance is raised considerably if low-quality protein is the main source of protein. Low-quality proteins including certain vegetable proteins do not support growth as well as protein from milk, eggs, or meat. The differences in the nutritional value of protein are largely due to the higher content of essential amino acids in animal proteins and to differences in digestibility. Protein requirements are highest during growth spurts and it is during these times that protein deficiency is most harmful. Protein requirements are raised if other energy sources are not ingested in adequate amounts— as is sometimes seen in athletes and other people who are dieting to decrease their body-fat levels. Amino acids ingested without other energy sources are not efficiently incorporated into protein partly because of the energy lost during amino acid metabolism. Moreover, incorporation of each amino acid molecule into peptides requires three high-energy phosphate bonds. Consequently, excess of dietary energy over basal needs improves the efficiency of nitrogen utilization. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 170 EFFECT 43803_C004 Final Proof page 170 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge OF DIETARY PROTEIN ON PROTEIN METABOLISM The dietary history of the individual is an important factor affecting protein turnover. This would include not only the long-term effects of the previous diet, but also the short-term effects of recent prior meals. Anabolic and catabolic activities can be influenced by the amount as well as the frequency and timing of protein intake. The overall and specific metabolic effects of diets containing inadequate amounts of protein have been extensively studied.164–174 Following short-term restriction of dietary protein, there is a decrease in the rates of whole-body and organ protein synthesis and turnover.175–179 Amino acids are catabolized less and used more as precursors for endogenous compounds such as nucleic acids and creatine and for necessary protein synthesis. High levels of protein in the diet usually result in increased hepatic amino acid catabolism, especially in the beginning when there is a dramatic increase in the level of dietary protein or individual amino acids. The liver, because of its ability to adapt intracellular metabolizing enzyme180182 and transport processes,183 has a major role in maintaining serum amino acid levels within a certain physiological range.175 However, even with this increased catabolic effect, higher-protein diets or diets modified for their amino acid content, result in higher serum and tissue levels of amino acids under a variety of conditions.184–189 Supplementation with amino or keto acids also usually results in increased serum and tissue levels.72,190–192 These higher tissue levels translate into increased protein synthesis as shown by a number of studies using both oral amino acids and amino acid infusions.58,60,193–196 For example, one study by using femoral arteriovenous catheterization and quadriceps muscle biopsies, measured muscleprotein synthesis and breakdown, and amino acid transport during intravenous infusion of an AAM in young and elderly subjects.197 Peripheral amino acid infusion significantly increased amino acid delivery to the leg, amino acid transport, and muscle protein synthesis, independently of the age of the volunteers. Despite no change in protein breakdown during amino acid infusion, a positive net balance of amino acids across the muscle was achieved. The same team of researchers published another study several years later and determined that a bolus oral ingestion of amino acids also produce a similar response in young and elderly individuals.198 For more information on the effects of oral supplementation of amino acids please see later under the individual amino acids. Altered intakes of protein and amino acids modulate the rates of the major systems (protein synthesis, protein degradation, and amino acid oxidation) responsible for the maintenance of organ and whole-body protein and amino acid homeostasis. Switching from a low-protein diet to a high-protein diet or vice versa, results in adaptive responses that reflect the metabolic state of the organism. Protein deficiency results in decreased proteolysis and decreased protein synthesis, changes that spare amino acids. Switching from a low-protein to a high-protein diet results in an adaptation phase in which there is a high rate of protein synthesis (a catch-up phase) and increased amino acid oxidation.177,199,200 Similarly, supplementing the diet with certain amino acids can also increase protein synthesis and amino acid oxidation.72,201 A review looked at the mechanisms and nutritional significance of metabolic responses to altered intakes of protein and amino acids, with reference to nutritional adaptation in humans.176 They concluded that for oxidation, amino acid availability is a primary determinant and protein synthesis is affected particularly at the initiation phase. Thus, the immediate response to a change in dietary protein levels is often the opposite that occurs once the body has adapted to the change. For example, the rate of degradation of myofibrillar protein in skeletal muscle rises just after the beginning of fasting,202 but decreases once the body is accustomed to the dietary change (see below). Studies have shown that the body feels the amino acid pinch after only two protein-restricted meals. In one study,203 the mechanism governing short-term adaptation to dietary protein restriction Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 171 7.11.2007 5:32pm Compositor Name: BMani 171 was investigated in nine normal adults by measuring their metabolic response to a standard mixed meal, first while they were adapted to a conventional, high-protein diet (day 1) and then again after they had eaten two low-protein meals (day 2). Urea appearance (measured as the sum of its urinary excretion and the change in body urea pool size), body retention of 15N-alanine included in each test meal, and whole-body protein turnover were calculated over the 9 h following meal consumption on each day. Postprandial urea nitrogen appearance decreased on day 2. Whole-body nitrogen flux, protein synthesis, and protein breakdown all decreased significantly on day two. The authors concluded that short-term metabolic adaptation occurs within two meals of reduced protein intake. The mechanism appears not to involve selectively an increased first-pass retention of dietary amino acids, but rather a general reduction in fed-state whole-body protein breakdown. It was also shown in the study that inadequate protein intake for as few as two meals induces a prompt compensatory increase in body protein retention when protein is returned to the diet. The authors of another study 204 relate: ‘‘The argument here is that body protein is lost in the post-absorptive state and that the extent of these losses is likely to be variable according to the extent to which changes in protein synthesis and degradation in response to fasting is conditioned by dietary protein intake.’’ Thus, if there is an induction of protein-oxidizing enzymes and a high rate of protein turnover as dietary protein intake rises and if this effect persists for an appreciable period of time when the diet is changed, after a period on a high-protein diet, a switch to a low-protein diet ‘‘should result in a marked negative balance with insufficient fed-state gain for balance whilst the high post-absorptive losses persist.’’ AMINO ACID METABOLISM Amino acids are not stored in the body other than as integral parts of protein. However, there is a small amount of extracellular and intracellular free amino acids that is referred to as the free amino acid pool. This free amino acid pool in muscle is labile, that is, constantly changing and is affected by the quantity and quality of dietary protein which in turn affects protein metabolism.187 During exercise, most protein synthesis is suppressed in muscle, although the synthesis of certain proteins remains unchanged or even increases. The general suppression of protein synthesis in muscle leaves much of the free amino acid pool unused. As a result of the suppression and an increase in muscle protein catabolism, an increased pool of available free amino acids is created. The main use of free amino acids is connected with the energy requirement of muscular activity, through the oxidation of BCAA and the use of alanine in gluconeogenesis (see later).205 The increases in the free amino acid pool, in the rate of the glucose–alanine cycle, and in the use of amino acids in the liver are stimulated by an increased level of glucocorticoids and a decreased level of insulin during exercise. During recovery after exercise, the use of amino acids for adaptive protein synthesis is intensified. Should any amino acids required for the synthesis of new contractile proteins be limiting in availability, the training response may suffer as a result. Moreover, as the dietary provision of amino acids is always inefficient,206 the need for a continual and ready supply of amino acids may be of particular importance to those individuals hoping to maximize training-induced lean tissue gains. Fasting causes energy levels to drop. Catabolism of muscle proteins is enhanced at first, and free amino acids such as alanine and glutamine are liberated which can be metabolized for energy via the TCA cycle or be used to form glucose via gluconeogenesis. In the postabsorptive state, amino acids are released from the periphery to provide precursors for protein synthesis in the splanchnic organs. In skin, for instance, integrity is maintained in the absence of amino acid intake by using amino acids resulting from the net breakdown of muscle protein as precursors for synthesis. The importance of the prevailing amino acid concentration has been suggested by studies showing that increased amino acid availability alone may augment skeletal muscle protein synthesis Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 172 43803_C004 Final Proof page 172 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge in healthy men.207,208 It appears that increasing amino acid availability is a major component in the resulting increase seen in protein synthesis. The hormonal response to protein and amino acids and the direct effects of these substrates constitute the two mechanisms by which feeding influences amino acid turnover and oxidation. In rats, it has been shown that the anabolic drive from dietary proteins and amino acids is mediated in part by their effects on insulin, thyroid hormone, and IGF-1.209 In humans, an increase seen in insulin secretion, along with increased levels of amino acids, has been shown to result in an overall increase in protein deposition. The hyperinsulinemia decreased proteolysis but did not stimulate protein synthesis, and the hyperaminoacidemia stimulated protein synthesis but did not suppress endogenous proteolysis.210,211 A study found that amino acids fed alone, which exert a minor stimulus to insulin secretion, increase plasma amino acid levels, markedly stimulate protein synthesis and oxidation, and inhibit protein degradation in humans.212 The authors of this study concluded that the feeding response of protein synthesis, degradation, and amino acid oxidation reflects the combined impact of insulin, inhibiting degradation, and tissue amino acids both inhibiting degradation and stimulating synthesis and oxidation. In another study, the authors concluded that in the human adult, amino acid catabolism is necessarily the major fate of dietary protein at low concentrations, with the shift to deposition requiring increasing dietary protein concentrations.213 Thus, with lower protein intake the stimulus for protein synthesis is depressed while with higher protein intakes there is an increase in protein synthesis. In this study, the regulatory influence of dietary amino acids on anabolic processes only occurred with higher protein intakes. It would appear that the anabolic drive induced by low-protein (0.36 g=kg=day) and medium-protein (0.77 g=kg=day) diets was inadequate. Changes in dietary protein or amino acid intake may alter transport of certain neutral amino acids into skeletal muscle via changes in plasma amino acid pools. In one study, amino acid transport systems A and L, which transfer preferentially small neutral amino acids and large neutral amino acids, respectively, were studied in the isolated soleus muscle.214 Selective differences were observed in transport by skeletal muscle of model amino acids for the A and L systems: increased transport resulting from various stimuli was limited to the model for the A system, and transport of both models was depressed with mixtures containing physiological levels of amino acids. THE DIETARY PROTEIN PARADOX—THE PROBABLE NEED FOR PROTEIN AND AMINO ACID SUPPLEMENTS EVEN IN DIETS HIGH IN DIETARY PROTEIN However, simply increasing dietary protein through the use of increased amounts of wholeprotein-containing foods may not be enough. Some studies have suggested that increased dietary intake of protein may inhibit growth. In one study, 28-day-old male Sprague–Dawley rats were fed, either ad libitum or in restricted amounts, isoenergetic diets containing 2%, 5%, 10%, 15%, 25%, or 50% lactalbumin protein and 5%, 11.9%, or 21.1% fat for 8 weeks, and were then killed.215 Weekly food consumption, body weight, terminal weight, body water and lipid, and liver weight, DNA, RNA, protein, and lipid were measured. The growth rate increased progressively with each increase in the level of dietary protein up to 25% protein and then declined. Growth was also accelerated by a high-fat diet but was retarded by restriction of energy intake. Total body lipid correlated directly with the level of fat in the diet. Multiple regression analysis showed that the maximum rate of weight gain of 58.8 g=week occurred when the diet contained 23% protein. Growth rate declined when the diet contained a higher protein level. More recently, Moundras et al.216 found that increasing the dietary protein level in rats led to a reduced availability of some amino acids for peripheral tissues. This was accompanied by a depressed weight gain in animals fed the highest protein diet (60% casein). The authors suggested that the depression in growth rate might be due to energy wastage caused by catabolism of excess Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge Dietary Protein and Amino Acids 43803_C004 Final Proof page 173 7.11.2007 5:32pm Compositor Name: BMani 173 amino acids, the reduction in the availability of certain amino acids, or decreased insulinemia in rats fed the high-protein diets. In this study, set up to evaluate the effect of changes in dietary protein level on overall availability of amino acids for tissues, rats were adapted to diets containing various concentrations of casein (7.5%, 15%, 30%, and 60%) and were sampled either during the postprandial or postabsorptive period. In rats fed the protein-deficient diet, glucogenic amino acids (except threonine) tended to accumulate in plasma, liver, and muscles. In rats fed high-protein diets, the hepatic balance of glucogenic amino acids was markedly enhanced and their liver concentrations were consistently depressed. This response was the result of a marked induction of amino acid catabolism (a 45-fold increase of liver threonine–serine dehydratase activity was observed with the 60% casein diet). The muscle concentrations of threonine, serine, and glycine underwent changes parallel to plasma and liver concentrations, and a significant reduction of glutamine was observed. During the postabsorptive period, adaptation to high-protein diets resulted in a sustained catabolism of most glucogenic amino acids, which accentuated the drop in their concentrations (especially threonine) in all the compartments studied. The time course of metabolic adaptation from a 60% to a 15% casein diet was also investigated. Adaptation of alanine and glutamine metabolism was rapid, whereas that of threonine, serine, and glycine was delayed and required 7–11 days. This was paralleled by a relatively slow decay of liver threonine–serine dehydratase activity in contrast to the rapid adaptation of pyruvate kinase activity after refeeding a high-carbohydrate diet. Thus in a high-protein diet in which the protein is obtained from high-protein whole foods, the decreased availability of amino acids, specifically threonine, glycine, and serine as well as glutamine and alanine, may result in a decrease in protein synthesis. The use of these amino acids in a protein supplement may correct the relative deficiency and increase the amount and efficiency of protein synthesis.217 It also seems that some amino acid deficiencies may result from the use of different protein foods in both isocaloric and hypocaloric diets. For example, in one study the substitution of soybean protein for casein in a high-protein diet resulted in a taurine deficiency in cats.218 When a caseinbased diet containing either 25% or 50% protein was given to cats for 6 weeks, no difference in plasma taurine concentration was observed; however, substituting soybean protein for casein resulted in a significant decrease in plasma taurine concentration of cats in the 50% soybean protein group, but not in the 25% soybean protein group. In a second part of this study, when the food intake of cats was limited, cats fed 60% soybean protein or casein diets had significantly lower plasma taurine concentrations than cats fed a 30% casein diet, with the 60% soybean protein diet causing the greater decrease. In high-protein diets there is an increased intake of BCAAs which have been shown to stimulate protein synthesis (see earlier or later). However, with a high-protein diet the BCAAs are quickly catabolized to alanine and glutamine in muscles, which in turn are used for gluconeogenesis and by the gut and immune system respectively (see below). Even with the formation of glutamine from BCAAs, a feature of high-protein, whole-food diets is the decrease in hepatic, plasma, and muscle glutamine concentrations. This decrease may have important implications for effective protein synthesis. Increasing the endogenous levels of glutamine by the use of exogenous glutamine might have important effects on increasing protein synthesis in high-protein, whole-food diets. Low levels of alanine in between high-protein meals results in low levels of plasma alanine.216 This decrease in plasma alanine may result in an increase in protein catabolism and decrease in protein synthesis. Supplemental alanine may be important for the regulation of protein metabolism and increasing the anabolic drive. High-protein diets may also result in decreased levels of threonine, glycine, and serine in liver, plasma, and muscle (see later). Since threonine is an essential amino acid, its decrease might cause severe metabolic alterations and significant decreases in protein synthesis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 174 43803_C004 Final Proof page 174 7.11.2007 5:32pm Compositor Name: BMani Amino Acids and Proteins for the Athlete: The Anabolic Edge PROTEIN NEEDS IN CALORIE-RESTRICTED DIETS Because of the increased oxidation of endogenous protein during weight loss, an increased level of exogenous protein is needed to minimize the autophagy (breakdown of muscle protein) that occurs. In a randomized trial in 17 healthy obese women, a diet containing 1.5 g protein per kilogram ideal body weight was found to result in significantly better protein sparing than an isocaloric diet providing only 0.8 g protein per kilogram ideal body weight.219 Patients lost weight at the same rate on the two diets, but since there was less nitrogen loss on the diet without carbohydrate, it can be assumed that more fat loss occurred than on the diet where carbohydrate replaced some of the protein. Bodybuilders who are dieting for a contest and undergoing glycogen depletion would oxidize more protein for energy and thus likely need even higher dietary protein to prevent muscle catabolism. Also, since it has been shown that individuals with lower glycogen stores oxidized more than twice the protein than those with high initial stores, this would further increase the need for dietary protein.220 Bodybuilders preparing for competition cut calories to lose body fat. In order to maintain and perhaps even build muscle tissue, dietary protein must be increased even further than with their normally high-protein diets. In addition, as has been stated earlier, those on calorie-restricted diets and those with increased muscle mass need more protein in their diets than normal. In order to increase dietary protein but minimize the caloric increase, foods must be chosen that are low in energy and high in protein. 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Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C004 Final Proof page 184 7.11.2007 5:32pm Compositor Name: BMani Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 185 29.10.2007 7:53pm Compositor Name: JGanesan Foods versus Protein 5 Protein and Amino Acid Supplements The source of dietary protein can be from whole foods, especially eggs, meat, fish, soy, and dairy products; whole-protein supplements that usually are inexpensive and contain one or more of soybean, milk, and egg protein; hydrolyzed protein containing variable amounts of di-, tri-, and polypeptides; and amino acid mixtures (AAM). The usual consensus is that there are no valid scientific or medical studies to show that supplements of intact protein have an anabolic advantage over high-quality protein foods. The advantages usually cited for the use of whole-protein supplements include: 1. 2. 3. 4. Convenience of preparation and storage, and long shelf life Replacement of dietary protein for those wishing to decrease dietary fat Ability to raise protein intake by those who wish to minimize caloric intake Increasing dietary protein by those who cannot eat the volume of food necessary to insure adequate or increased protein intake 5. In some cases the cost of protein supplements is lower than corresponding high-protein foods While these are valid points, protein supplements have other distinct advantages over whole-food protein in hypocaloric, isocaloric, and hypercaloric diets. For example, several studies involving Refit (a milk powder containing about 90% proteins and 5% mineral salts—Ca, P, K, Na) have shown the ergogenic effects of this protein supplement.1,2 In one study, nine male and eight female, top Olympic athletes were given 1.2–1.5 g of milk protein daily, per kilogram of body weight, during a period of 6 months.3 The milk proteins were consumed in addition to 2.2–2.5 g of proteins per kilogram body weight in their diet. A control group, with the same number of athletes and the same sports, was fed on the same diet, but without extra addition of milk protein. The effects of the addition of extra milk proteins were monitored by estimating some parameters such as lean body mass, fat mass, muscle strength, protein and lipid composition in the blood serum, calcium metabolism, urinary mucoproteins, liver and kidney functional tests. All the athletes had been under medical supervision during the experiment and no side effects were registered. The results indicated that extra milk proteins significantly improved physiological condition and led to better sports performance, even when compared to the controls. In another study, 66 Romanian Olympic endurance athletes (30 kayak canoeing, 36 rowing—45 males and 21 females) participated in a trial to determine the possible biological effects of an isolated soy-protein supplement given at 1.5 g=kg=day, for 8 weeks.4 A control group was used that did not receive the supplemental protein. The soy-protein supplementation resulted in significant increases of lean body mass and strength. The same author also found significant effects from the use of a combination vitamin supplement compound that contained amino acids.5 A study examined the effects of food supplements designed to foster muscular gains when combined with weight training.6 The study included 28 weight-training subjects, who took one of 185 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 186 29.10.2007 7:53pm Compositor Name: JGanesan 186 Amino Acids and Proteins for the Athlete: The Anabolic Edge the three supplements daily: 190 g of maltodextrin, a carbohydrate source that acted as a source of calories only (about 760 calories); Gainer’s Fuel 1000, a weight-gain powder providing both calories and protein (just over 1400 calories and 60 g of protein per day); and Phosphagain, a powder advertised to help increase muscles without gaining excess body fat, providing less calories and more protein in the form of whole-food protein and high levels of creatine monohydrate and of certain amino acids such as glutamine and taurine (about 600 calories and 80 g of whole-food protein and amino acids per day). Results showed that those on either Phosphagain or Gainer’s Fuel gained more lean mass compared to those consuming maltodextrin, showing that increased dietary protein and other nutrients resulted in a superior gain in muscle as compared to those who simply took in the equivalent amount of calories. In addition, those on Phosphagain gained more lean body mass and less fat than those on Gainer’s Fuel. Why the difference? Phosphagain contains nutrients not found in the Gainer’s Fuel product. These nutrients include creatine monohydrate, taurine, and L-glutamine. These three nutrients may have, along with the differences in the macronutrients, made the difference. For more information on them see the relevant sections in this book. Besides having more protein (due mainly to the large amounts of taurine and glutamine), Phosphagain also had a much lower carbohydrate content—69 g versus 290 g per daily serving. Both had low levels of fat. The increased carbohydrate and subsequent calorie content is likely responsible in the increased fat gain seen with Gainer’s Fuel, a problem that is also seen when whole foods are used to increase protein intake and other nutrients. Both Gainer’s Fuel and Phosphagain and others like Met-Rx, Hot Stuff, and a host of others contain a large number of nutrients and can be used as meal replacements or as a source of extra nutrients beyond the three squares a day. Some of these products are advertised as weight gainers while others are more specific and claim to be lean body mass weight gainers. While the difference in products may not be as extreme as the advertiser’s claim, there is some validity to the differentiation of the two product types and their effect on fat accretion and gains in lean body mass. The meal replacement products, whether for weight loss or for weight gain, provide the standard macro- and micronutrients at different calorie levels. They may be convenient and either more or less costly than whole foods one can get at the supermarket, but as an all-in-one package they are usually more convenient and provide better nutrition than many of us get with our junk food meals and calorie-full but nutrient-deficient snacks. But on the whole, if one is conscientious about what to buy and eat and is willing to put in the time and energy, one can do as well or better by just buying the whole foods and planning one’s own diet for weight gain or weight loss. However, some of the newer complete nutrient supplements, such as Phosphagain, definitely have an edge over even the most meticulously prepared diets. In order to get pharmacological levels of some of the nutrients present in these products, you would have to consume an unrealistic amount of certain foods resulting in a much larger than desired calorie intake and increase in body fat. HYDROLYSATES—COMPARISON TO WHOLE PROTEIN AND INTACT WHOLE-PROTEIN SUPPLEMENTS Special processing can make certain protein supplements more effective and less problematic than whole foods. One example is the whey protein presently available on the market. About 20% of the protein in milk (milk has about 6.25% protein) is whey protein. In the course of production of cheese, quargs, and casein a lot of whey results. At one time whey was considered waste and was often dumped into ponds. Whey is rich in BCAAs, lactose, minerals, vitamins, and contains lactalbumin (similar to serum albumin) and traces of fat. Milk and milk products (including whey) can cause intestinal bloating and gas in certain individuals. This is due to the lactose content and the fact that soon after birth, the production of the enzyme lactase, responsible for hydrolysis of lactose into galactose and glucose (its constituent monosaccharides), is reduced. By age 3–7 in most humans the ability to digest lactose is Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 187 29.10.2007 7:53pm Compositor Name: JGanesan Protein Foods versus Protein and Amino Acid Supplements 187 substantially decreased.7 Because of the possibility of lactose intolerance the manufacturers of some whey proteins remove the lactose from the whey. In addition, most whey proteins on the market are low in fat since the fat is extracted through centrifugation. In the last decade it has been shown that whey and whey protein may possess special properties not seen in other proteins including casein, the other milk protein.8,9 In one study, a whey protein diet appeared to enhance the liver and heart glutathione concentration in aging mice and to increase longevity.10 Undenatured whey protein has also been shown to enhance the immune system,11–13 and to lower serum cholesterol14 as compared to casein and soy proteins. Conversely, a study has shown that casein, but not whey or soy protein, has antimutagenic effects15 and may thus be protective against certain cancer-producing genotoxic compounds. A study done this year compared the effects of four isoenergetic and isonitrogenous diets on the nitrogen utilization, total serum protein concentration, and serum amino acid profile in starved rats at weaning.16 These diets differed only in the molecular form of two milk proteins (whey protein and casein), which were either native or partly hydrolyzed. Nitrogen retention was higher in the two groups of rats given the protein-hydrolysate-based diets compared with those given the intact-protein-based diets. This was associated with a lower urinary nitrogen excretion in rats given the whey protein hydrolysate and the casein-hydrolysate diets. Despite this fact, the serum amino acid pattern of rats given the hydrolyzed protein diet was very similar to that of those given the corresponding native protein diet. In conclusion, the study proved that enzymic hydrolysates from milk proteins have equivalent effects to native proteins in recovery after starvation in rats at weaning, on nitrogen absorption, total serum protein concentration, and serum amino acid profile, and even give higher nitrogen retention. The study did not show any harmful effect in using protein hydrolysates instead of native proteins. On the other hand, another study using the same four nitrogenous diets, done by the same authors, evaluated the effect of the molecular form of dietary protein (native or enzymatically hydrolyzed) on the total serum protein concentrations and the serum amino acid profile of growing rats at weaning.17 In this study, differences in the serum amino acid profiles exclusively related to the amino acid composition of the protein were observed (casein or whey proteins), but differences due to their molecular form were not observed. The authors concluded that the use of enzymatic hydrolysates of whey proteins and casein has the same effects as their native proteins on nitrogen intake, body weight gain, and serum amino acid profile of growing rats at weaning. The higher nitrogen retention seen in the previous study might have been due to the poor nutritional state of the starved rats. It is possible that some athletes on low-calorie, fat-reducing diets might also have a higher nitrogen retention with the use of enzymatic hydrolysates of whey proteins and casein. An example would be competitive bodybuilders who are often on semistarvation diets when preparing for a competition. WHEY PROTEIN HYDROLYSATES AND THE ATHLETE Whey protein, because of its increased bioavailability and solubility, and its higher percentage content of BCAAs, is felt to be a superior form of protein supplementation for athletes. An early study pointed out the possible ergogenic effects of whey protein.18 Over the past several years a number of improved whey protein concentrates have become available at a reasonable, although still relatively expensive price. The predigested whey protein powder that is made by low-temperature, high-speed sonic drying, is one of the best protein powders available. These protein powders contain protein that has not been denatured due to hightemperature preparations (as are many of the protein powders now on the market). Some of this whey protein has the lactose removed through the use of micropore membrane filters and ionexchange columns and therefore is better tolerated than many milk (either casein or whey) proteins that contain variable amounts of lactose. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 188 29.10.2007 7:53pm Compositor Name: JGanesan 188 Amino Acids and Proteins for the Athlete: The Anabolic Edge Protein powders with a high biological value (BV) and improved solubility may be worth the extra bucks because they are so convenient to use and you do not have to take as much to significantly increase your protein intake. Another advantage of the hydrolyzed ion exchange whey proteins is that they are much more soluble than the others and thus can be prepared with a minimum of fuss. In addition to those who are lactose-intolerant, the use of whey protein supplements allows these people to use this quality source of protein. In diets that are low calorie and relatively protein deficient, whey protein may offer some advantages over whole-food proteins and other kinds of protein supplements. We will discuss some of these points in more detail later. BV OF WHEY HYDROLYSATES In a study it was shown that whey protein hydrolysate has a significantly higher BV than the whole protein.19 This corroborates an earlier study that found a higher nitrogen balance in rats fed a diet containing whey protein hydrolysate compared with the native protein.20 In this earlier study the effects of alimentary whey proteins given, as whole proteins, controlledtrypsin and chymotrypsin hydrolysate oligopeptides (WPH), or a free amino acid mixture (AAM), on growth, nitrogen retention, and steatorrhea were assessed in 24 Wistar rats after 72 h of starvation and 24–96 h of realimentation, and also in 24 controls. The three diets had the same caloric, nitrogen, vitamin, and mineral contents. Rats had free access to the liquid diets. Only rats which ate the whole diet (90 calories) were included in the study. No differences in steatorrhea and fecal nitrogen were observed. The absorption rate was over 95% on the three diets. In contrast, weight gain was statistically better on WPH than on whole proteins or on AAM. This was associated with statistically higher nitrogen retention at all time periods studied, which was a result of a significant lower nitrogen urinary excretion. The authors concluded that this better growth was a result of a better protein synthesis and lower ureagenesis. WHEY PROTEIN AND THE IMMUNE SYSTEM Arginine, the BCAAs, and glutamine are the most important amino acids involved in immune system function.21 Whey protein is naturally high in BCAA, and certain whey protein powders currently available are said to be fortified with glutamine. Glutamine supplementation may also be beneficial in the treatment of athletes affected by exercise-induced stress and overtraining. EFFECTS OF WHEY PROTEIN ON GLUTATHIONE LEVELS Glutathione is a peptide that contains one amino acid residue each of glutamic acid, cysteine, and glycine. Glutathione occurs widely in plant and animal tissues, and plays a major role in protecting skeletal muscle and other body tissues from oxidative damage.22 The author of a review stresses the importance of an augmented antioxidant defense system and suggests that all athletes, especially the ‘‘weekened’’ ones, regularly or occasionally ingest foods rich in antioxidants.23 The muscular injury models in which reactive oxygen species are supposed to play a role are ischemia=reperfusion syndrome, exercise-induced myopathy, heart and skeletal muscle diseases related to the nutritional deficiency of selenium and vitamin E and related disorders, and genetic muscular dystrophies. These models provide evidence that mitochondrial function and the glutathione-dependent antioxidant system are important for the maintenance of the structural and functional integrity of muscular tissues. Studies have shown that exhaustive physical exercise causes a change in glutathione redox status in blood. It has also been found that antioxidant administration, that is, oral vitamin C, N-acetyl-L-cysteine, or glutathione, is effective in preventing oxidation of the blood glutathione pool Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 189 29.10.2007 7:53pm Compositor Name: JGanesan Protein Foods versus Protein and Amino Acid Supplements 189 after physical exercise in rats.24 Cysteine is rate limiting for glutathione synthesis, and can become conditionally essential in intensive care unit (ICU) patients under physiological stress. In one study, researchers, realizing that protein in enteral products may be their sole source of cysteine, decided to determine the effects of two different proteins, casein and whey, on glutathione status in ICU patients.25 The results indicated that mean glutathione concentration increased in those fed whey protein, but not in those fed casein. According to the authors, casein contains 0.3% cysteine, whereas whey protein contains 2.0%– 2.5% cysteine. It was concluded that protein sources containing high levels of cysteine may be effective in maintaining or repleting whole-blood glutathione. WHEY AND CASEIN AND ANTIGENICITY—ADVANTAGES OF HYDROLYSATES In addition to superior absorption and higher BV, protein supplements may also be superior to whole foods for use in people who have certain food allergies. For example, it has been shown that early introduction to formula instead of breast-feeding can lead to allergic reactions against milk proteins as well as other foods. All the major cow-milk proteins in their native states are potential allergens in infants with milk allergy. A lot of research has gone on in finding solutions to this problem so that the hypoallergenic formulas could be used for infants, nursing mothers, and for allergic children and adults, and also for enteral nutrition.26–29 For example, studies have shown that there are significant effects of even a partial whey hydrolysate formula on the prophylaxis of atopic disease.30 Heat treatment can reduce the antigenicity of whey proteins considerably, but it has virtually no effect on the antigenicity of casein. Infants allergic to milk still react to heat-denatured whey proteins. Therefore heat denaturation alone cannot produce a formula with low allergenicity. Enzyme hydrolysis reduces the antigenicity and allergenicity of protein. Partial hydrolysis produces hydrolysate consisting mainly of large peptides, whereas extensive hydrolysis produces hydrolysate containing a mixture of large and small peptides and free amino acids. When hydrolysate formula is made, casein or whey protein hydrolysate is ultrafiltered to remove large residual peptides. Certain amino acids are fortified to provide a balanced amino acid profile. In the United States the final formulation must comply with the recommendations of the Codex Alimentarius Commission (FAO=WHO) and the U.S. Infant Formula Act to provide adequate infant nutrition. Protein-hydrolysate-based formula can be improved by further reducing the residual allergenicity, increasing the small peptide content, and improving the taste. Thus protein hydrolysates if properly prepared are much less antigenic. Hypoallergenic protein hydrolysates are prepared in such a way that the quantity of antigenic material is drastically reduced and thus can be used by those who are normally allergic to milk proteins. The enzymatic hydrolysis of proteins coupled with ultrafiltration is the best way to reduce its antigenicity, and also of preserving the nutritional qualities of the amino acids and the peptides produced. In one study, two milk protein (whey protein and casein) hydrolysates were obtained by enzymatic hydrolysis.19 In the casein hydrolysate, an ultrafiltration stage was performed to remove all the peptides with molecular weights more than 2500. A nutritional value study of both hydrolysates was undertaken. No differences were found between the native proteins and their enzymatic hydrolysates. The casein hydrolysate, after being ultrafiltered, was devoid of sensitizing capacity by the oral route and was also ineffective in producing local or systemic anaphylaxis in previously sensitized animals, as shown by in vivo assays. Amino acid formulas, because of their decreased antigenicity, would likely be useful for those who have severe cows’ milk intolerance and those who have already reacted to whey or casein hydrolysate formula.31 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 190 29.10.2007 7:53pm Compositor Name: JGanesan 190 Amino Acids and Proteins for the Athlete: The Anabolic Edge WHEY PROTEIN—CONCLUSIONS From the discussion above it can be seen that there are substantial possible benefits of whey protein use by athletes including increased protein synthesis, enhanced immune system performance, and antioxidant activity. FREE-FORM AMINO ACIDS VERSUS DI- AND TRIPEPTIDES A controversy exists in the athletic community over the absorption of free amino acids versus small peptides. The process by which protein is degraded and absorbed provides us with the answer to this controversy. Less than 1% of total protein that passes through the GI tract is lost in the stools, usually in the form of tough fibrous and indigestible forms of protein. The makeup of the protein as presented to the digestive tract determines how much is absorbed and how much is lost. For example, whole peanuts may pass right through, while the protein from peanut butter is almost totally absorbed. Protein digestion is traditionally thought to proceed to hydrolysation of protein to free amino acids which are subsequently absorbed and enter the circulation. Current research, however, has shown that protein fragments, mainly small peptides, are absorbed intact and transported by the systemic circulation to peripheral tissues. Several studies have shown that hydrolysates made up mostly of di- and tripeptides are absorbed more rapidly than amino acids and hydrolysates containing larger peptides, and much more rapidly than whole foods.32,33 There are transport systems in the gut for both individual amino acids (usually functionally similar amino acids such as the neutral ones will share a carrier) and for small peptides. The considerably greater absorption rate of amino acids from the dipeptide than from the AAM appears to be the result of uptake by a system that has a greater transport capacity than amino acid carrier systems, thus minimizing competition among its substrates.34 The di- and tripeptides are absorbed intact, hydrolyzed intercellularly, and released as free amino acids or released intact into the circulation. It is the kinetics of the absorption, not the net absorption of amino acids from whole protein, small peptide mixture (produced by enzymatic hydrolysis), and a mixture of amino acids that determine the greater nutritive value of the mixture of small peptides. In one study, six nonanesthetized pigs were used to study the intestinal absorption of amino acids from either an enzymic hydrolysate of milk containing a large percentage of small peptides (about 50% with less than five amino acids residues) and very few free amino acids (8%), or from a mixture of free amino acids with an identical pattern infused intraduodenally.35 The absorption of amino acids was greater, more rapid, and more homogeneous after the hydrolysate infusion than after the free amino acid infusion, independent of the amount infused. Thus enzymic hydrolysates are superior to a mixture of an equivalent pattern of free amino acids. Another study has shown similar results.36 In this study, the nutritive value of two nitrogencontaining mixtures, one formed from small peptides (milk-protein mild enzymatic hydrolysates) and the other consisting of a mixture of free amino acids having the same pattern except for glutamine, was measured in rats. In this study, protein utilization was found to be greater for the mixtures of small peptides than for the mixture of free amino acids. The authors felt that under conditions of discontinuous enteral feedings, a mixture of small peptides is of greater nutritive value than a mixture of amino acids having a close composition, probably because some amino acids from peptides (especially leucine and arginine) are more rapidly absorbed and may produce greater insulin reaction. In another study, it was found that this greater rate of absorption appears to be buffered, although not abolished, if carbohydrates are present.37 Human studies have been done on surgical or trauma patients. In a prospective study, 12 intensive care patients, after abdominal surgery, received three alternate 6-day courses of two enteral diets with identical nitrogen (0.3 g N=kg=day) and energy (60 kcal=kg=day) supply.38 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 191 29.10.2007 7:53pm Compositor Name: JGanesan Protein Foods versus Protein and Amino Acid Supplements 191 The protein-hydrolysate diet (PH) contained enzyme-hydrolysed casein and lactoserum (60% small peptides), while the nondegraded protein diet (NDP) contained a nitrogen source of similar amino acid composition, but in the form of NDP. The patients were randomized to receive either PH–NDP–PH or NDP–PH–NDP. Parameters reflecting protein metabolism were assessed in the plasma, urine, and stomal effluent on days 1, 6, 12, and 18, 3 h after stopping the nutrition (t0), and 1 h after restarting it (t1). Comparisons of t1 and t0 values showed that 13 amino acids (including the eight essential ones) increased significantly with the protein-hydrolysate diet, but only two increased with the NDP diet. Similarly, with protein hydrolysate, insulinemia at t1 was significantly higher than at t0 and correlated with plasma leucine, phenylalanine, alanine, and lysine concentrations. In addition, significant improvements in plasma albumin, transferrin, and retinol-binding protein concentrations were seen with protein hydrolysate, together with a significant decrease in the plasma phenylalanine to tyrosine ratio and urinary 3-methylhistidine excretion. The authors of this study concluded that in patients in intensive care after abdominal surgery, enteral support containing small peptides is more effective than an equivalent diet containing whole proteins in restoring plasma amino acid and protein levels. On the other hand, another study compared the effects on nitrogen balance, diets containing either lactalbumin whole protein, its peptide-rich enzymic hydrolysate, or an equivalent mixture of free amino acids as the sole source of dietary nitrogen were fed to two healthy subjects, each studied for 38 days on two separate occasions.39 The results of this study showed no significant differences in the effect of the three nitrogen sources on nitrogen balance. The authors concluded that there is no nutritional evidence to support the current practice of prescribing expensive enteral diets containing peptides or amino acids rather than the much cheaper whole protein to patients with normal GI function. As we have seen, however, this conclusion does not apply to those under trauma, surgery, or exercise-induced catabolic stress. The bottom line is that hydrolysates containing a mixture of small peptides and free amino acids are absorbed faster and produce more pharmacological effects (increasing GH and insulin response) than mixtures of free amino acids and much faster than whole-food protein. However, this still does not give us the complete picture since commercial preparations have confused the issue. Most peptide-bound amino acids are formed from whole-protein foods usually by either enzyme or acid hydrolysis. Both processes create some free-form amino acids along with some small-chain peptides. Although any protein-containing foods or tissues (including skin, hair, hooves, etc.) can be used as a starting point for hydrolysis, the most commonly used are the milk proteins casein and lactalbumin, and egg. Although it is possible to produce a hydrolysate that is made up of mostly di- and tripeptides along with some free amino acids, such is not usually the case. Many preparations state that they have small peptides when in fact the amount of these peptides is very low, with most of the preparations being in the form of large oligopeptides. Free-form amino acid preparations are absorbed more rapidly from most of the peptide preparations now on the market. On the other hand, amino acids are more expensive than the most costly whole-protein supplements and some free-form amino acids are difficult to take because of their bad smell and taste. Notwithstanding the misrepresentation that can occur with some commercial preparations, and the possible problems with palatability, ideally in order to make the best use of the few hours of hypermetabolism that exists after training, a protein supplement high in di- and tripeptides would seem to be absorbed the fastest and thus give the most immediate anabolic and anticatabolic effects. PRACTICAL GUIDE TO COMMERCIAL AMINO ACID PREPARATIONS There are a large number of products commercially available that contain amino acids in one form or another. The athlete is often confused both by the large variety and the claims made for various products. I have summarized some basic facts that will help the buyer make better choices. Amino acids are commercially available in both free and peptide-bonded forms. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 192 29.10.2007 7:53pm Compositor Name: JGanesan 192 Amino Acids and Proteins for the Athlete: The Anabolic Edge FREE-FORM AMINO ACID MIXTURES These crystalline amino formulations can consist of individual or combinations of two or more amino acids. They should be reserved for those times when their rapid absorption can make a difference or for their pharmacological effects (as outlined below). Comprehensive mixtures of free-form amino acids can be used as an adjunct to food or as an alternative to whole-food protein supplements to increase dietary protein. There is no evidence, however, that shows any advantage to using the much more expensive free-form AAMs simply to augment protein intake. There are times when the use of a free-form AAM or a product that contains free-form amino acids together with di- and tripeptides may be more useful than either high-protein foods or wholefood protein supplements. The free-form amino acid and small peptides are absorbed much faster and more expeditiously from the GI tract than whole-food protein and even mixtures of free amino acids. The use of specific combinations of free-form AAMs before, during, and after training may also be useful because of their pharmacological effects (as discussed below). The kind and quantity of amino acid ingested, and the form it is in, have an effect on the portion that enters the systemic circulation. Normally, most of the amino acids, except for the BCAAs, entering the liver from the digestive tract via the portal vein are deaminated and the resulting ammonia is converted to urea. A lesser amount is released into the general circulation as free amino acids. Large amounts of oral powdered crystalline amino acids present a bolus of amino acids to the liver, allowing a larger portion to escape processing by the liver and reach the systemic circulation. Less would reach the systemic circulation if the same amount were ingested as whole-protein powder or tablets of free amino acids, because although the same quantity of amino acids is presented to the liver, it is presented over a longer period of time so that more of the amino acids are catabolized to urea. If the powder is held in the mouth before swallowing, some of the amino acids are absorbed from the buccal mucosa directly into the systemic circulation, bypassing the liver; the rest is absorbed through the gastric and intestinal mucosa. The overall result of buccal and GI absorption of powdered amino acids is of a relatively higher level than of the amino acids in the systemic circulation. This results in higher tissue levels, and a more pronounced systemic effect. PEPTIDE-BONDED AMINOS Peptide-bonded amino acids are useful for increasing dietary protein intake, for improving the protein efficiency ratio of dietary protein, and at specific times after training (as explained below). Since they are a mixture of amino acids, they cannot be used for pharmacological effects. General protein supplements and mixtures of multiple amino acids can be used to increase dietary protein intake and after training to make use of the window of opportunity. They are used by all athletes but more by strength athletes—track and field (Olympic), powerlifters, and bodybuilders to maximize lean body mass and strength. Taken by themselves or mixed with a variety of other supplements and foods they can be a valuable, easy to use, palatable addition to athletes’ diets. The better, higher priced products usually offer more value. These products usually have a higher biological value, are more palatable, hypoallergenic, and are more natural usually containing no chemical additives, artificial colors, or sweeteners. FACTORS AFFECTING AMINO ACID BIOAVAILABILITY The length of time it takes to digest a protein food and for the digested amino acids to be available for use by the body is determined by a number of factors, which include: Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 193 29.10.2007 7:53pm Compositor Name: JGanesan Protein Foods versus Protein and Amino Acid Supplements . . . . . 193 Cooking: Amino acids are sensitive to heat although the degree of sensitivity varies with each amino acid. For example, arginine is extremely stable and will decompose only if exposed to sustained temperatures above 4708F. Carnitine decomposes at temperatures of 2848F. Cooking, in addition to killing microorganisms, makes the long spiral polypeptide chains unwind, causing the amino acid to become more exposed when it reaches the digestive system. Physical nature of the food: Whether solid, liquid, powder, or tablet; whether and to what extent chemically predigested; and the type and amounts of binders, fillers, and other nutritive and nonnutritive materials. Status of the digestive system: Genetics, age, overall health, and specific diseases and illnesses. Metabolism or utilization by the intestine before absorption: Such as occurs with glutamine. Metabolism or utilization in the liver before transfer to the general circulation: For maximal directed effects, amino acids should be taken on an empty stomach and in a dosage that enables significant quantities to reach the target tissues. ROLE OF AMINO ACID SUPPLEMENTATION IN MUSCLE HYPERTROPHY AND STRENGTH The use of commercial amino acid supplements by athletes has increased dramatically in the last decade. While a certain amount of research has been done to determine the amount of dietary protein needed by strength and endurance athletes (see above), less is known about how the quality of protein or specific amino acid supplements affects metabolic and physiological responses to strength and endurance training. Amino acids serve two rather different functions. Dietary amino acids are metabolized in a number of ways and are used to replenish the free amino acid pools in tissues and plasma. Oxidative losses of free amino acids occur both during and after feeding. The oxidative losses occur during feeding because they are consumed at a rate which is usually in excess of the rate in which net protein synthesis can occur, so that oxidation occurs as part of the process of maintaining the relatively small but important tissue-free amino acid pools. Any excess amino acids, that is, above the amounts needed for immediate protein synthesis and replenishment of the free amino acid pool, are oxidized, transformed, or metabolized, and stored as glycogen or fat. Amino acids exert an important regulatory influence on growth, development, and protein turnover through their activation of various hormonal and metabolic responses. This effect is primarily aimed at the stimulation of growth, including the stimulation of oxidative losses. These anabolic influences may be exerted for short periods after feeding and under the influence of specific hormones. With the manipulation of diet and of the hormonal milieu, the anabolic influences can be prolonged. Amino acid supplementation has also been shown to enhance recovery from exercise by its effects on increasing protein synthesis and decreasing degradation. In a recent study amino acids were found to be effective in dealing with the adverse effects of overreaching, where there is suboptimal recovery by athletes.40 The major findings of this study are the following: . . . . Initial (first week) response to high-volume resistance training overreaching is a reduction in muscle strength; however, this phenomenon is attenuated by amino acid supplementation. Creatine kinase (CK) and uric acid were elevated only in the placebo group after the first week of overreaching. Total testosterone to SHBG ratio was maintained with amino acid supplementation during overreaching. Amino acid supplementation provided no further benefit to resistance-training performance once training volume was reduced (after T3) and participants adapted to the training stress. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 194 29.10.2007 7:53pm Compositor Name: JGanesan 194 Amino Acids and Proteins for the Athlete: The Anabolic Edge In this study, muscle strength was reduced initially only in the placebo group, thus showing the importance of increased amino acid intake during the initial phase of overreaching.The authors felt that amino acid supplementation is likely most helpful during high-volume and high-intensity resistance training. It has been my experience that amino acid supplementation has the most effects when periodized with the most intense phases of training and precompetition. REFERENCES 1. Dragan, G.I., Wagner, W., and Ploesteanu, E., Studies concerning the ergogenic value of protein supply and L-carnitine in elite junior cyclists, Physiologie, 25(3), 129–132, 1988. 2. Dragan, G.I., Vasiliu, A., and Georgescu, E., Research concerning the effects of Refit on elite weightlifters, J. Sports Med. Phys. Fitness, 25(4), 246–250, 1985. 3. Dragan, G.I., Vasiliu, A., and Georgescu, E., Effects of Refit on Olimpic (sic) athletes, Sportorvosi Szemle=Hungarian Rev. Sports Med., 26(2), 107–113, 1985. 4. Dragan, I., Stroescu, V., Stoian, I., Georgescu, E., and Baloescu, R., Studies regarding the efficiency of Supro isolated soy protein in Olympic athletes, Rev. Roum. Physiol., 29(3–4), 63–70, 1992. 5. Dragan, G.I., Ploesteanu, E., and Selejan, V., Studies concerning the ergogenic value of Cantamega-2000 supply in top junior cyclists, Rev. Roum. Physiol., 28(1–2), 13–16, 1991. 6. Kreider, R.B., Klesges, K.H., Grindstaff, P., et al., Effects of ingesting supplements designed to promote lean tissue accretion on body composition during resistance training, Int. J. Sports Nutr., 6, 234–246, 1996. 7. Lowenthal, D.T. and Karni, Y., The nutritional needs of athletes, in V. Herbert and G.J. Subak-Sharpe (Eds.), Total Nutrition: The Only Guide You’ll Ever Need, St. Martin’s Press, New York, 1995, p. 406. 8. Belford, D.A., Rogers, M.L., Regester, G.O., et al., Milk-derived growth factors as serum supplements for the growth of fibroblast and epithelial cells, In Vitro Cell. Dev. Biol. Anim., 31(10), 752–760, 1995. 9. Horton, B.S., Commercial utilization of minor milk components in the health and food industries, J. Dairy Sci., 78(11), 2584–2589, 1995. 10. Bounous, G., Gervais, F., Amer, V., Batist, G., and Gold, P., The influence of dietary whey protein on tissue glutathione and the diseases of aging, Clin. Invest. Med., 12(6), 343–349, 1989. 11. Bounous, G. and Gold, P., The biological activity of undenatured dietary whey proteins: Role of glutathione, Clin. Invest. Med., 14(4), 296–309, 1991. 12. Bounous, G., Batist, G., and Gold, P., Immunoenhancing property of dietary whey protein in mice: Role of glutathione, Clin. Invest. Med., 12(3), 154–161, 1989. 13. Costantino, A.M., Balzola, F., and Bounous, G. [Changes in biliary secretory immunoglobulins A in mice fed whey proteins], Minerva-Dietol-Gastroenterol., 35(4), 241–245, 1989. 14. Zhang, X. and Beynen, A.C., Lowering effect of dietary milk-whey protein v. casein on plasma and liver cholesterol concentrations in rats, Br. J. Nutr., 70(1), 139–146, 1993. 15. Bosselaers, I.E., Caessens, P.W., Van Boekel, M.A., and Alink, G.M., Differential effects of milk proteins, BSA and soy protein on 4NQO- or MNNG-induced SCEs in V79 cells, Food Chem. Toxicol., 32(10), 905–909, 1994. 16. Boza, J.J., Martinez-Augustin, O., Baro, L., Suarez, M.D., and Gil, A., Protein v. enzymic protein hydrolysates. Nitrogen utilization in starved rats, Br. J. Nutr., 73(1), 65–71, 1995. 17. Baro, L., Guadix, E.M., Martinez-Augustin, O., Boza, J.J., and Gil, A., Serum amino acid concentrations in growing rats fed intact protein versus enzymatic protein hydrolysate-based diets, Biol. Neonate, 68(1), 55–61, 1995. 18. Battermann, W., Whey protein for athletes, Deutsche Milchwirtschaft, 37(33), 1010–1012, 1986. 19. Boza, J.J., Jimenez, J., Martinez, O., Suarez, M.D., and Gil, A., Nutritional value and antigenicity of two milk protein hydrolysates in rats and guinea pigs, J. Nutr., 124(10), 1978–1986, 1994. 20. Poullain, M.G., Cezard, J.P., Roger, L., and Mendy, F., Effect of whey proteins, their oligopeptide hydrolysates and free amino acid mixtures on growth and nitrogen retention in fed and starved rats [published erratum appears in JPEN 1989, 13(6), 595], J. Parenter. Enteral Nutr., 13(4), 382–386, 1989. 21. Blackburn, G.L., Nutrition and inflammatory events: Highly unsaturated fatty acids (omega-3 vs omega-6) in surgical injury, Proc. Soc. Exp. Biol. Med., 200(2), 183–188, 1992. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 195 29.10.2007 7:53pm Compositor Name: JGanesan Protein Foods versus Protein and Amino Acid Supplements 195 22. Asayama, K. and Kato, K., Oxidative muscular injury and its relevance to hyperthyroidism, Free Radic. Biol. Med., 8(3), 293–303, 1990. 23. Clarkson, P.M., Antioxidants and physical performance, Crit. Rev. Food Sci. Nutr., 35(1–2), 131–141, 1995. 24. Sastre, J., Asensi, M., Gasco, E., Pallardo, F.V., Ferrero, J.A., Furukawa, T., and Vina, J., Exhaustive physical exercise causes oxidation of glutathione status in blood: Prevention by antioxidant administration, Am. J. Physiol., 263(5, Pt 2), R992–R995, 1992. 25. Rowe, B., Kudsk, K., Borum, P., et al., Effects of whey- and casein-based diets on glutathione and cysteine metabolism in ICU patients, J. Amer. Coll. Nutr., 254, 535, 1994. 26. Odze, R.D. and Wershil, B.K., Allergic Colitis in Infants, J. Pediatr., 126, 163–170, 1995. 27. Gmoshinskii, I.V., Kruglik, V.I., Samenkova, N.F., Krzhechkovskaia, V.V., Zorin, S.N., and Mazo, V.K. [Characteristics of peptide preparations obtained during enzymatic hydrolysis and ultrafiltration of milk proteins for use in specialized nutrition products], Voprosy Pitaniia, (3), 21–27, 1991. 28. Lee, Y.H., Food-processing approaches to altering allergenic potential of milk-based formula, J. Pediatr., 121(5, Pt 2), S47–S50, 1990. 29. Schmidt, D.G., Meijer, R.J., Slangen, C.J., and van Beresteijn, E.C., Raising the pH of the pepsincatalysed hydrolysis of bovine whey proteins increases the antigenicity of the hydrolysates, Clin. Exp. Allergy, 25(10), 1007–1017, 1995. 30. Vandenplas, Y., Hauser, B., Van den Borre, C., and Clybouw, C., The long-term effect of a partial whey hydrolysate formula on the prophylaxis of atopic disease, Eur. J. Pediatr., 154(6), 488–494, 1995. 31. McLeish, C.M., MacDonald, A., and Booth, I.W., Comparison of an elemental with a hydrolysed whey formula in intolerance to cows’ milk, Arch. Dis. Child., 73(3), 211–215, 1995. 32. Grimble, G.K. and Silk, D.B., The optimum form of dietary nitrogen in gastrointestinal disease: Proteins, peptides or amino acids? Verhandlungen der Deutschen Gesellschaft für Innere Medizin, 92, 674–685, 1986. 33. Grimble, G.K., Rees, R.G., Keohane, P.P., Cartwright, T., Desreumaux, M., and Silk, D.B., Effect of peptide chain length on absorption of egg protein hydrolysates in the normal human jejunum, Gastroenterology, 92(1), 136–142, 1987. 34. Steinhardt, H.J. and Adibi, S.A., Kinetics and characteristics of absorption from an equimolar mixture of 12 glycyl-dipeptides in human jejunum, Gastroenterology, 90(3), 577–582, 1986. 35. Rerat, A., Nunes, C.S., Mendy, F., and Roger, L., Amino acid absorption and production of pancreatic hormones in non-anaesthetized pigs after duodenal infusions of a milk enzymic hydrolysate or of free amino acids, Br. J. Nutr., 60(1), 121–136, 1988. 36. Monchi, M. and Rerat, A.A., Comparison of net protein utilization of milk protein mild enzymatic hydrolysates and free amino acid mixtures with a close pattern in the rat, J. Parenter. Enteral Nutr., 17(4), 355–363, 1993. 37. Rerat, A., Simoes-Nunes, C., Mendy, F., Vaissade, P., and Vaugelade, P., Splanchnic fluxes of amino acids after duodenal infusion of carbohydrate solutions containing free amino acids or oligopeptides in the non-anaesthetized pig, Br. J. Nutr., 68(1), 111–138, 1992. 38. Ziegler, F., Ollivier, J.M., Cynober, L., Masini, J.P., Coudray-Lucas, C., Levy, E., and Giboudeau, J., Efficiency of enteral nitrogen support in surgical patients: Small peptides v non-degraded proteins, Gut, 31(11), 1277–1283, 1990. 39. Moriarty, K.J., Hegarty, J.E., Fairclough, P.D., Kelly, M.J., Clark, M.L., and Dawson, A.M., Relative nutritional value of whole protein, hydrolysed protein and free amino acids in man, Gut, 26(7), 694–699, 1985. 40. Kraemer, W.J., Ratamess, N.A., Volek, J.S., Hakkinen, K., Rubin, M.R., French, D.N., Gomez, A.L., McGuigan, M.R., Scheett, T.P., Newton, R.U., Spiering, B.A., Izquierdo, M., and Dioguardi, F.S., The effects of amino acid supplementation on hormonal responses to resistance training overreaching, Metabolism, 55(3), 282–291, 2006. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C005 Final Proof page 196 29.10.2007 7:53pm Compositor Name: JGanesan Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 197 29.10.2007 7:54pm Compositor Name: JGanesan and 6 Physiological Pharmacological Actions of Amino Acids INTRODUCTION Amino acids are the building blocks of body proteins, including structural and contractile proteins, enzymes, and certain hormones and neurotransmitters. They also function in many other metabolic activities, are used as substrates for energy production, are involved in immune function, ammonia detoxification, the synthesis of nucleic acids, and also function as antioxidants. Amino acids, either free form or in the form of small peptides, are potent physiological and pharmacological agents. For example, amino acid availability rapidly regulates protein synthesis and degradation. In one study endogenous leucine flux, reflecting proteolysis, decreased while leucine oxidation increased in protocols where amino acids were infused.1 In addition, nonoxidative leucine flux reflecting protein synthesis was stimulated by amino acids. Increasing amino acid concentrations stimulates protein synthesis in a dose-dependent manner at the level of mRNA translation initiation and inhibits protein degradation by inhibiting lysosomal autophagy.2 Supplying energy alone (i.e., carbohydrate and lipids) cannot prevent negative nitrogen balance (net protein catabolism) in animals or humans; only the provision of certain amino acids allows the attainment of nitrogen balance.3 BCAAs, and in particular leucine, have been shown to have anabolic effects on protein metabolism by increasing protein synthesis and decreasing protein breakdown.4 Certain neurotransmitters (i.e., acetylcholine, catecholamines, and serotonin) are formed from dietary constituents (i.e., choline, tyrosine, and tryptophan). Changing the consumption of these precursors alters release of their respective neurotransmitter products. Many amino acids have also been shown to act as CNS neurotransmitters.2,5–21 Acute dietary indispensable amino acid deficiency may increase vulnerability to seizures by repeated activation of the anterior piriform cortex of the brain.22 Glutamine23 and taurine24 have hepatoprotective and antihepatotoxic effects. Several amino acids such as cysteine (which decreases cross linkage of proteins) and histidine25 have significant antioxidant effects. Several amino acids, including the essential BCAAs, especially leucine, are used directly as oxidizable fuels during exercise. Depending on the duration and intensity of exercise and other factors such as glycogen stores and energy intake, amino acids can provide from a few to approximately 10% of the total energy for sustained exercise.26 Arginine, glycine, and methionine are the amino acids that are required for the hepatic synthesis of creatine. Creatine is taken up by skeletal muscle, where it is phosphorylated to form phosphocreatine (PC), a high-energy phosphate compound and an important energy source for cellular energy and muscular contraction. Lysine has been shown useful for treating herpes infections in some patients, although there is considerable controversy on its effectiveness, perhaps because other factors, such as dietary intake, have not been taken into account.27 197 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 198 29.10.2007 7:54pm Compositor Name: JGanesan 198 Amino Acids and Proteins for the Athlete: The Anabolic Edge Cysteine, a precursor for reduced glutathione, may be of some use in reducing hepatotoxicity secondary to drugs (such as alcohol and anabolic steroids) and infections.28,29 Studies have shown that muscle wasting postoperatively can be countered by the use of enteral or parenteral glutamine, AKG, and the BCAAs. Intravenous or oral feedings rich in BCAAs are said to improve nutrition in chronic liver disease and to exert anticatabolic effects on protein degradation, thus sparing muscle protein.30 All amino acids have the capacity to accept or release hydrogen ions. Only glutamate, aspartate, histidine, and perhaps arginine serve as buffers with respect to regulation of hydrogen ions in the body. Glutamine is especially important as a buffer in the kidney. Studies have shown a dose-related suppression of food intake by phenylalanine and an anorexic effect of aspartame.31 Some of the amino acids may also be effective for the treatment and prevention of certain diseases. For example, a study in rabbits has shown that L-arginine, the precursor of endotheliumderived relaxing factor (EDRF), decreases atherosclerosis in hypercholesterolemic animals.32 It has even been shown that certain amino acids may have protective effects on lactational performance against toxic compounds.33 In this study glycine, which increases GH release, and tryptophan, which enhances and mimics prolactin secretion, negate the harmful effects of 7,12dimethylbenzanthracene by stimulating mammary gland growth and secretion. Apart from acting as a protein supplement, amino acids alone or in various combinations can have specific and different pharmacological and physiological effects including effects on immunomodulation, neurotransmitters, hormones, control of protein turnover, renal function, and maintenance of gut trophicity.34 Certain amino acids exert pharmacological effects similar to hormones and drugs. Thus amino acids (and their metabolites and analogues) can transcend their roles of being just the building blocks of protein but can actually regulate protein synthesis by acting as anabolic and anticatabolic agents and affecting hormonal functions. For example, in one study examining the effects of certain amino acid groups on renal hemodynamics, glomerular filtration rate, and renal plasma flow were increased by infusion of mixed gluconeogenic amino acids (arginine, glycine, proline, cysteine, methionine, serine) but not by either alanine, another gluconeogenic amino acid, or the BCAAs.35 In another study accepted for publication but not as yet published, synthesis and degradation of globular and myofibrillar proteins across arm and leg muscles were examined during stepwise increased intravenous infusion of amino acids (0.1, 0.2, 0.4, and 0.8 g nitrogen=kg=day) to healthy volunteers.36 In this study protein dynamics were measured by a primed constant infusion of L-phenylalanine and the release of 3-methylhistidine from skeletal muscles. Amino acid infusion caused a significant uptake of the majority of amino acids across arm and leg tissues, except tyrosine, tryptophan, and cysteine, probably due to low concentrations of these amino acids in the formulation. The balance of globular proteins improved significantly due to stimulation of synthesis and attenuation of degradation across arm and leg tissues, despite insignificant uptake of tyrosine, tryptophan, and cysteine. Degradation of myofibrillar proteins was uninfluenced by provision of amino acids. Arterial concentrations and flux of glucose, lactate, and FFAs were unchanged despite increasing concentrations of plasma amino acids from 2.6 to 5.8 mM. Plasma insulin, IGF-1, and plasma concentrations of IGF-1 binding proteins-1 and 3 remained at fasting levels throughout the investigation. The results of this study demonstrate that neither insulin nor circulating IGF-1 explained improved protein balance in skeletal muscles following elevation of plasma amino acids. Rather, some amino acids in themselves trigger cellular reactions that initiate peptide formation. In this study the limited availability of some extracellular amino acids was overcome by increased reutilization of the intracellular amino acid. There is evidence that specific amino acids play a role in the etiology of fatigue and the overtraining syndrome in athletes because of their flux during exercise causing a change in certain metabolic kinetics. The metabolism of BCAAs, glutamine, and tryptophan may be the key to Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 199 29.10.2007 7:54pm Compositor Name: JGanesan Physiological and Pharmacological Actions of Amino Acids 199 understanding some aspects of central fatigue37 and some aspects of immunosuppression that are very relevant to athletic endeavor.38 According to the central fatigue hypothesis the increase of the neurotransmitter serotonin during prolonged exercise causes fatigue and a decreased ability to sustain work output. The basis of the hypothesis is that an increase in blood levels of the amino acid tryptophan leads to an increase in serotonin synthesis in the brain and as a result there is an increased feeling of fatigue and sleepiness. Unbound tryptophan, which is normally bound to serum albumin, competes with the BCAAs for transportation across the blood–brain barrier. Thus an increase in unbound tryptophan and=or a decrease in BCAAs in the blood leads to an increase in the amount of tryptophan that is available for conversion into serotonin. It has been shown that plasma BCAA levels decrease, and plasma FFAs increase during prolonged exercise leading to increased free tryptophan and an increase in tryptophan transport into the brain. BCAA supplementation, in theory, competes with plasma free tryptophan and as such less tryptophan makes it to the brain and less serotonin is produced resulting in less fatigue.39 Recently, other neurotransmitters have also been implicated.40 Complex AAMs do not give the same pharmacological effects as do the use of individual or combinations of a few or more select amino acids. In the case of the effects on the CNS (in regulating hypothalamic and pituitary function, and in affecting the level of certain neurotransmitters), the reason is partly due to the nature of the transport mechanism that carries the amino acids across the blood–brain barrier into the specific CNS neurons. The large neutral amino acids (LNAA) such as tryptophan, tyrosine, the BCAAs (valine, leucine, isoleucine), phenylalanine, and methionine compete with each other for the same available transport molecules. Thus the presence in plasma of several of these amino acids in any significant amounts results in a decrease in the amounts of each amino acid that are transported into the body and the CNS.41 On the other hand, if only one or a small number of specific amino acids were ingested, serum levels of these amino acids would rise appreciably over the serum levels of the other amino acids, so that more of the ingested amino acids would enter the CNS. It follows that individual or selectively combined amino acids should be taken on an empty stomach by themselves. If taken with food, the amino acids derived from natural sources compete with any individual or combination of specific amino acids—diluting the physiological and pharmacological effects of these amino acids. CENTRAL NERVOUS SYSTEM EFFECTS OF AMINO ACIDS The amount of neurotransmitter released when a neuron fires normally varies and is dependent to some degree on changes on neurotransmitter synthesis resulting from the metabolic consequences of normal activities such as eating and exercise.42 These activities produce their effects by varying the plasma levels of nutrients that are the precursors of the neurotransmitter. It has been shown that increasing the precursors for the neurotransmitters often leads to an increased synthesis of the specific neurotransmitter.43–45 Other studies have shown that specific brain regions may respond uniquely to amino acid ingestion and that dietary composition may influence the amino acid profiles of the extracellular fluid in brain.46 Thus, many individual and combinations of amino acids may enhance athletic performance when they are used to produce pharmacological changes in behavior and physiological functions that precursor-dependent neurons happen to subserve. For example, tyrosine and phenylalanine (which can be converted to tyrosine) are precursors for the neurotransmitters dopamine, epinephrine, and norepinephrine, thus potentially possessing some antidepressant, stimulating, and anorexiant properties. Studies have shown that the use of exogenous tyrosine can transiently increase plasma and CNS catecholamines.47 Tryptophan can be converted by the body to niacin. Also, along with other nutrients, it is a precursor for the neurotransmitter serotonin. There seems to be some evidence that large doses Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 200 29.10.2007 7:54pm Compositor Name: JGanesan 200 Amino Acids and Proteins for the Athlete: The Anabolic Edge of tryptophan may act as a relaxant, and may be useful as a mild nighttime sedative48 and antidepressant.49 The amino acids alanine, asparagine, g-aminobutyrate (GABA), glutamine, glycine, and taurine have been shown to act as CNS neurotransmitters.50,51 GABA also stimulates GH secretion, likely mediated via dopamine release at a suprapituitary level.52 Mixtures of amino acids have been shown to enhance the secretion of pituitary hormones, including ACTH, gonadotropins, and GH. Specific amino acids are so effective in this regard that a paper published several years ago53 concluded that. In an ethical light, because of the effects of amino acid supplementation on hormonal secretion in athletes, we believe that amino acid supplementation . . . in humans has to be examined in terms of safety and effects on the endocrine system, should occur only when necessary and with medical supervision, and has to be reconsidered in the athletic world in terms of doping substances. EFFECTS OF AMINO ACIDS ON GROWTH HORMONE RELEASE One of the reasons that amino acid supplements are popular among bodybuilders and other athletes is because of claims that pharmacologic dosages of one or combinations of amino acids can increase the GH release. Studies have shown that pharmacologic dosages of some amino acids can indeed increase the release of GH. Histidine, arginine, L-dopa, tryptophan, valine, leucine, ornithine, tyrosine, phenylalanine, lysine, methionine, and GABA all may potentiate GH secretion.54–60 The amino acids can also stimulate other hormones. For example, arginine, like other dibasic amino acids, stimulates pituitary release of GH and prolactin, and pancreatic release of glucagon and insulin.61 Ornithine, on the other hand while stimulating GH release, does not stimulate insulin release even in relatively high dosages.62 While several studies have shown that certain amino acids increase GH secretion, others studies have not. In general, however, larger doses of the GH-stimulating amino acids show more of a GH response than some of the doses used in studies that have not shown this effect.63 For example, in one study the authors used quantities of various amino acids as recommended by the manufacturers. Unfortunately, most manufacturers, because of expense, recommend less than therapeutically effective dosages. In this study, seven male bodybuilders fasted for 8 h and were then given, in random order, either a placebo—a 2.4 g arginine=lysine supplement, a 1.85 g ornithine=tyrosine supplement—or a 20 g BovrilR drink.64 Blood was collected before each treatment and again every 30 min for 3 h for the measurement of serum GH concentration. On a separate occasion, subjects had an intravenous infusion of 0.5 mg GHreleasing hormone per kilogram body weight to confirm that GH secretory response was normal. The main finding was that serum GH concentrations were not altered consistently in healthy young males following the ingestion of the amino acid supplements in the quantities recommended by the manufacturers. Athletes often use combinations of GH stimulators in hopes of increasing GH secretion even further. There is in fact some basis for this belief—the synergistic effect of clonidine, propanolol, 65 L-dopa, and others has been documented. The GH-releasing effect of both arginine and pyridostigmine is mediated via the same mechanism, namely, by suppression of endogenous somatostatin release. Combined administration of either arginine or pyridostigmine with GHRH has a similar striking GH-releasing effect which is clearly higher than that of GHRH alone.66 On the other hand, the mechanisms by which insulin-induced hypoglycemia, L-dopa, and arginine stimulate GH secretion are all different.67 The effect of various amino acids on GH will also be discussed later under the specific amino acids. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 201 29.10.2007 7:54pm Compositor Name: JGanesan Physiological and Pharmacological Actions of Amino Acids 201 HEPATOPROTECTANT AND CYTOPROTECTIVE EFFECTS OF AMINO ACIDS Glycine has been shown to have important specific and general cytoprotective and hepatoprotective effects. In one study glycine suppressed some hepatocellular injury, as reflected in a reduction in the concentration of serum enzymes.68 Another study showed the importance of glycine in protecting hepatocytes from noxious insult in general as well as from valproate (VPA), an antiepileptic drug that causes hepatotoxicity, in particular.69 In this study, VPA caused a dose-dependent increase in leakage of lactic acid dehydrogenase (LDH), and glycine prevented this toxic response. L-Carnitine, L-alanine, and L-cysteine did not protect hepatocytes from VPA. Glycine also partially antagonized inhibition of fatty acid b-oxidation by VPA, as estimated by the generation of acid-soluble products from [14C]palmitic acid. These results are consistent with the hypothesis that glycine prevents VPA toxicity by removing acyl-CoA esters, which accumulate during VPA exposure and interfere with fatty acid b-oxidation. In this same study, glycine also antagonized the toxic effects of acetaminophen on hepatocytes, although at higher concentrations than required to protect hepatocytes from VPA. Because the mechanism of toxicity of acetaminophen probably is different from that of VPA, a nonspecific cytoprotective effect may contribute to glycine antagonism of valproate toxicity. One study investigating the mechanism by which glycine protects against hepatocyte death during anoxia found that glycine may exert its cytoprotective activity during lethal anoxic hepatocyte injury, in part by inhibiting Ca2þ-dependent degradative, nonlysosomal proteases, including calpains.70 The results also pointed to the hepatoprotectant effects of alanine since glycine and alanine (but not valine) markedly improved cell viability during anoxia in a concentration-dependent manner. Glycine has been shown to have significant cytoprotective effects on other tissues, including protective effects on cardiomyocytes, renal tubule cells, and hepatocytes from ischemia, ATP depletion, and cold-storage injury.71–75 For example, some protection against hypoxic injury by supraphysiological glycine and alanine concentrations has been found in the isolated perfused rat kidney.76 Intravenous glutamine protects liver cells from oxidant injury by increasing intracellular glutathione (GSH) content. In one study oral glutamine supplementation was shown to enhance the selectivity of antitumor drugs by protecting normal tissues, including liver, from and possibly sensitizing tumor cells to chemotherapy treatment-related injury.77 Another study has alluded to the hepatoprotectant effects of an arginine–aspartate mixture.78 Moreover, several amino acid derivates have been shown to have variable effects on liver health79 and reducing hepatic damage secondary to drug toxicity.80,81 A recent study, for example, found that S-adenosylmethionine (SAMe) was more effective than N-acetyl cysteine (NAC) in reducing hepatic toxicity secondary to the use of acetaminophen.82 The following chapters will cover some of the relevant biological and pharmacological effects of each amino acid in detail, and will limit the coverage to those amino acids that may have an effect on anabolic and catabolic processes in the body and that may influence athletic performance. REFERENCES 1. Charlton, M.R., Adey, D.B., and Nair, K.S., Evidence for a catabolic role of glucagon during an amino acid load, J. Clin. Invest., 98(1), 90–99, 1996. 2. Smith, K., Reynolds, N., Downie, S., Patel, A., and Rennie, M.J., Effects of flooding amino acids on incorporation of labeled amino acids into human muscle protein, Am. J. Physiol., 275, E73–E78, 1998. 3. May, M.E. and Buse, M.G., Effects of branched-chain amino acids on protein turnover, Diabetes Metab. Rev., 5(3), 227–245, 1989. 4. Blomstrand, E., Eliasson, J., Karlsson, H.K., and Kohnke, R., Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise, J. Nutr., 136(Suppl. 1), 269S–273S, 2006. 5. Herrling, P.L., Synaptic physiology of excitatory amino acids, Arzneimittelforschung, 42(2A), 202–208, 1992. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 202 29.10.2007 7:54pm Compositor Name: JGanesan 202 Amino Acids and Proteins for the Athlete: The Anabolic Edge 6. Watkins, J.C., Some chemical highlights in the development of excitatory amino acid pharmacology, Can. J. Physiol. Pharmacol., 69(7), 1064–1075, 1991. 7. Tsumoto, T., Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex, Neurosci. Res., 9(2), 79–102, 1990. 8. Nieoullon, A. [Excitatory amino acids, central nervous system neurotransmitters], Therapie, 45(3), 281–285, 1990. 9. McEntee, W.J. and Crook, T.H., Glutamate: Its role in learning, memory, and the aging brain, Psychopharmacology, 111(4), 391–401, 1993. 10. Danbolt, N.C., The high affinity uptake system for excitatory amino acids in the brain, Prog. Neurobiol., 44(4), 377–396, 1994. 11. Headley, P.M. and Grillner, S., Excitatory amino acids and synaptic transmission: The evidence for a physiological function, Trends Pharmacol. Sci., 11(5), 205–211, 1990. 12. Shinozaki, H. and Ishida, M., Excitatory amino acids: Physiological and pharmacological probes for neuroscience research, Acta Neurobiol. Exp., 53(1), 43–51, 1993. 13. Krebs, M.O. [Excitatory amino-acids, a new class of neurotransmitters. Pharmacology and functional properties], Encephale., 18(3), 271–279, 1992. 14. D’Angelo, E. and Rossi, P., Excitatory amino acid regulation of neuronal functions, Funct. Neurol., 7(2), 145–161, 1992. 15. Farooqui, A.A. and Horrocks, L.A., Excitatory amino acid receptors, neural membrane phospholipid metabolism and neurological disorders, Brain Res. Brain Res. Rev., 16(2), 171–191, 1991. 16. Rothstein, J.D., Kuncl, R., Chaudhry, V., et al., Excitatory amino acids in amyotrophic lateral sclerosis: An update, Ann. Neurol., 30, 224–225, 1991. 17. Fonnum, F., Glutamate: A neurotransmitter in mammalian brain, J. Neurochem., 42, 1–11, 1984. 18. Nicholls, D. and Attwell, D., The release and uptake of excitatory amino acids, Trends Pharmacol. Sci., 11, 462–468, 1990. 19. Haber, S.N., Neurotransmitters in the human and nonhuman primate basal ganglia, Hum. Neurobiol., 5(3), 159–168, 1986. 20. D’Souza, S.W. and Slater, P., Excitatory amino acids in neonatal brain: Contributions to pathology and therapeutic strategies, Arch. Dis. Child. Fetal Neonatal Ed., 72(3), F147–F150, 1995. 21. Singewald, N., Zhou, G.Y., and Schneider, C., Release of excitatory and inhibitory amino acids from the locus coeruleus of conscious rats by cardiovascular stimuli and various forms of acute stress, Brain Res., 704(1), 42–50, 1995. 22. Gietzen, D.W., Dixon, K.D., Truong, B.G., Jones, A.C., Barrett, J.A., and Washburn, D.S., Indispensable amino acid deficiency and increased seizure susceptibility in rats, Am. J. Physiol., 271(1, Pt 2), R18–R24, 1996. 23. Ostroverkhov, G.E., Khokhlov, A.P., Maliugin, E.F., Terent’eva, V.B., and Rykov, V.I. [Hepatoprotective effect of glutamine], Ter Arkh, 46(1), 89–96, 1974. 24. Kendler, B.S., Taurine: An overview of its role in preventive medicine, Prev. Med., 18(1), 79–100, 1989. 25. Johnson, P. and Hammer, J.L., Histidine dipeptide levels in ageing and hypertensive rat skeletal and cardiac muscles, Comp. Biochem. Physiol. B Comp. Biochem., 103(4), 981–984, 1992. 26. Brooks, G.A., Amino acid and protein metabolism during exercise and recovery, Med. Sci. Sports Exerc., 19(5), S150–S156, 1987. 27. Algert, S.J., Stubblefield, N.E., Grasse, B.J., Shragg, G.P., and Connor, J.D., Assessment of dietary intake of lysine and arginine in patients with herpes simplex, J. Am. Diet. Assoc., 87(11), 1560–1561, 1987. 28. Mgbodile, M.U.K., Holscher, M., and Neal, R.A., Possible protective role for reduced glutathione in aflatoxin B1 toxicity: Effect of pretreatment of rats with phenobarbital and 3-methylcholanthrene on aflatoxin toxicity, Toxicol. Appl. Pharmacol., 34, 128–142, 1975. 29. Ryle, P.R., Chakraborty, J., and Thomson, A.D., Effects of cysteine and antioxidants on the hepatic redoxstate, acetaldehyde and triglyceride levels after acute ethanol dosing, Alcohol Alcohol, 1(Suppl.), 289–293, 1987. 30. Buse, M.G. and Reid, S.S., Leucine: A possible regulator of protein turnover in muscle, J. Clin. Invest., 56, 1250, 1975. 31. Rogers, P.J. and Blundell, J.E., Reanalysis of the effects of phenylalanine, alanine, and aspartame on food intake in human subjects [comment], Physiol. Behav., 56(2), 247–250, 1994. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 203 29.10.2007 7:54pm Compositor Name: JGanesan Physiological and Pharmacological Actions of Amino Acids 203 32. Cooke, J.P., Singer, A.H., Tsao, P., Zera, P., Rowan, R.A., and Billingham, M.E., Antiatherogenic effects of L-arginine in the hypercholesterolemic rabbit, J. Clin. Invest., 90, 1168–1172, 1992. 33. Pitkow, H.S., Rainieri, J.J., and Dwyer, P., Hormone potentiating capability of amino acids on lactational performance in rats injected with 7,12-dimethylbenzanthracene (DMBA) during gestation, Drug Chem. Toxicol., 9, 15–23, 1986. 34. Cynober, L. [Role of new nitrogen substrates during peri-operative artificial nutrition in adults] [French], Ann. Fran. Anesth. Reanim., 14(Suppl. 2), 102–106, 1995. 35. Castellino, P., Levin, R., Shohat, J., and DeFronzo, R.A., Effect of specific amino acid groups on renal hemodynamics in humans, Am. J. Physiol. Renal Fluid Electrolyte Physiol., 258(4), F992–F997, 1990. 36. Svanberg, E., Moller-Loswick, A.C., Matthews, D.E., Korner, U., Andersson, M., and Lundholm, K., Effects of amino acids on synthesis and degradation of skeletal muscle proteins in humans, Am. J. Physiol., 271(4, Pt 1), E718–E724, 1996. 37. Newsholme, E.A. and Blomstrand, E., Branched-chain amino acids and central fatigue, J. Nutr., 136(Suppl. 1), 274S–276S, 2006. 38. Parry-Billings, M., Blomstrand, E., McAndrew, N., and Newsholme, E.A., A communicational link between skeletal muscle, brain, and cells of the immune system, Int. J. Sports Med., 11(Suppl. 2), S122–S128, 1990. 39. Davis, J.M., Carbohydrates, branched-chain amino acids, and endurance: The central fatigue hypothesis, Int. J. Sport Nutr., 5(Suppl.), S29–S38, 1995. 40. Meeusen, R., Watson, P., Hasegawa, H., Roelands, B., and Piacentini, M.F., Central fatigue: The serotonin hypothesis and beyond, Sports Med., 36(10), 881–909, 2006. 41. Fernstrom, J.D., Dietary amino acids and brain function, J. Am. Dietetic Assoc., 94(1), 71–77, 1994. 42. Richard, J.W. and Martin, C.L., Exercise, plasma composition, and neurotransmission: Advances in nutrition and top sports, Med. Sport Sci., 32, 94–109, 1991. 43. Lehnert, H. and Wurtman, R.J., Amino acid control of neurotransmitter synthesis and release: Physiological and clinical implications, Psychother. Psychosom., 60(1), 18–32, 1993. 44. Harper, A.E. and Peters, J.C., Protein intake, brain amino acid and serotonin concentrations and protein self-selection, J. Nutr., 119(5), 677–689, 1989. 45. Pogson, C.I., Knowles, R.G., and Salter, M.A.F., The control of aromatic amino acid catabolism and its relationship to neurotransmitter amine synthesis, Crit. Rev. Neurobiol., 5(1), 29–64, 1989. 46. Currie, P.J., Chang, N., Luo, S., and Anderson, G.H., Microdialysis as a tool to measure dietary and regional effects on the complete profile of extracellular amino acids in the hypothalamus of rats, Life Sci., 57(21), 1911–1923, 1995. 47. Rasmussen, D., Effects of tyrosine and tryptophan ingestion on plasma catechloamine concentrations, J. Clin. Endocrinol. Metab., 57(4), 760–763, 1983. 48. Lehnert, H., Beyer, J., Cloer, E., Gutberlet, I., and Hellhammer, D.H., Effects of L-tryptophan and various diets on behavioral functions in essential hypertensives, Neuropsychobiology, 21(2), 84–89, 1989. 49. Feinberg, S.S. and Halbreich, U., Treatment-resistant depression: Emerging options, Drug Ther., 15, 106–107; 110–111; 115–118, 1985. 50. Crawford, P.M., Lloyd, K.G., and Chadwick, D.W., CSF gradients for amino acid neurotransmitters, J. Neurol. Neurosur. Psychiat., 51(9), 1193–1200, 1988. 51. Huxtable, R.J., Taurine in the central nervous system and the mammalian actions of taurine, Prog. Neurobiol., 32(6), 471–533, 1989. 52. Cavagnini, F., Benetti, G., Invitti, C., Ramella, G., Pinto, M., Lazza, M., Dubini, A., Marelli, A., and Muller, E.E., Effect of gamma-aminobutyric acid on growth hormone and prolactin secretion in man: Influence of pimozide and domperidone, J. Clin. Endocrinol. Metab., 51(4), 789–792, 1980. 53. di Luigi, L., Guidetti, L., Pigozzi, F., Baldari, C., Casini, A., Nordio, M., and Romanelli, F., Acute amino acids supplementation enhances pituitary responsiveness in athletes, Med. Sci. Sports Exerc., 31(12), 1748–1754, 1999. 54. Merimee, T.J., Rabinowitz, D., and Fineberg, S.E., Arginine-initiated release of human growth hormone, New Engl. J. Med., 280(26), 1434–1438, 1969. 55. Martin, J., Neuroendocrine regulation of growth hormone secretion, Pediat. Adolesc. Endocrinol., 12, 1–26, 1983. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 204 29.10.2007 7:54pm Compositor Name: JGanesan 204 Amino Acids and Proteins for the Athlete: The Anabolic Edge 56. Jacobson, B., Effect of amino acids on growth hormone release, Physician Sportsmed., 18(1), 63–70, 1990. 57. Merimee, T.J., Lillicrap, D.A., and Rabinowitz, D., Effect of arginine on serum-levels of growth-hormone, Lancet, 2, 668, 1965. 58. Merimee, T.J., Rabinowitz, D., Riggs, L., et al., Plasma growth hormone after arginine infusion: Clinical experiences, New Engl. J. Med., 276, 434, 1967. 59. Knopf, R.F., Conn, J.W., Fajans, S.S., et al., Plasma growth hormone response to intravenous administration of amino acids, J. Clin. Endocrinol. Metab., 25, 1140, 1965. 60. Bucci, L.R., Hickson, J.F. Jr., Pivarnik, J.M., et al., Ornithine ingestion and growth hormone release in bodybuilders, Nutr. Res., 10(3), 239–245, 1990. 61. Iwasaki, K., Mano, K., Ishihara, M., Yugari, Y., and Matsuzawa, T., Effects of ornithine or arginine administration on serum amino acid levels, Biochem. Int., 14(5), 971–976, 1987. 62. Bucci, L.R., Hickson, J.F. Jr., Wolinsky, I., and Pivarnik, J.M., Ornithine supplementation and insulin release in bodybuilders, Int. J. Sport Nutr., 2(3), 287–291, 1992. 63. Fogelholm, G.M., Naveri, H.K., Kiilavuori, K.T., and Harkonen, M.H., Low-dose amino acid supplementation: No effects on serum human growth hormone and insulin in male weightlifters, Int. J. Sport Nutr., 3(3), 290–297, 1993. 64. Lambert, M.I., Hefer, J.A., Millar, R.P., and Macfarlane, P.W., Failure of commercial oral amino acid supplements to increase serum growth hormone concentrations in male body-builders, Int. J. Sport Nutr., 3(3), 298–305, 1993. 65. Rodriguez, D.O.L., Valeron, M.C., Carrillo, D.A., et al. [Evaluation of growth hormone stimulation tests using clonidine, glucagon, propanolol, hypoglycemia, arginine and L-dopa in 267 children of short stature], Rev. Clin. Esp., 173(2), 113–116, 1984. 66. Ghigo, E., Bellone, J., Mazza, E., et al., Arginine potentiates the GHRH- but not the pyridostigmineinduced GH secretion in normal short children: Further evidence for a somatostatin suppressing effect of arginine, Clin. Endocrinol., 32(6), 763–767, 1990. 67. Masuda, A., Shibasaki, T., Hotta, M., et al., Insulin-induced hypoglycemia, L-dopa and arginine stimulate GH secretion through different mechanisms in man, Regul. Pept., 31(1), 53–64, 1990. 68. den Butter, G., Lindell, S.L., et al., Effect of glycine in dog and rat liver transplantation, Transplantation, 56(4), 817–822, 1993. 69. Vance, M.A., Gray, P.D., and Tolman, K.G., Effect of glycine on valproate toxicity in rat hepatocytes, Epilepsia, 35(5), 101–122, 1994. 70. Nichols, J.C., Bronk, S.F., Mellgren, R.L., and Gores, G.J., Inhibition of nonlysosomal calcium-dependent proteolysis by glycine during anoxic injury of rat hepatocytes, Gastroenterology, 106(1), 168–176, 1994. 71. Schemmer, P., Bradford, B.U., Rose, M.L., Bunzendahl, H., Raleigh, J.A., Lemasters, J.J., and Thurman, R.G., Intravenous glycine improves survival in rat liver transplantation, Am. J. Physiol., 276, G924–G932, 1999. 72. Nishimura, Y., Romer, L.H., and Lemasters, J.J., Mitochondrial dysfunction and cytoskeletal disruption during chemical hypoxia to cultured rat hepatic sinusoidal endothelial cells: The pH paradox and cytoprotection by glucose, acidotic pH, and glycine, Hepatology, 27, 1039–1049, 1998. 73. Dong, Z., Venkatachalam, M.A., Weinberg, J.M., Saikumar, P., and Patel, Y., Protection of ATP-depleted cells by impermeant strychnine derivatives: Implications for glycine cytoprotection, Am. J. Pathol., 158, 1021–1028, 2001. 74. Zhang, K., Weinberg, J.M., Venkatachalam, M.A., and Dong, Z., Glycine protection of PC-12 cells against injury by ATP-depletion, Neurochem. Res., 28, 893–901, 2003. 75. Ruiz-Meana, M., Pina, P., Garcia-Dorado, D., Rodriguez-Sinovas, A., Barba, I., Miro-Casas, E., Mirabet, M., and Soler-Soler, J., Glycine protects cardiomyocytes against lethal reoxygenation injury by inhibiting mitochondrial permeability transition, J. Physiol., 558(Pt 3), 873–882, 2004. 76. Endre, Z.H., Cowin, G.J., Stewart-Richardson, P., Cross, M., Willgoss, D.A., and Duggleby, R.G., 23Na NMR detects protection by glycine and alanine against hypoxic injury in the isolated perfused rat kidney, Biochem. Biophys. Res. Commun., 202(3), 1639–1644, 1994. 77. Rouse, K., Nwokedi, E., Woodliff, J.E., Epstein, J., and Klimberg, V.S., Glutamine enhances selectivity of chemotherapy through changes in glutathione metabolism, Ann. Surg., 221(4), 420–426, 1995. 78. Dragan, I., Georgescu, E., and Bendiu, P., Comparative studies concerning the efficiency of some hepatotropic drugs in top athletes, suffering from chronic hepatitis, Medecine du sport, 62(4), 199–201, 1988. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 205 29.10.2007 7:54pm Compositor Name: JGanesan Physiological and Pharmacological Actions of Amino Acids 205 79. Mato, J.M. and Lu, S.C., Role of S-adenosyl-L-methionine in liver health and injury, Hepatology, 45(5), 1306–1312, 2007. 80. Valentovic, M.A., Terneus, M.V., Harmon, R.C., and Carpenter, A.B., S-Adenosylmethionine (SAMe) attenuates acetaminophen hepatotoxicity in C57BL=6 mice, Toxicol. Lett., 154, 165–174, 2004. 81. Smyrniotis, V., Arkadopoulos, N., Kostopanagiotou, G., Theodoropoulos, T., Theodoraki, K., Farantos, C., Kairi, E., and Paphiti, A., Attenuation of ischemic injury by N-acetylcysteine preconditioning of the liver, J. Surg. Res., 129(1), 31–37, 2005. 82. Terneus, M.V., Kiningham, K.K., Carpenter, A.B., Sullivan, S.B., and Valentovic, M.A., Comparison of S-adenosyl-L-methionine and N-acetylcysteine protective effects on acetaminophen hepatic toxicity, J. Pharmacol. Exp. Ther., 320(1), 99–107, 2007. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C006 Final Proof page 206 29.10.2007 7:54pm Compositor Name: JGanesan Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 207 29.10.2007 7:57pm Compositor Name: JGanesan 7 Essential Amino Acids Essential amino acids, also called indispensable amino acids, are not synthesized in the body in humans and must be supplied in the diet either as free amino acids or as constituents of dietary proteins. By this criterion, the eight amino acids essential in man are: . . . . . . . . Isoleucine* Leucine* Lysine Methionine Phenylalanine Threonine Tryptophan Valine* USE OF ESSENTIAL AMINO ACIDS AFTER TRAINING INCREASES 24 H PROTEIN BALANCE The use of amino acids in and around training and at other times, especially the essential amino acids, has been shown to acutely increase muscle protein synthesis.1,2 The question in many people’s minds, however, is whether the body compensates for this acute increase after training by decreasing protein synthesis later on during the day or even at night. In other words, does the increase in protein synthesis in response to amino acid ingestion that is seen after training result in a long-term anabolic response, one that is above what we would normally see without their use after training. The answer is yes, according to a recent study.3 This study, the first to describe the response of muscle protein balance to exercise and amino acids over a 24 h period, was designed to see if the response of net muscle protein balance to resistance exercise and amino acid ingestion that occurs on an acute basis, reflects the response of net muscle protein balance over an extended period of time. The essential AAM used included histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, and valine—the same mixture that in a previous study was shown to maximally enhance post-training protein synthesis. The results of this study show that muscle protein balance is increased, primarily because of an increase in muscle protein synthesis, when measured acutely and found that this response is additive to the basal response over a full 24 h period. This increase in muscle protein synthesis is part of the key to maximizing the anabolic results of training. However, that does not mean that the other amino acids are not important. The conditionally essential and nonessential amino acids that have been shown to be most used for protein synthesis in * Branched chain amino acids (BCAAs). 207 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 208 29.10.2007 7:57pm Compositor Name: JGanesan 208 Amino Acids and Proteins for the Athlete: The Anabolic Edge muscle serve not only as direct substrates but also spare the conversion of these amino acids from the essential ones, thus effectively increasing the levels of essential amino acids. But there is more to it than that. As we have seen earlier, amino acids, leucine, and insulin have additive and synergistic effects on protein synthesis and protein catabolism. As such the combination of the BCAAs, plus a mixture of amino acids, plus amino acids, and other nutrients that elevate insulin, may maximize protein metabolism and muscle accretion. BRANCHED-CHAIN AMINO ACIDS: ISOLEUCINE, LEUCINE, AND VALINE The BCAAs are thus named because they each have a carbon chain which deviates or branches from the main linear carbon backbone. Although structurally similar, with only slight differences in their side chains, they, and their metabolites, have varied functions in protein and neurotransmitter synthesis, and in energy metabolism. BCAAs are important sources of nitrogen for the synthesis of nondispensable and conditionally dispensable amino acids, such as glutamine and alanine. These three amino acids—isoleucine, leucine, and valine—are involved in the regulation of protein metabolism,4–6 weight loss,7 body composition,8 and may be useful ergogenic aids9–12 and for treatments of wasting and sarcopenia.13–19 The potent effects of leucine supplementation is apparent in one study in which it was found that low-dose leucine supplementation increased body fat loss and improved liver-protein status and the capacity of muscle protein synthesis in rats submitted to food restriction.20 However, that is not to discount the effects of adequate dietary energy on protein synthesis. Studies have shown that energy supplementation increases protein synthesis even if one or more amino acids are limiting,21 and that this response is independent of energy source.21 The effects of the BCAAs have recently been the subject of several studies and symposiums. The renewed interest has shed new light on their importance and effects, especially as signaling molecules on protein and energy metabolism.22 For example, the mechanism by which leucine regulates muscle protein synthesis involves the initiation of mRNA translation via activation of the mTOR signaling pathway.23 While several new signaling molecules have been identified recently that may be involved in mediating the effect of leucine on mTOR, whether a leucine metabolite is involved in the signaling mechanism is one of the many questions that remain to be answered.22 Earlier studies have shown much of what we need to know about the anabolic and anticatabolic effects of BCAAs, both alone and in concert with other factors such as GH and IGF-1. In heart and skeletal muscle in vitro, increasing the concentration of the three BCAAs or of leucine alone reproduces the effects of increasing the supply of all amino acids in stimulating protein synthesis and inhibiting protein degradation.24 Several studies indicated that leucine is an in vivo regulator of protein metabolism by decreasing protein degradation and increasing protein synthesis. In one study, leucine infusion decreased plasma concentrations of several amino acids.25 The authors concluded that leucine decreases protein degradation in humans and that this decrease during leucine infusion contributes to the decrease in plasma essential amino acids. In another study, excessive intake of a single BCAA led rapidly to elevated plasma concentration of both the amino acid administered and its corresponding a-keto acid and, if the rats had previously been fed a low-protein diet, to an increase in liver branched-chain a-keto acid dehydrogenase activity.26 Leucine caused decreased plasma isoleucine, valine, and a-keto-b-methylvaleric acid and a-keto isovaleric acid concentrations. These decreases were not caused by increased degradation of these metabolites to carbon dioxide as BCAA oxidation rates in vivo were unchanged by leucine loading and the degradative enzymes were unchanged in adequately fed rats. The authors suggest that the decreased concentrations of these amino and keto acids may be the result of decreased protein degradation or increased protein synthesis, possibly mediated by insulin. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 209 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 209 In another study, the mechanisms of protein gain during protein feeding were investigated using a combination of oral and intravenous labeled leucine in healthy young men.27 The oral labeled leucine was administered, either as a free oral tracer (13C- or 2H3-leucine) added to unlabeled whey protein, or as whey protein intrinsically labeled with L-[1–13C] leucine. When the oral tracer was free leucine, it appeared in the plasma more rapidly than the unlabeled leucine derived from the whey protein, and this resulted in an artifactual 88% decrease of protein breakdown. When the oral tracer was protein bound, protein breakdown did not change significantly after the meal. By contrast, nonoxidative leucine disposal (i.e., protein synthesis) was stimulated by 63% by the meal. The authors concluded that: 1. An intrinsically labeled protein is more appropriate than an oral free tracer to study postprandial leucine kinetics under nonsteady state conditions. 2. Protein gain after a single whey-protein meal solely results from an increased protein synthesis with no modification of protein breakdown. Louard et al. examined the effects of infused BCAAs on whole-body and skeletal muscle amino acid kinetics in 10 postabsorptive normal subjects.28 Ten control subjects received only saline. Infusion of BCAAs caused a fourfold rise in arterial BCAA levels and a twofold rise in BCAA keto acids. Plasma insulin levels were unchanged from basal levels. Their results implied that BCAA infusion results in a suppression of whole-body and forearm muscle proteolysis. Since insulin levels were unaffected by the infusion of BCAAs, the observed alterations in muscle and whole-body amino acid kinetics appear to be independent of insulin, and are probably mediated by the elevated BCAA concentrations themselves. A study looked at the effects of long-term BCAA administration on metabolic and respiratory parameters.29 The authors concluded that the physical fitness of BCAA-treated subjects was improved, and that BCAA seemed to promote protein synthesis in the fat-free body mass. Additionally, leucine has been shown to increase the release of acetoacetate and 3-hydroxybutyrate (as a result of the partial oxidation of leucine).30 The ketone bodies, b-hydroxybutyrate and acetoacetate, are known to be a useful source of energy, and also show a glucose-sparing effect.31 It has been shown experimentally that oral or parenteral intake of b-hydroxybutyrate decreases the amount of protein catabolized in obese people on starvation diets.32 Both a-ketoisocaproic (KIC) acid and b-hydroxy-b-methylbutyrate (HMB; see later) also seem to have some ketone-like anticatabolic effects. In a review the authors concluded that:33 1. The ketone bodies, D-b-hydroxybutyrate and acetoacetate, inhibit glycolysis thereby reducing pyruvate availability which leads to a marked inhibition of BCAA metabolism and alanine synthesis in skeletal muscles from fasted mammalian and avian species. 2. The rate of glutamine release from skeletal muscles from fasted birds is increased at the expense of alanine in the presence of elevated concentrations of ketone bodies because of an increase in the availability of glutamate for glutamine synthesis. 3. Ketone bodies inhibit both protein synthesis and protein degradation in skeletal muscles from fasted mammalian and avian species in vitro. The mechanisms involved remain unknown. 4. Inhibition of amino acid metabolism and protein turnover in skeletal muscle by ketone bodies may be an important survival mechanism during adaptation to catabolic states such as prolonged fasting. Alvestrand et al.34 found leucine to decrease the net degradation of muscle protein even in normal resting subjects, something Blomstrand and Newsholme35 suggested might aid repair and Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 210 29.10.2007 7:57pm Compositor Name: JGanesan 210 Amino Acids and Proteins for the Athlete: The Anabolic Edge recovery and be of importance for athletes doing regular physical training. Alvestrand found that approximately 40% of the leucine taken up by muscle was accumulated in the intracellular free pool, some 20% could have been incorporated into protein, and 40% was probably oxidized. Studies using BCAAs have found them to have a beneficial effect on the synthesis of proteins under special circumstances. For example, one study showed that BCAAs have a specific effect on the synthesis of plasma proteins by cultured hepatocytes.36 Another study found a positive benefit from BCAAs as the protein component of total parenteral nutrition in cancer cachexia.37 On the other hand, while we have learned a lot in the last decade, the role of the BCAAs in protein synthesis is not completely clear. The concepts for explaining the effects of BCAAs on protein turnover in skeletal muscle were once based on the assumption that the BCAA or leucine alone might become rate-limiting for protein synthesis in muscle under catabolic conditions. However, even back then in one study using suckling rats, the use of the leucine analogue norleucine was found to stimulate protein synthesis even though norleucine cannot replace any of the BCAAs in protein.38 Several amino acids show changes in intracellular and extracellular concentrations in response to exercise, suggesting that skeletal protein and amino acid catabolism, and gluconeogenesis is increased by exercise.39 The BCAAs (leucine, isoleucine, and valine) are specifically utilized by muscle metabolism, although some evidence indicates their use by other organ tissues.40 Leucine and other BCAAs, unlike most other amino acids, are oxidized (used as energy) by muscle cells, and are a source of cellular energy in the form of ATP and PC. They are also involved in the glucose–alanine cycle (see section on alanine). There is a significant activation of BCAA metabolism with prolonged exercise, and current studies indicate that this is more pronounced in endurance-trained subjects than in untrained controls.41 Plasma concentrations of the BCAAs (leucine, isoleucine, and valine) are more prominently affected than the concentrations of other amino acids by changes in dietary caloric, protein, fat, and carbohydrate intake in humans.42 BCAAs administered before exercise affect the response of some anabolic hormones, mainly GH, insulin, and testosterone.43 A study conducted by Carli et al.43 examined the effects of BCAA administration on the hormonal response in male marathon runners through a 1 h running test at constant speed. In this study, using 14 male long-distance runners, two trials were carried out at 1-week intervals. In each trial, 1 h of running was performed at a predetermined speed for each athlete. In the first trial (E) the athletes were given a commercial diet product containing BCAAs (5.14 g leucine, 2.57 g isoleucine, and 2.57 g valine), with milk proteins (12 g), fructose (20 g), other carbohydrates (8.8 g), and fats (1.08 g), the total energy content being 216 kcal. In the second trial (P), the athletes received a similar mixture with the same energy content but without BCAAs, which were replaced by additional milk proteins (10 g). In both the trials the mixture was consumed approximately 90 min before the running test. The BCAA mixture ingested by the athletes in the E trial elicited, in resting conditions, a sustained elevation in plasma BCAA levels which lasted several hours. In the whole-milk protein without the BCAA trial there was a decrease in serum-free testosterone (as measured by the decreased testosterone to SHBG ratio) and SHBG. This was thought to have been an indication of increased metabolic clearance of testosterone. In the E trial, testosterone was unchanged at the end of the exercise period and it actually increased in the following rest period. The authors concluded, ‘‘we would suggest that exogenous administration of BCAA and their uptake by muscle fibers could limit endogenous amino acid oxidation and indirectly prevent testosterone muscle clearance. This suggestion is also supported by the direct anabolic effect of BCAA, mainly leucine, on muscle proteins.’’44 In this study, the authors suggest that BCAAs may be rate-limiting for muscle protein synthesis. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 211 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 211 An anticatabolic effect of BCAA administration was also indicated by the values of the testosterone to cortisol ratio, an indicator of changes in the anabolic–androgenic activity of the body,45 in the last sample. In summary, the study showed that the hormonal response to exercise was modified by BCAA ingestion and that the anabolic hormones insulin and especially testosterone were favorably affected when BCAAs were substituted for equivalent amounts of whole-milk proteins. These findings are extremely important and point out the advantages of BCAA supplementation over the use of wholefood proteins and even whole-food protein supplements. Of the BCAAs, leucine appears to be the most important for athletes. Leucine affects various anabolic hormones and has anabolic and anticatabolic effects. It is also involved in nitrogen metabolism and ammonia removal. A study found that leucine infusion depressed muscle proteolysis.46 The use of large amounts of leucine and not valine and isoleucine, while decreasing the serum levels of valine and isoleucine, does not seem to adversely affect the rate of protein synthesis.47 In another study, the plasma and muscle concentrations of tyrosine, phenylalanine, and BCAAs were measured before and after two types of sustained intense exercise during which the subjects were given a mixture of BCAAs or a placebo.35 An increase in the muscle concentration of tyrosine and=or phenylalanine could be an indication of net protein degradation in skeletal muscle since these amino acids are neither taken up nor metabolized by this tissue. The investigation comprised two separate studies, both involving male subjects. In one aspect of the study, 26 subjects participated in a 30 km cross-country race. In a second aspect, 32 subjects participated in a full-length marathon race (42.2 km). All subjects were well trained. In both the races the subjects were randomly divided into two groups. One group, the test group, was given a drink containing a mixture of BCAAs in a 5% (cross-country) or 6% (marathon) carbohydrate solution. The other group (placebo) was given 5% and 6% carbohydrate solutions in the cross-country and marathon races, respectively. The total amount of BCAAs given to each subject in the cross-country race was 7.5 g (2.6 g leucine, 3.8 g valine, and 1.1 g isoleucine) and in the marathon race 12 g (4.2 g leucine, 4.8 g valine, and 3.0 g isoleucine). The results of the study showed that an intake of the BCAA mixture prevented an exerciseinduced increase in the muscle concentration of tyrosine and phenylalanine which was found in the placebo groups. This, the authors remarked, might indicate an inhibitory effect of BCAAs on the net rate of protein degradation in the working muscles during exercise. Furthermore, they added ‘‘ . . . a decreased net rate of protein degradation during exercise might improve physical performance and would probably aid repair and recovery after such intense exertion. Hence, this finding could be of some importance for athletes doing regular physical training.’’ The authors suggest that the intake of BCAAs during exercise might decrease protein degradation that occurs in human skeletal muscle during that exercise. The intake of BCAAs during exercise might also alleviate some of the fatigue seen in prolonged exercise. Plasma levels of free tryptophan have been shown to increase during exercise and the levels of BCAAs to decrease. Thus the ratio of free tryptophan to BCAAs increases markedly with some forms of exercise. It is thought that this increased ratio may lead to an increase in the transport of tryptophan across the blood–brain barrier and hence to an increase in the synthesis of 5-hydroxytryptophan (5-HTP or serotonin) in specific areas of the brain and thus may be responsible for the development of some of the physical=mental fatigue seen during prolonged exercise.48 There are a number of compounds that potentially may have more of an anticatabolic and anabolic effect than the BCAA. For example, one study on amino acid metabolism with exercise49 found that plasma concentration of glutamine, the branched-chain keto acids (BCKA) and shortchain acyl carnitines were elevated with exercise. Utilization of compounds such as carnitine with or without the use of amino acids may spare muscle glutamine and BCAAs and thus may have a greater muscle-sparing effect than just the use of amino acid combinations. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 212 29.10.2007 7:57pm Compositor Name: JGanesan 212 Amino Acids and Proteins for the Athlete: The Anabolic Edge The significance of the serum changes of amino acids with exercise has not been fully explored. Serum amino acid levels can change for a number of reasons. For example, increases in the serum level of any amino acid could be due to decreased use, increased release, or increased formation of that amino acid and might be associated with increased use of that amino acid by other tissues. Thus, in some instances of increased levels of an amino acid, such as glutamine, supplementation of that amino acid might be necessary to decrease protein catabolism and increase protein synthesis. In other instances, increased levels of an amino acid might simply show decreased use of that amino acid and supplementation would not be of any value. In one study, the quantitative relationship between insulin and plasma amino acid levels was characterized in five healthy young men during a number of euglycemic insulin infusions.50 While 8 of 10 amino acids measured decreased in a dose-responsive pattern to increasing levels of insulin—alanine and glycine concentrations remained unaffected. In this instance, the use of exogenous alanine would not affect protein metabolism, whereas the use of some of the other amino acids, such as the BCAA would. Alanine supplementation (since it is a major gluconeogenic precursor) might be useful before and=or during exercise since alanine flux would increase as exercise time and intensity increased. BCAAS AND MUSCLE HYPERTROPHY Protein synthesis is the process in which amino acids are linked together by peptide bonds to form one or more long polypeptide chains. Once a chain or group of chains has achieved a stable threedimensional configuration and is biologically active as a result, it is termed a protein. In a given cell, all proteins are fabricated from a common pool of free amino acids. As synthesis proceeds, the required amino acids are obtained from this cellular pool and are carried to the growing polypeptide chain by tRNA molecules. When the intracellular free amino acid pool is limiting, protein synthesis would be expected to suffer. Indeed, the apparent importance of the prevailing amino acid concentration has been demonstrated by several studies which found that increased amino acid availability alone may augment skeletal muscle as well as whole-body protein synthesis in young healthy men.51–53 Thus, supplemental BCAAs, by helping to maintain the free amino acid pool in skeletal muscle fibers, could conceivably increase the rate of protein synthesis beyond that achievable by foodstuffs alone. BCAA supplements may also be helpful in reducing protein catabolism.54 The degradation of muscle proteins is a normal part of animal metabolism. An important early adaptation to fasting is the mobilization of amino acids from skeletal muscle protein for gluconeogenesis or direct oxidation.55 This net loss of muscle protein in fasting is due to both a decrease in the overall rate of protein synthesis and an enhancement of proteolysis. Several hormones, in particular glucocorticoids (e.g., cortisol) and insulin, appear to be involved in mediating these changes in protein turnover. Interestingly, some researchers have suggested that anabolic steroids may exert some of the observed muscle-building effects by antagonizing the catabolic effects of glucocorticoids. Insulin deprivation also leads to activated muscle-protein catabolism, and this effect likely plays a role in mediating the increased proteolysis seen during fasting. During exercise, a greater availability of BCAAs in muscle tissue could help to maintain a more favorable testosterone to cortisol ratio. This would be expected to enhance the productivity of the training session in terms of its growth-stimulating capacity. Additionally, leucine (as well as arginine) are potent insulin secretagogues, and so in this way may be of help in reducing the catabolism of muscle protein. The transamination of leucine to a-KIC acid appears to be important for its anticatabolic effects on muscle protein. Hence, supplements containing this compound may prove more effective in this regard. Muscle protein accretion is the net difference between protein synthesis and degradation and results in muscle hypertrophy during growth and development. Hence, in order to achieve optimal Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 213 29.10.2007 7:57pm Compositor Name: JGanesan 213 Essential Amino Acids gains in muscle mass, it would seem prudent to optimize muscle protein synthesis while minimizing muscle protein catabolism. As the results of the aforementioned studies suggest, the use of BCAAs could prove helpful in this regard. BCAAS AND HYPOXIA Exposure to high altitude often results in a loss of body mass. This has been shown to be due to a reduction of both lean body mass, attributable to skeletal muscle wasting, and to fat mass.56 On the ultrastructural level, it has been shown that the muscle fibers apparently shrink, causing the relative capillary density to increase. This effectively reduces the distance that oxygen must diffuse to reach the site of cellular respiration (i.e., the mitochondria), thereby effectively enhancing the performance of the muscle fiber under conditions of chronic hypoxia.57 The cause of muscle wasting under hypoxic conditions is not entirely understood. Reduced caloric intake and=or a direct effect of hypoxia on protein metabolism could play a role. Another possible explanation for high-altitude muscle loss could be a relative lack in the intake or absorption of some essential amino acids, which would therefore be mobilized from skeletal muscles. Since this can be limited by a dietary supplement of BCAAs, the authors of one study thought that this may also be the case for subjects exposed to chronic hypoxia.58 To test the hypothesis, they administered BCAAs to a group of healthy human subjects during a 21-day trek through the Peruvian Andes. In all, 16 subjects (11 men, 5 women) participated in the study. The subjects were subdivided into two groups for age, sex, and fitness factors by an independent researcher unaware of the aim of the study and not involved in the project. All the members of the two groups received, in a double blind manner, envelopes containing either a mixture of BCAAs (per envelope 1.2 g leucine, 0.6 g isoleucine, and 0.6 g valine) or an inert substance with the same appearance and taste (placebo). The dosage was two envelopes three times a day for the duration of the trek (total per day 7.2 g leucine, 3.6 g isoleucine, and 3.6 g valine). Upon return from the trek, all subjects (with the exception of one subject from the BCAA group) had lost a significant amount of body mass. In the placebo group, the loss amounted to 1.8 kg, which skin fold measurements revealed was partitioned as 90% fat mass and 10% lean body mass. In the BCAA group, the loss of body mass was less (1.07 kg), but still significant compared to pre-trek values. In this group, however, the decrease in body mass was only due to loss of fat, whereas the lean mass tended to increase by 0.71 kg. The increase was nonsignificant. The authors remarked ‘‘From the differences in lean body mass one may conclude that on the whole the placebo group was catabolizing, whereas the BCAA group was synthesizing muscle tissue.’’ The muscle-sparing response to the BCAAs under hypoxic conditions may involve an interplay between mTOR and AMPK signaling pathways,59 and the ubiquitin-proteasome system,60 resulting in an increase in protein synthesis and a decrease in degradation. REGULATION OF MUSCLE PROTEIN SYNTHESIS BY LEUCINE Among the amino acids, leucine seems to be the most important in regulating protein synthesis.61 In recent years, the involvement of leucine in the regulation of muscle protein metabolism (increasing synthesis and decreasing degradation), both directly and indirectly, has been reexamined and has led to advances on understanding of the mechanisms involved, including the mTOR signaling pathway and the skeletal muscle protein degradation by the ubiquitin-proteosome pathway.23,62,63 The protein anabolic role of BCAAs seems to be mediated not only by their important role as a promoter of the translation process (and possibly acting at the transcription level) but also by inhibition of protein degradation.64 Leucine may play a critical role in these signaling pathways.65 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 214 29.10.2007 7:57pm Compositor Name: JGanesan 214 Amino Acids and Proteins for the Athlete: The Anabolic Edge Supplementation with BCAAs spares lean body mass during weight loss, promotes wound healing, may decrease muscle wasting with aging, and may have beneficial effects in renal and liver disease. BCAA supplementation is extensively used in the athletic field with the assumption of improved performance and muscle mass. Leucine stimulates and has a regulatory effect on protein synthesis, either directly or indirectly via the metabolites KIC acid and HMB.66,67 Protein synthesis in mammalian cells is regulated through alterations in the states of phosphorylation of eukaryotic initiation factors and elongation factors and of other regulatory proteins.68 Leucine can stimulate translation via the mTOR in part through AMPK activation,69 or independently.70 The latter is supported by the observation that skeletal muscle protein synthesis and eIF4E– eIF4G assembly are enhanced by the administration of leucine in the presence of rapamycin (a specific inhibitor of mTOR). Additionally, oral leucine administration enhanced the phosphorylation of 4E-BP1, showing the stimulation effect of leucine on protein synthesis in skeletal muscle of normal rats.71 We are now beginning to understand the cellular mechanisms by which amino acids modulate protein synthesis. Amino acids, and leucine in particular, consistently activate S6K1 and promote translation.72 Among BCAAs, leucine apparently has the greatest effect in activating S6K1.73,74 Since increased S6K1 activity has been associated with increased protein synthesis and enhanced translation of mRNA,75 the results of a recent study concluded that the oral administration of leucine-rich diet induced the increase of the protein synthesis in skeletal muscle in tumor-bearing rats possibly through the activation of eIF factors and=or the S6 kinase pathway.76 In this study, leucine tended to increase the expression of eIF4G in skeletal muscle. This slight increase in the amount of eIF4G could result in greater binding to eIF4E. The regulation of the initiation of translation depends on the availability of eIF4E, the levels of which were maintained in the leucine-rich diet group. LEUCINE AND BODY COMPOSITION Most people want to look good and fit and that means having less body fat and more muscle mass. Athletes, while not immune to looking fit and lean, are mainly looking to maximize body composition in order to maximize performance. While bodybuilding is the ultimate sport for maximizing muscle mass and minimizing body fat, for athletes in all sports, including both the power and endurance events, carrying excess body fat is detrimental to performance and maximizing muscle mass per pound of bodyweight is a universal goal. As we have seen, increased protein intake coupled with exercise has been shown to result in improvements in body composition, an increase in muscle mass, and a decrease in body fat. Increasing protein intake results in increased satiety (secondary to various influences of dietary protein including suppression of ghrelin and elevation of cholecystokinin), and subsequently decreased calorie intake.77–79 Coupled with calorie restriction with and without exercise, increasing protein intake has been shown to result in effective weight loss with a preservation of muscle mass, and more of the weight loss represented by the loss of body fat.80–86 Increasing dietary protein also helps to keep the weight off.87,88 And decreasing dietary carbohydrate intake seems to have an additive effect in increasing weight and fat loss and improving body composition.89–92 The BCAAs, and especially leucine, even at low doses20 have favorable effects on weight and fat loss, and body composition.7,8 The positive effects of leucine on protein metabolism, increasing protein synthesis, and decreasing catabolism, have been known for over three decades.93,94 Recently, however, some of the mechanisms involved in leucine’s effects on protein metabolism have been elucidated, including its effects on translation initiation of protein synthesis (through initiation factors eIF4E and eIF4G and ribosomal protein S6), effects on mTOR and other signaling Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 215 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 215 pathways (including AMPK, PI3-kinase, and PKB activation of rpS6), insulin signaling, and other processes.95 Several studies reporting an increase in protein synthesis have used high doses of leucine, often equivalent to the daily intake, administered in a single dose. However, in a recent study, amounts corresponding to 10% of daily leucine intake were sufficient to acutely increase the protein synthesis rate in the gastrocnemius muscle of rats.96 Although the use of leucine supplementation increases protein synthesis there may be one caveat with overusing it. It is possible that supplementation with leucine alone may have side effects due to impairment in the availability of valine and isoleucine, because the activity of the ratelimiting enzymatic complex in BCAA degradation, that is, branched-chain a-keto acid dehydrogenase is markedly stimulated by the presence of leucine or its keto acid, a-ketoisocaproate.97–99 A recent study, evaluated the effects of chronic leucine supplementation on body composition and some indicators of protein nutritional status.20 The results of this study indicate that long-term, low-dose leucine supplementation increases body fat loss and improves liver-protein status and the capacity of muscle protein synthesis in rats that are subjected to food restriction. BRANCHED-CHAIN KETO ACIDS BCKAs are both made from, and converted into, BCAAs. The only difference being the presence of a ‘‘keto’’ group instead of an ‘‘amino’’ group. In essence, BCKAs can be viewed as ammonia-free sources of BCAAs. The amino groups from the BCAAs are transferred to AKG to form the corresponding BCKA and glutamate. This transfer is reversible and for all three BCAAs is catalyzed by the BCAA aminotransferase, which has a central role in the regulation of BCAA catabolism.100 The next step is the decarboxylation of the carboxyl group, which is irreversible, and then via either ketogenic or gluconeogenic processes end up in the Krebs cycle.101 BCKAs (like BCAAs) can increase lean body mass and improve nitrogen retention. They are also essential for energy production in muscle and for the removal (detoxification) of ammonia.102 The most pronounced results are obtained in athletes during strenuous and=or prolonged bouts of exercise, as well as in individuals consuming high-protein diets. BCKAs can function as dietary supplements for the essential BCAAs, although their capacity to generate BCAAs is well below 100%. In one study, for example, the efficiency for conversion in normal rats (and in rats with impaired kidney function) was 50% (meaning that only half of the ingested BCKAs were made into BCAAs).103 Other studies have reported similar findings in both humans and animals (between 35% and 65% conversion). The likely reason that this value is so low is that BCKAs are picked up and burned (oxidized) in various organs before they can be made into BCAAs. Exercise has profound effects on the intracellular and extracellular levels of amino acids and their metabolites. In one study using rats, plasma concentration of glutamine, the BCKAs, and shortchain acyl carnitines were elevated with exercise. Ratios of plasma BCAAs to BCKAs were dramatically lowered by exercise in the trained rats. A decrease in plasma-free carnitine levels was also observed. The data from this study suggests that amino acid metabolism is enhanced by exercise even in the trained state. Moreover, BCAAs may only be partially metabolized within muscle and some of their carbon skeletons are released into the circulation in forms of BCKAs and short-chain acyl carnitines. It is possible that supplying these compounds may decrease the catabolism of BCAAs so that they can be used for protein synthesis. a-KETOISOCAPROIC ACID a-Ketoisocaproate—the keto acid of leucine—is clearly the most important BCKA. KIC is anabolic, and anticatabolic, particularly during catabolic states. Since any intensive, strenuous activity Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 216 29.10.2007 7:57pm Compositor Name: JGanesan 216 Amino Acids and Proteins for the Athlete: The Anabolic Edge is also catabolic, there is reason to believe that the BCKAs will prove to be of value to bodybuilders, power-lifters, and aerobic athletes. Both leucine and KIC are metabolized further to a-amino-n-butyrate and HMB. These compounds have not as yet been investigated in humans for their anabolic and protein-sparing properties. However they may have some anabolic and anticatabolic effects. Although KIC is the transamination product of the BCAA leucine, and both tend to increase in parallel in the serum when exogenous leucine is used,34 there is substantial evidence that KIC has anabolic and protein-sparing properties separate from leucine.104,105 On the other hand, leucine has properties apart from its transamination to KIC. In one study, the effects of leucine, its metabolites, and the 2-oxo acids of valine and isoleucine on protein synthesis and degradation in incubated limb muscles of immature and adult rats were investigated.106 The results of this study seemed to point to an anabolic and anticatabolic effect of leucine which was found to stimulate protein synthesis and reduce protein catabolism. However, the anticatabolic effects of leucine, in contrast to its anabolic effects, required its transamination to the 2-oxo acid, 4-methyl-2-oxopentanoate, or KIC. While the oxo acid of leucine was found to have significant effects, the 2-oxo acids of valine and isoleucine did not affect protein synthesis or degradation. Interestingly, the authors of this 1984 study felt that the effect of KIC in preventing muscle degradation was likely due to the accumulation within cells of a metabolite of KIC (such as HMB). They also felt that it is KIC rather than leucine that reduced the catabolic process. Supplements containing KIC in pharmacological doses have been shown to decrease the rate of 3-methylhistidine excretion by patients with Duchene’s muscular dystrophy.107 3-Methylhistidine, which occurs only in the myofibrillar proteins, is often used as an indicator of contractile protein degradation.108 The results of these studies confirmed the results obtained from an earlier study that concluded that KIC showed anticatabolic but not anabolic effects.109 The results also showed that KIC reduced muscle catabolism but did not stimulate protein synthesis. Leucine, on the other hand, seemed to have effects on protein synthesis. Interestingly, the authors concluded that neither of the other two BCAAs (isoleucine and valine) promoted protein synthesis. Perhaps, one of the more important actions of KIC (and also leucine) is its enhancement of the anabolic drive via stimulation of insulin secretion. Both leucine110 and KIC (and other amino acid compounds such as arginine, ornithine, and ornithine a-ketoglutarate [OKG]) stimulate insulin secretion. Insulin increases the intercellular transport of amino acids, and has both anabolic and anticatabolic effects. A few recent studies have suggested that KIC, alone or in combination with other amino acids, may improve athletic performance,111 and reduce signs and symptoms of exercise-induced muscle damage.112 However, more studies are needed to confirm these results. b-HYDROXY-b-METHYLBUTYRATE HMB, a leucine catabolite, may also have ergogenic effects. The classical pathway of leucine metabolism involves conversion to a-ketoisocaproate, transport into the mitochondria, and oxidation to 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA), which can then be converted to ketones. An alternative leucine metabolic pathway has been described. In this pathway, leucine is converted to KIC, and KIC is oxidized to HMB by a cytosolic enzyme called KIC-dioxygenase.113 The HMB produced is further metabolized in at least three ways: The first fate is further metabolism to HMG-CoA. This cytosolic source of HMG-CoA plays at least some role in cholesterol synthesis. The second fate of HMB produced in the cell is loss in the urine. Previous feeding studies indicate that up to 40% of the HMB fed is lost in the urine and likely accounts for the relatively short half-life of HMB of about 1 h. The third fate of HMB appears to be the further use by the cell. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 217 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 217 HMB meets the criteria of a dietary supplement because it is found in some foods in small amounts (such as in catfish and some citrus fruits), it is found in breast milk, and it is used and produced by body tissues. As yet, there has been no evidence that HMB is a necessary nutrient and that the body utilizes it preferentially for modulating protein synthesis. However, it seems to have effects on protein synthesis and may help maximize the anabolic effects of exercise. HMB has been seriously studied for more than a decade and found to have an effect on protein synthesis and lean body mass. Animal studies have shown that HMB appears nontoxic and may counteract the effects of stress, and increase growth and health of animals.114–117 One study looked at effects of leucine and its catabolites on in vitro, mitogen-stimulated DNA synthesis by bovine lymphocytes.118 The results of the study suggest that leucine is necessary for mitogen-induced DNA synthesis by bovine lymphocytes, and that this requirement for leucine can be partially met by KIC. When leucine was not limiting, KIC, HMB, and HMG at concentrations that might occur in vivo did not alter lymphocyte DNA synthesis in vitro. Thus, it would appear that in some circumstances and in some tissues, HMB is lower down on the anabolic and anticatabolic scale than either leucine or KIC. However, a few studies suggest that HMB has significant anticatabolic potential in skeletal muscle. Especially promising is a study that has been accepted for publication in which supplementation with either 1.5 or 3.0 g HMB=day was shown to partly prevent exercise-induced proteolysis=muscle damage and result in larger gains in muscle function associated with resistance training in humans.67 In this study, the effects of dietary supplementation of HMB were studied in two experiments. In Study 1, 41 untrained (no participation in a resistance-training program for at least 3 months) subjects were randomized among three levels of HMB supplementation (0, 1.5, or 3.0 g HMB=day) and two protein levels, and who weight lifted (WL) 1.5 h, 3 days a week, for 3 weeks. In Study 2, subjects were fed either 0 or 3.0 g HMB=day for and who WL for 2–3 h, 6 days a week, for 7 weeks. In Study 1, HMB significantly decreased the exercise-induced rise in muscle proteolysis, as measured by urine 3-methylhistidine, during the first 2 weeks of exercise. Plasma CPK, was also decreased with HMB supplementation. WL increased by HMB supplementation when compared with the unsupplemented subjects during each week of the study. After a 1-week adaptation period, the 3-week supplementation=exercise protocol was started. After only 1 week of training=supplementation, muscle-protein breakdown in the group fed 3.0 g HMB was decreased by 44% (compared to placebo). Muscle-protein breakdown continued to be lower in the HMB group for the entire 3-week study. A second indicator of muscle damage and muscle breakdown is the muscle-specific enzyme called CPK. This enzyme was also markedly decreased with HMB supplementation. The biochemical indicators of muscle damage were also accompanied by demonstrable increases in muscle strength and muscle mass. Lean body mass increases for the 0, 1.5, and 3.0 g HMB=day were 0.4, 0.8, and 1.2 kg=3 weeks, respectively. Strength increases for the 0, 1.5, and 3.0 g HMB=day were 10%, 23%, and 29%, respectively. In Study 2, in which the intensity of exercise was much higher, fat-free mass was significantly increased in HMB-supplemented subjects compared with the unsupplemented group at 2, 4, 5, and 6 weeks of the study. This second part of the study may be of even more significance than the first part in that proteolysis would ordinarily be expected to be higher, with a danger of overtraining, and therefore the anticatabolic effects of HMB would be even more prominent. Nissen et al. have subsequently carried out another study to see if HMB has similar effects in trained subjects. The results of this study were presented at the 1996 Experimental Biology Meeting on April 15, 1996. In this 4-week, double-blind study, 3.0 g of HMB per day in capsules were given in three divided doses to trained and untrained subjects while they participated in an intense weighttraining program. HMB was equally effective both for the previously trained and untrained groups, so results were pooled for HMB supplementation. Overall, HMB increased lean body mass 4.44 lb or 3.1% and decreased body fat by 2.17 lb or 7.3% (both significantly better than control groups). The Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 218 29.10.2007 7:57pm Compositor Name: JGanesan 218 Amino Acids and Proteins for the Athlete: The Anabolic Edge HMB-supplemented subjects also showed an increase in their bench-press strength of 22 lb, which represents a 37% absolute increase over that of controls, and similar results were found for other exercises. Although it seems from the available studies that HMB has some influence on protein metabolism, the mechanisms behind any effects are unknown. HMB may be a compound that regulates protein synthesis either through hormonal receptor effects (cortisol, testosterone, GH, IGF-1, insulin) or by modulating the enzymes responsible for muscle tissue breakdown. HMB may have effects on the metabolism of leucine and glutamine, and perhaps other anabolic and anticatabolic amino acids, or may decrease gluconeogenesis and the subsequent oxidation of amino acids in the intracellular amino acid pool and catabolism of skeletal muscle cellular protein. While HMB has generally been found not to have any useful ergogenic or body composition effects,119–123 there are some studies that show it has some potential in increasing athletic performance. For example, several studies have shown that the use of HMB with resistance exercise may reduce protein degradation and increase recovery of damaged muscle cells,124 may increase performance by increasing the onset of blood lactate accumulation,125 and may improve body composition of both young and old when used along with resistance training.67,126 Moreover, a recent study found that while HMB had no effect on body composition, it did result in gains in maximum oxygen consumption, thereby possibly affecting aerobic performance.127 There is also some evidence that the use of HMB with other nutrients, by acting via different mechanisms, may have a synergistic or additive effect on body composition and muscle strength.128 In summary, it appears that while HMB may have some effects under some circumstances on body composition, strength, and athletic performance, the overall consensus is that it is not an especially effective ergogenic aid. LYSINE While lysine’s involvement in protein synthesis is its main role, it is also a precursor for the biosynthesis of carnitine, which plays an important role in b-oxidation.129 Intake recommendations to meet lysine requirements in infants and adults have recently been established.130 Low levels (as seen in some vegetarian diets since lysine levels are low in all cereal proteins— see earlier) can slow protein synthesis, affecting muscle and connective tissue. It has an inhibitory effect on viral growth and is used in the treatment of herpes simplex.131 Information on effects of lysine has been documented in other sections of this book. In this section, we will concentrate on the lysine–carnitine connection and the effects of carnitine and acetyl-L-carnitine. L-CARNITINE More than 95% of the body’s total carnitine (L-3-hydroxy-4-N,N,N-trimethylaminobutyric acid) store exists within skeletal muscle tissue as either free or acyl carnitine.132 L-carnitine (LCAR) is considered by many to be essential nutrient with critical roles in energy and macronutrient metabolism. 1. It is the substrate for carnitine palmitoyltransferase 1 (CPT1) and thus plays an essential role in the translocation of long-chain fatty acyl groups into the mitochondrial matrix for subsequent b-oxidation (see Figure 7.1). CPT1, which is located on the outer membrane of mitochondria, functions as a limiting enzyme in a long-chain, fatty-acid oxidation pathway. Malonyl-CoA, a competitive inhibitor of CPT1, is formed from acetyl-CoA by a reaction catalyzed by acetyl-CoA carboxylase (ACC). Decreasing levels of malonyl-CoA—whether by downregulating the synthesis of acetyl-CoA carboxylase or upregulating malonyl-CoA decarboxylase (MCD)—increase CPT levels. AMPK appears to regulate both enzymes, inactivating ACC while activating MCD.133,134 2. LCAR buffers excess mitochondrial acetyl-CoA units (in a reaction catalyzed by carnitine acetyltransferase), generally produced when pyruvate dehydrogenase complex Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 219 29.10.2007 7:57pm Compositor Name: JGanesan 219 Essential Amino Acids ATP+CoA AMP + PPi Acyl-CoA FFA Carnitine palmitoyltransferase 1 Acyl-CoA synthetase Outer mitochondrial membrane Acyl-CoA Carnitine Carnitine acylcarnitine translocase Carnitine palmitoyltransferase 2 CoA Acylcarnitine Acylcarnitine Carnitine Inner mitochondrial membrane Acylcarnitine Acyl-CoA Transport of fatty acids from the cytoplasm to the inner mitochondrial space for oxidation. Following activation to a fatty-CoA, the CoA is exchanged for carnitine by carnitine-palmitoyltransferase 1. The fatty carnitine is then transported to the inside of the mitochondrion where a reversal exchange takes place through the action of carnitine-palmitoyltransferase 2. Once inside the mitochondrion, the fatty-CoA is a substrate for the β-oxidation machinery. FIGURE 7.1 Transportation of long-chain fatty acyl-CoA across the mitochondrial membrane. (From http:== isu.indstate.edu=mwking) flux (production of acetyl-CoA from pyruvate) is greater than the rate of acetyl-CoA use by the TCA cycle. The depletion of LCAR by this acetylation can deplete the free carnitine pool and thus decrease the transport of long-chain fatty acids into the mitochondria and subsequently decrease the oxidation of fatty acids.135 Supplementing with LCAR also increases CPT1 activity, perhaps by acting as a buffer and preventing the buildup of unbound short- and medium-chain acyl-CoA which may be inhibitory to CPT1. Ketogenesis is likely reduced as the increase in CPT1 activity allows the use of fatty acids as fuel by most tissues, decreasing the overall need for ketones for energy metabolism. Several studies have shown that LCAR decreases the production of some of the pro-inflammatory cytokines and has anti-inflammatory and immunomodulating effects.136,137 In a recent study, the authors concluded that the use of LCAR can improve cellular defense against chronic inflammation and oxidative stress, most likely by modulating the specific signal transduction cascade activated by an overproduction of pro-inflammatory cytokines and oxidative stress.138 Carnitine is essential for fatty acid transport and proper muscle function, and some studies have shown that carnitine supplementation improves exercise performance.139 L-Carnitine Metabolism LCAR ( (g)-trimethylamino-3-hydroxybutyrate), an endogenous compound widely distributed in mammalian tissues, is metabolized from lysine and methionine, although its true point of origin is Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 220 29.10.2007 7:57pm Compositor Name: JGanesan 220 Amino Acids and Proteins for the Athlete: The Anabolic Edge trimethyllysine. This molecule is either obtained from the diet or is synthesized in the body from L-lysine which is methylated three consecutive times by a SAMe. Trimethyllysine is transformed into hydroxy-trimethyllysine, then into trimethylaminobutyraldehyde, and finally into trimethylaminobutyrate (or g-butyrobetaine). The g-butyrobetaine is hydroxylated into carnitine. This reaction chain only functions well when three vitamins—ascorbic acid, pyridoxine, and niacin—are present.140 Studies on rats have shown that skeletal muscle, heart, intestines, testis, and especially kidneys, insure the transformation of trimethyllysine into g-butyrobetaine but that happens only in the testis, and especially liver, and can transform hydroxylate g-butyrobetaine into carnitine. However, in rats the relative importance of the kidneys and liver in total carnitine synthesis has not yet been determined. The situation is the same in humans, although it has been proven that human brain and kidneys, as liver, have g-butyrobetaine hydroxylase. It is known that the rate of carnitine synthesis depends on three factors—the amount of trimethyllysine available, the rate of g-butyrobetaine transfer to tissue(s) hydroxylating it, and g-butyrobetaine hydroxylase activity.141 Butyrobetaine undergoes hydroxylation in the liver, brain, and kidney to form carnitine, which in turn is transported via the plasma to the heart and skeletal muscle where it is important for allowing b-oxidation of fatty acids. Carnitine is synthesized in liver and kidney, stored in skeletal muscle, and excreted mainly in urine. Dietary lysine deficiency results in tissue carnitine deficiency.142–146 Vitamin C deficiency results in decreased carnitine synthesis in the liver and decreased carnitine levels in heart and skeletal muscle.147,148 Most of the carnitine needed is ingested through the diet, in particular by meat and dairy products. The rest is biosynthesized. In most cases, endogenous synthetic pathways are sufficient to provide adequate amounts of carnitine to meet the needs of the body. However, circulating carnitine levels of strict vegetarian adults and children, and of infants fed carnitine-free formulas, are significantly lower than normal. In one study, excess amounts of the carnitine precursors, lysine plus methionine, e-N-trimethyllysine or g-butyrobetaine were fed as supplements to a low-carnitine diet for 10 days.149 In this study, dietary g-butyrobetaine dramatically increased carnitine production, e-N-trimethyllysine had a somewhat smaller effect, and lysine plus methionine had even less effect on carnitine synthesis. The authors concluded that carnitine synthesis is not limited by the activity of g-butyrobetaine hydroxylase but that carnitine synthesis from exogenous e-N-trimethyllysine is limited either by enzymatic processes that lead to the final intermediate, g-butyrobetaine, or by the ability of this substrate to enter tissues capable of carrying out these transformations. L-Carnitine Functions The carnitine acyltransferases (acetyl-, octanoyl-, palmitoyl-, and possibly others) esterify tissue acyl-CoA to carnitine-producing acylcarnitine which readily passes into mitochondria. Once in the mitochondria, the long-chain fatty acids are oxidized and used as an energy substrate for many tissues, and are also metabolized to ketones in the mitochondria of liver cells. The main consequence of carnitine deficiency is impaired energy metabolism. LCAR is present in plasma, skeletal muscle, and to a lesser extent in other tissues and is necessary for the transport of long-chain fatty acids across the inner mitochondrial membrane. Besides being necessary for the b-oxidation of long-chain fatty acids at the mitochondrial level, LCAR seems also to have a role in the metabolism of the BCAAs, in ammonia detoxification, and in urea production.150–152 One study concluded that carnitine stimulates decarboxylation of BCAAs by increasing the conversion of their ketoanalogues into carnitine esters, and that a greater carnitine acyltransferase activity in muscle than in liver may be responsible for the greater carnitine effect in muscle.153 Although, carnitine has the ability to be ketogenic154 once formed, ketones are carnitine-independent energy carriers for different tissues. The ketone bodies, b-hydroxybutyrate and acetoacetate, are known to Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 221 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 221 be a useful source of energy, and also show a glucose-sparing effect.31 It has been shown experimentally that oral or parenteral intake of b-hydroxybutyrate decreases the amount of protein catabolized in obese people on starvation diets.32 The CNS can use only glucose and ketones for its energy requirements. Ketones are products of lipid metabolism, and can be readily oxidized in muscle, and in case of glucose deficiency, in the CNS. The two ketones which are biologically important are acetoacetic acid and b-hydroxybutyric acid. During fasting or sustained exercise, once the initial glycogen store is used up by the body the serum glucose must be kept within certain safe limits until the ketones can be formed in adequate amounts. The processes involved in producing sources of energy for the body are all hormonally regulated with insulin, glucagon, GH, epinephrine, and cortisol involved (see earlier discussion). Exercise, fasting, and hypoglycemia increase glucagon, GH, and epinephrine levels. These higher levels stimulate adipose lipase,155 which cleaves triglyceride into glycerol and three FFA molecules. This process leads to increased FFA and ketone production, thus sparing the skeletal muscle protein from catabolism. Glucagon (which increases when insulin decreases), causes a rise in hepatic carnitine concentration, and thus activates the ketone-forming machinery of the liver. Once activated, the rate of ketone formulation is dependent on, and proportional to, the amount of FFA derived from the breakdown of storage fat. In rats, plasma and hepatic carnitine deficiency has been associated with steatosis and retarded growth.146,156 In human infants, systemic carnitine deficiency is characterized by hepatic steatosis, hypoglycemia, and a defect in fasting-induced ketogenesis. In this disorder, hepatic carnitine deficiency leads to a defect in the transport of long-chain fatty acids into the mitochondria, limiting fatty acid oxidation, and resulting in increased reesterification of fatty acids to triglyceride. Carnitine deficiency can result in muscle weakness and hypotonia in patients with cystinosis and renal Fanconi syndrome that are carnitine deficient.157 Carnitine deficiencies are common and can be due to insufficient intake, for example in strict vegetarians as mentioned earlier. Currently, these deficiencies are classified into two groups.158,159 In deficiencies with myopathy only the muscles are deficient in LCAR, perhaps as a result of a primary anomaly of the LCAR transport system in muscles. In systemic deficiencies, LCAR levels are low in the plasma and in all body tissues. Systemic LCAR deficiencies are usually the result of a variety of disease states including deficient intake in premature infants or long-term parenteral nutrition; renal failure; organic acidemias; and Reye’s syndrome. Modifications in LCAR metabolism have also been reported in patients with diabetes mellitus, malignancies, myocardial ischemia, and alcohol abuse.172 Carnitine deficiency is also commonly seen in those following a low-carbohydrate, ketogenic diet. In a recent study,160 the carnitine levels of 19 patients who were treated with ketogenic diet because of pharmacoresistent epilepsy, were evaluated retrospectively. Carnitine deficiency during intake of ketogenic diet was detected in 26% of the patients and in 57% of the patients without carnitine substitution. Decreased carnitine level occurred also with carnitine substitution independent from additional valproate therapy. The time of appearance of carnitine deficiency on ketogenic diet was on average about 32 weeks but varied between 3 days and 248 weeks. The authors concluded that carnitine levels should be checked during the treatment with ketogenic diet, both at the beginning and during long-term-therapy. LCAR Effects on Body Composition and Exercise Performance Research over the last decade has shed new light on the importance of carnitine as a regulator of skeletal muscle fuel selection and physiological function. A recent review161 summarized that: It has been established that free carnitine availability may be limiting to the CPT1 reaction, and thus the rate of fat oxidation, during high-intensity submaximal exercise when the rate of acetylCoA production from the PDC (pyruvate dehydrogenase complex) reaction is in excess of its rate of utilization by the TCA cycle. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 222 29.10.2007 7:57pm Compositor Name: JGanesan 222 Amino Acids and Proteins for the Athlete: The Anabolic Edge Increasing muscle total-carnitine content in resting healthy humans (via insulin-mediated stimulation of Naþ-dependent skeletal muscle carnitine transport) reduces muscle glycolysis, increases glycogen storage, and is accompanied by an apparent increase in fat oxidation. It is important to acknowledge that an increase in human skeletal muscle carnitine content had previously not been achievable. By manipulating skeletal muscle PDC activity and acetylcarnitine availability, we have established that acetyl group delivery is limiting to mitochondrial ATP resynthesis at the onset of exercise. Moreover, this ‘‘acetyl group deficit’’ can be overcome by activating the PDC and=or stockpiling acetyl groups (mainly in the form of acetylcarnitine) at rest, which can then provide substrate to the TCA cycle from the immediate onset of intense exercise, resulting in a reduction in oxygenindependent ATP resynthesis (PCr hydrolysis and lactate accumulation) and fatigue development. There are several reasons why LCAR might affect body composition and exercise performance including facilitating fatty acid transport and oxidation, decreasing fat formation, ramping up energy metabolism, decreasing lactate, improving cell membrane function, and anti-inflammatory, immunomodulating, and cytoprotective effects.135–139,162–165 Synthetic carnitine is available in oral and parenteral forms and usually used in doses ranging from 100 to 5000 mg=day. Oral carnitine supplementation results in increased plasma and tissue levels of carnitine166,167 and trimethyllysine.168 The higher doses of carnitine lead to increased systemic and hepatic carnitine, and may lead to increased ketone production, FFA utilization, and increased adipose-tissue breakdown. It is possible that ketones inhibit essential protein catabolism. These facts may be useful to athletes wishing to reduce their weight and fat content, and retain maximum muscular size and strength. Utilization of these substances might not increase the rate of weight loss, but might increase the ratio of fat to lean tissue loss. These compounds might also be useful in reducing muscle breakdown during strenuous prolonged exercise. Thus, the most useful feature of exogenous carnitine and ketones is their postulated ability to increase the production and utilization of FFAs and ketones and thus preserve muscle protein while fasting or exercising. There may be a synergistic effect of LCAR and androgens, although no studies have linked effects of carnitine on fat metabolism with the anabolic and lipolytic effects of testosterone. Testosterone injections, however, have been shown to raise some tissue levels of carnitine,169 but not that in liver, heart, and skeletal muscle, although testosterone decreases the urinary excretion of carnitine.170 LCAR is used by some athletes because of its supposed antifatigue effect (thus increasing exercise capacity and enhancing performance) and in an attempt to minimize the loss of essential muscle cellular protein during prolonged exercise and in dieting. Although there are people who feel that there is no evidence supporting the concept that acute administration of carnitine is associated with increased physical performance in athletes,171 LCAR administration has been shown to: (1) increase lipid turnover in working muscles leading to glycogen saving and, as a consequence, allow longer performances for given heavy workloads; (2) contribute to the homeostasis of free and esterified LCAR in plasma and muscle, the feeling being that the levels of one or more of these compounds decrease in the course of heavy repetitive exercise.172 Although exogenous LCAR has been shown to be useful in treating carnitine deficiency and the sequelae of this deficiency (such as cardiomyopathy, and skeletal muscle pain and weakness),173–175 and in cases of deficient-tissue perfusion,176 there is limited evidence that it is useful in normal individuals.177 However, since carnitine is an essential cofactor in the catabolism of fats as an energy source, studies have been carried out to see if carnitine loading has effects on plasma and tissue carnitine and on performance. Studies have shown that there is an increased urinary excretion of esterified and total carnitine after physical exercise (due to an increased overflow of short-chain carnitine esters in urine) and that both these potentially unfavorable effects were prevented by oral administration of LCAR.178 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 223 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 223 There are studies that show the possible beneficial effects of LCAR on athletic performance,179–182 perhaps by stimulating pyruvate-dehydrogenase activity, thus diverting pyruvate from lactate to acetylcarnitine formation,183,184 and by allowing a larger quantity of FFA to enter the mitochondria and to be more extensively used as an energy source.185 Studies have shown that carnitine may induce an increase of the respiratory chain enzyme activities in muscle, probably by mechanisms involving mitochondrial DNA.186 It is also possible that carnitine may have a hypertrophic effect on skeletal muscle.187 In this study, the authors investigated the effect of long-term intravenous administration of LCAR on human-muscle fibers. They found that in these patients undergoing hemodialysis, the use of carnitine for 12 months resulted in a marked increase in serum and muscle-carnitine levels together with hypertrophy and predominance of type 1 fibers. The authors suggest that carnitine has a specific trophic effect on type 1 fibers which are characterized by an oxidative metabolism. In one study, running a marathon caused a fall in the plasma content of unesterified carnitine and an increase in the level of acetylcarnitine present.188 Loading of the athletes with LCAR for 10 days before running a marathon abolished the exercise-induced fall in plasma-free carnitine whilst amplifying the production of acetylcarnitine. However, in this study carnitine loading of the athletes made no detectable improvement in performance of the marathon. A study examined the effect of diet supplementation of oxaloacetate precursors (aspartate and asparagine) and carnitine on muscle metabolism and exercise endurance in rats.189 The results suggest that the diet supplementation increased the capacity of the muscle to utilize FFA and spare glycogen. Time to exhaustion was about 40% longer in the experimental group compared to the control, which received commercial diet only. These findings suggest that oxaloacetate may be important to determine the time to exhaustion during a prolonged and moderate exercise. Hyperammonia has been shown to alter lipid metabolism and decrease body fat content in rats.190 However, ammonia can be toxic. LCAR and its analogues, such as acetyl-L-carnitine (ALCAR), however, have been reported to have protective effects against ammonia toxicity.191 Studies are needed to explore the possible anabolic potential of the ammonia and LCAR=ALCAR interaction. Thus, while some studies have pointed to the possible ergogenic effects of carnitine, other studies, have found that LCAR supplementation does not seem to enhance athletic performance.192–195 The different findings may be due to the dose and length of carnitine supplementation since the effects of LCAR seem to be dependent on these two factors. Carnitine, however, might have more to offer to athletes under certain conditions. In one study, 10 moderately trained male subjects were submitted to two bouts of maximal cycle ergometer exercise separated by a 3-day interval.183 Each subject was randomly given either LCAR (2 g) or placebo orally 1 h before the beginning of each exercise session. At rest LCAR supplementation resulted in an increase of plasma-free carnitine without a change in acid-soluble carnitine esters. Treatment with LCAR induced a significant postexercise decrease of plasma lactate and pyruvate and a concurrent increase of acetylcarnitine. Carnitine might be an effective supplement in high-fat, high-protein diets.196 Certainly, carnitine appears to be an important nutrient for newborns whose main energy fuel is constituted by fat (colostrum is particularly rich in carnitine).151 It has also been shown that carnitine excretion increases on a high-fat diet.197 In a study on semi-starved rats, supplementation with carnitine increased plasma-free carnitine, improved lipid metabolism, and a decrease in urine excretion of ketone bodies, all pointing to an increased efficiency of fatty acid transport and oxidation.198 While on a high-fat, low-carbohydrate diet the skeletal muscles use lipids for energy during rest and exercise. Activated fatty acids are transferred across muscle mitochondrial membranes by esterification with carnitine. Several transferase enzymes, including carnitine palmitoyltransferase, release the fatty acids in the mitochondria, where they then undergo b-oxidation. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 224 29.10.2007 7:57pm Compositor Name: JGanesan 224 Amino Acids and Proteins for the Athlete: The Anabolic Edge Endogenous carnitine levels may be enhanced to some extent by the use of the amino acids lysine and methionine (see earlier), and the use of some of the essential nutrients involved in carnitine synthesis; such as iron and vitamins C, B6, and niacin. L-Carnitine and Choline Choline and carnitine are commonly used nutritional supplements, generally used alone or in combination and usually for the purposes of fat loss. Carnitine is essential for fatty acid transport and proper muscle function, and some studies have shown that carnitine supplementation improves exercise performance.139 Choline is a lipotropic agent that prevents deposition of fat in the liver. It is an essential nutrient for humans, which gives cell membranes structural strength and provides structure and is involved in the signaling across these membranes as well as synthesis and release of the neurotransmitter acetylcholine. Like carnitine, choline supplementation may also improve physical performance.199 Choline is the major source of methyl-groups in the diet (one of choline’s metabolites, betaine, participates in the methylation of homocysteine to form methionine). The possible synergistic interaction between carnitine and choline has been known for several years. A study on rats clearly showed that choline in combination with caffeine plus carnitine resulted in a significant conservation of carnitine and augmented exercise performance and promoted fatty acid oxidation as well as disposal in urine.200 The use of the three supplements resulted in two other beneficial effects, a decrease in blood triglycerides and an increase in the triglyceride content of skeletal muscles. Since muscle fat is used to supply energy to exercising muscles, and actually increases muscle size, much like glycogen, this is a completely different thing in increasing fat in subcutaneous skin and other fatty areas of the body. The increase in intramuscular fat also helps endurance and recovery and would be useful on any diet. Also choline and carnitine supplementation together have been shown to decrease oxidative stress.201 But there is more to the story. A recent study has shown that choline and carnitine supplementation, with or without exercise, alter carnitine status, body fat, and biochemical markers of fat oxidation in humans.202 In this study, 19 women were placed in three groups: (1) placebo, (2) choline during week 1 followed by the addition of carnitine in week 2 and week 3, and (3) carnitine during week 1 followed by the addition of choline in week 2 and week 3. There were several interesting results of this study outlining the effects of choline, and choline plus carnitine supplementation. Without going into all the details, the study showed that the cholineinduced decrease in serum and urinary carnitine is buffered by carnitine preloading, and these supplements shift tissue partitioning of carnitine that favors fat mobilization, incomplete oxidation of fatty acids, and disposal of their carbons in urine as acylcarnitines in humans. ACETYL-L-CARNITINE When LCAR is introduced in the body, an increase in the level of acetylcarnitine (acetyl-L-carnitine, ALC, ALCAR), the acetyl ester of carnitine, also occurs. However, despite this biochemical connection, ALCAR seems to have some powerful, beneficial properties of its own. ALCAR has shown protective effects in brain,203 myocardial,204 and tissue ischemia,205 positive effects on aspects of aging (especially in the brain),206–208 and its ability to lower cholesterol levels that rise as a result of age.209 Systemic administration of ALCAR, in a dose-dependent manner, stimulates ACh release in the striatum and hippocampus of freely moving rats, suggesting that the increase in ACh release is the result of ALCAR activation of a physiological mechanism in cholinergic neurons.210 The findings of one study confirmed the multiple effects of ALCAR in brain, and suggest that its administration can have a positive effect on age-related changes in the dopaminergic system.211 In this study, long-term ALCAR treatment had significant effects on age-related changes in striatal Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 225 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 225 dopamine receptors and brain amino acid levels. Another study found that ALCAR, when administered intracerebrally at fairly high concentrations, can affect the level and the release of not only such neurotransmitters as acetylcholine and dopamine, but also of amino acids.212 Other studies have documented the neuroprotective effects of ALCAR, its effects on metabolites involved in energy and phospholipid metabolism, and the possible therapeutic effects on neurodegenerative disease such as Alzheimer’s.213 The results of one study suggest that treatment with ALCAR is responsible for a reduction in brain glycolytic flow and for enhancement of the utilization of alternative energy sources, such as lipid substrates or ketone bodies.214 Studies have also shown that ALCAR has antioxidant actions that decrease free-radical-mediated, site-specific, protein oxidation, implicated in the pathophysiology of ischemia=reperfusion brain injury. The results of one study indicate that brain-protein oxidation does occur in a clinically relevant model of complete global cerebral ischemia and reperfusion, and that oxidation is inhibited under treatment conditions that improve neurological outcome.215 ALCAR has been considered of potential use in senile dementia of the Alzheimer type because of its ability to serve as a precursor for acetylcholine.216 However, pharmacological studies with ALCAR in animals have demonstrated its facility to maximize energy production and promote cellular membrane stability, particularly its ability to restore membranal changes that are age related. Since investigations have implicated abnormal energy processing leading to cell death, and severity-dependent membrane disruption in the pathology of Alzheimer’s disease,217 the beneficial effects associated with ALCAR administration in Alzheimer patients may not only be due to its cholinergic properties, but also to its ability to support physiological cellular functioning at the mitochondrial level. This hypothetical mechanism of action is discussed in one study with respect to compelling supportive animal studies and observations of significant decrease of carnitine acetyltransferase (the catalyst of LCAR acylation to ALCAR) in autopsied Alzheimer brains.218 ALCAR has a protective effect in brain,203,219,220 myocardial,204 and tissue ischemia. In one study, it was found that in animals L-acetylcarnitine markedly enhances functional muscle reinnervation, which was unaffected by LCAR.221 In another study, by the same principal author, L-acetylcarnitine was shown to markedly favor the process of reinnervation of the oculomotor nerve sectioned at intracranial level in guinea pigs.222 ALCAR produces a significant increase in the survival time course of adult rat sensory neurons maintained in primary cultures for up to 40 days, and thus might be involved in reducing neuronal loss observed in nervous system aging.219 Aging results in decrease of carnitine content in muscle of mice and humans.223 Analysis of muscle samples of healthy humans of different ages showed a drastic reduction of carnitine and ALCAR in the older subjects with a strong reverse correlation between age and carnitine levels. ALCAR lowers free and esterified cholesterol and arachidonic acid levels in the plasma often seen with aging.209 It is also possible that acetylcarnitine has antidepressant effects.224,225 A study set up to examine the effects of LCAR and ALCAR in rats and mice with experimental alloxan diabetes found that ALCAR may be more effective against diabetes in increasing glucose tolerance, restoring the impaired response of glucagon to glucose, showing glycogen-sparing action than LCAR.226 Besides these effects, ALCAR has several other properties that are of interest to athletes. With increasing work intensity, the level of carnitine in muscle decreases and that of ALCAR increases.227–229 The results of these studies indicate that acetylcarnitine is a major metabolite formed during intense muscular effort and that carnitine functions in the regulation of the intramitochondrial acetyl-CoA to CoA ratio by buffering excess production of acetyl units. Modulating the rise in intramitochondrial acyl-CoA to CoA ratio relieves the inhibition of many intramitochondrial enzymes involving glucose and amino acid catabolism.230 The use of ALCAR may have implications in the recovery from training and injuries in that it has beneficial effects on spontaneous and induced lipoperoxidation in rat skeletal muscle,231 and accelerates the disappearance of DNA single-strand breaks induced by oxygen radicals and alkylating agents.232 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 226 29.10.2007 7:57pm Compositor Name: JGanesan 226 Amino Acids and Proteins for the Athlete: The Anabolic Edge Acetylcarnitine also seems to have an effect on the hypothalamic–pituitary–testicular axis. Studies have shown that ALCAR (like the BCAA—see section earlier) prevented the decrease in plasma-testosterone levels after chronic exercise stress such as swimming,233,238 and that ALCAR stimulates testosterone production.234 It may also have a protective effect on selected body tissues, including skeletal muscle.231 This compound is part of a new technique to increase nighttime GH release, along with the amino acid ornithine.235 It is postulated that 500 mg of ALCAR and 25–100 mg of ornithine taken before bed will produce increased nighttime GH release. There is evidence to show that ALCAR also directly increases IGF-1 levels. A study found that ALCAR significantly increased levels of free IGF-1 (the bioactive component of total IGF-1). In addition, all the treated patients reported an improved sense of well-being by the second and third week of ALCAR therapy.236 For example, a recent study found that carnitines including ALCAR worked as well as replacement testosterone therapy in improving sexual dysfunction, depressed mood, and fatigue in aging men.237 Cognitive and Antiaging Effects of ALCAR ALCAR has been extensively studied and found to have significant cognitive and antiaging effects. It can be effective in improving memory, mood, and response to stress. ALCAR is a cognitive enhancer and neuroprotective agent that protects against a wide range of age-related degenerative changes in the brain and nervous system. ALCAR is an ester of carnitine that modulates cellular concentrations of free CoA and acetyl-CoA, two compounds integrally involved in numerous cellular functions, including the transfer of fatty acids across mitochondrial membranes for energy production. ALCAR is found in various concentrations in the brain and its levels are significantly reduced with aging. Several studies suggest that ALCAR delays onset of age-related cognitive decline and improves overall cognitive function in the elderly. ALCAR protects against brain degeneration, helps with energy production in mitochondria of cells, and removes toxins from the mitochondria. Its effects on brain cells include: Increasing neural energy production Protecting neurons from toxins Maintaining neuron receptors Increasing availability of the neurotransmitter acetylcholine ALCAR also has the ability to cross into the brain where it acts as a potent antioxidant, preventing the deterioration of brain cells that normally occurs with age. Because of this protective effect, ALCAR may be beneficial in the prevention and treatment of free-radical-induced diseases, such as Alzheimer’s and Parkinson’s disease. Several clinical trials suggest that ALCAR improves overall mental functioning and mood. In one study, ALCAR was given to elderly people with mild cognitive impairment. After 45 days, significant improvements in cognitive function (especially memory) were observed. Another large trial of ALCAR for mild cognitive impairment in the elderly found that supplementation significantly improved memory, mood, and responses to stress. The favorable effects persisted at least 30 days after treatment was discontinued. ALCAR also has effects on alleviating depression. Studies have shown that its supplementation is effective at relieving depression in elderly people, particularly those showing more serious clinical symptoms. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 227 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 227 ALCAR also significantly reduces damaged fats, such as lipofuscin, in the brains of aged rats. In addition to accumulating in the aging brain, lipofuscin also accumulates in the skin as aging spots, those brownish pigmented blemishes that accumulate in the backs of hands of many people over 50 years. The reduction of these deposits following consumption of ALCAR may be evidence of a slowing in the aging process in the brain. METHIONINE Methionine, cysteine, taurine, and homocysteine are the four common sulfur-containing amino acids. While methionine and cysteine are the only two incorporated into proteins, taurine and homocyteine also play vital physiological roles. Methionine and cysteine are the principal sources of sulfur in our diets. The body can make cysteine from methionine but not vice versa, so that methionine is a dietary essential. Methionine is mainly metabolized in the liver through the transmethylation–transsulfuration pathway. This pathway leads to the synthesis of homocysteine which in turn can be remethylated to form methionine or catabolized via the transsulfuration pathway to form cysteine.239,240 Cysteine requirements increase with injury and inflammation, which is understandable since cysteine is required for the synthesis of taurine and glutathione, both of which are important for host defense against oxidative stress.241 Cysteine requirements also increase with aging.242 Methionine has many functions in the body and the metabolic consequences of methionine deficiency are many and varied. The multifunctional role of methionine includes its participation in methyl-group metabolism, the synthesis of carnitine, creatine, glutathione, nucleic acids, polyamines, and catecholamines, as well as serving as a substrate for protein synthesis. Methionine deficiency has been shown to adversely affect serum cholesterol levels. In one study, very-low-density lipoprotein (VLDL) plus low-density lipoprotein (LDL) susceptibility to peroxidation was higher in rats fed soy protein than in casein-fed rats, which could reflect in part the lack of sulfur-containing amino acid availability, since methionine supplementation led to a partial recovery of lipoprotein resistance to peroxidation.243 These findings suggest that amino acid imbalance could be atherogenic by increasing circulating cholesterol and leading to a higher lipoprotein susceptibility to peroxidation. Ethanol feeding to rats alters methionine metabolism by decreasing the activity of methionine synthetase.244 This is the enzyme that converts homocysteine in the presence of vitamin B12 and N,5-methyltetrahydrofolate to methionine. Methionine may mitigate some of the harmful hepatic effects of alcohol. Deficiency of choline and methionine produces hepatic steatosis similar to that seen with ethanol, and supplementation with these lipotropes can prevent ethanol-induced fatty liver.245 Dietary deficiency of methionine in rats results in a fatty liver which can be cured by restoring this amino acid to the diet, or alternatively by giving the choline which the body would normally form from it, or by giving betaine, another methyl donor.244,246 Methionine, among its other roles, is also concerned with the important process of transmethylation and provides methyl groups (methyl donors) for the synthesis of choline which contains three such groups in each molecule. The chief dietary sources of methyl groups appear to be methionine and betaine. Methionine gives up the terminal CH3 group attached to its sulfur atom. Methionine has a high intracellular turnover, because it is involved in the initiation of peptide synthesis and in a variety of transmethylation and transsulfuration reactions.247,248 Methionine is the initiating amino acid in the synthesis of proteins translation. Initiation involves the association of the initiator tRNA (met-tRNAimet) with eIF-2 and the 40S ribosomal subunit together with a molecule of mRNA.249 Methionine may be the endogenous amino acid that is most limiting for maintenance of body protein and nitrogen balance and for effective reutilization of other amino acids.250,251 Since methionine is a nutritionally indispensable amino acid, its continued availability in free amino Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 228 29.10.2007 7:57pm Compositor Name: JGanesan 228 Amino Acids and Proteins for the Athlete: The Anabolic Edge acid tissue pools is important for maintaining an anabolic drive for protein synthesis252 under conditions of greater protein turnover. Methionine deficiency has been known to decrease protein synthesis while its reintroduction increases protein synthesis. In one study, small amounts of methionine, along with threonine, significantly reduced nitrogen excretion in rats fed a protein-free diet.253 Much of the work involving the need for methionine in protein synthesis has been done with burn and trauma patients. Major trauma, including thermal injury of the skin, results in a profound loss of body nitrogen, altered rates of whole-body protein synthesis and breakdown, and increased nitrogen and amino acid requirements relative to those in normal, nonstressed individuals.254,255 However, the in vivo mechanisms responsible for the altered nitrogen economy of the host and their relationship to the metabolism of specific nutritionally indispensable (essential) and dispensable (nonessential) amino acids are not fully defined.256 The fluxes of the major components of the methionine cycle in vivo include transmethylation (Tm), homocysteine remethylation (Rm) (recycling of homocysteine), oxidation or transsulfuration, the disposal of methionine via nonoxidative pathways (assumed to be protein synthesis [S] and its appearance via protein breakdown [B]). In a study that explored quantitative aspects of methionine metabolism in patients recovering from severe burn injury,257 (1-carbon-13, hydrogen-2-sub-3-methyl) methionine was given by continuous intravenous infusion to 12 adult patients. Each of the adults was studied in the fasted and in the fed state while receiving parenteral nutrition. Compared with findings obtained in other studies in healthy adults using a similar protocol,258,259 the rates of Tm, Rm, and methionine oxidation were all substantially increased in burn patients. From the relationships between these systems, it appears that there is a relative increase in the recycling of methionine carbon via Rm during the fasted state. This increased recycling of methionine, via Rm of homocysteine, would promote the availability of SAMe, required in numerous Tm reactions within the body.260 Thus, because methionine is the major methyl donor, and the increased rates of Rm and Tm indicate a high level of methyl group transfer and utilization in these patients, it seems likely that the synthesis rates of polyamines, choline, creatinine, and carnitine are increased.261–263 In this context, the polyamines are involved in normal growth and cell proliferation, and a higher turnover of these amines might well be expected where there is active and extensive tissue repair.264 Increased requirements of carnitine have also been claimed in traumatized patients,265 and the methylation of DNA is important for the production of acute-phase proteins and other proteins involved in renewal of tissues.266 Finally, the methionine–homocysteine cycle is also connected with the de novo synthesis of a wide range of dispensable amino acids, including glutamate, glycine, serine, and histidine.267 The increased activity of the methionine cycle therefore is probably causally linked to the enhanced metabolic requirements indicated earlier, and also, perhaps, to the higher rates of utilization and catabolism of these nutritionally dispensable amino acids and of histidine in traumatized patients.268 For these reasons, the methionine cycle would appear to play an important metabolic role in response to burn injury. However, despite the increased activity of the cycle, it appears to be linked with a relative reduction in the fraction of homocysteine entering the transsulfuration pathway, leading to the formation of cysteine. Perhaps the need to maintain high rates of methyl transfer reactions is of greater priority than is the synthesis of cysteine. Optimum body function and maintenance of protein homeostasis therefore may require an exogenous supply of cysteine for these patients. In summary, this investigation has shown that methionine kinetics are profoundly altered by burn injury and that the nutritional requirements for sulfur-containing amino acids are significantly increased. This increase is, in part, caused by or associated with enhanced activity of methyl transfer reactions within the body. This leads to increased rates of homocysteine formation and the question arises whether an exogenous supply of cysteine might further reduce the degradation of Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 229 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 229 homocysteine via the transsulfuration pathway and potentially promote whole-body protein synthesis and the maintenance of sulfur amino acid homeostasis in these patients. Clearly, additional tracer studies are needed to explore the metabolic and nutritional importance of methionine–cysteine interrelationships in burn and trauma patients, and for our purposes in athletes secondary to intense and prolonged exercise. Methionine is also essential for the formation of creatine. S-ADENOSYL-L-METHIONINE SAMe is a key intermediate in methionine metabolism.269 In addition to its role as a methyl donor, SAMe serves as a source of methylene groups (for the synthesis of cyclopropyl fatty acids and nucleosides), amino groups (in biotin synthesis), aminoisopropyl groups (in the synthesis of polyamines), and as a source of sulfur atoms in the synthesis of biotin and lipoic acid.270 Methyl donors are important for the methylation reaction, which adds a methyl group (one carbon atom and three hydrogen atoms), on proteins, enzymes, chemicals, DNA, and amino acids like homocysteine. Methylation is important for maintaining many functions in the body including genetic expression, and neurological and musculoskeletal function. Usually, this methylation process occurs through SAMe. SAMe, is synthesized from the amino acid methionine and its level in the body is increased by dietary methyl donors such as folic acid, B12, B6, and betaine (trimethyl glycine). These nutrients are also needed to reduce homocysteine levels and decrease cardiovascular disease. Various clinical trials and animal studies suggest that SAMe may be effective, among other things, in reducing pain and inflammation in the joints, in promoting cartilage repair, and helping with arthritic symptoms.271,272 It is also felt to have significant direct and indirect (by increasing glutathione synthesis) antioxidant effects.273 Some of these studies have shown that SAMe supplements were as effective as nonsteroidal anti-inflammatory drugs (NSAIDs) in people with arthritis in diminishing morning stiffness, decreasing pain, reducing swelling, improving range of motion, and increasing walking pace. Several of the studies also suggest that SAMe, while as effective as NSAIDs, has fewer side effects.274 CREATINE Creatine is an amino acid derivative that is obtained from food (especially red meat—2 lb of steak has about 4 g of creatine) and is also formed in the liver from the amino acids arginine, glycine, and methionine. Creatine is then taken up by skeletal muscle where it forms phosphocreatine, the high-energy phosphate compound. In humans, over 95% of the total creatine content is located in skeletal muscle, of which approximately a third is in its free form. The remainder is present in a phosphorylated form. Phosphocreatine serves as a backup source of energy for ATP, the immediate source of energy for muscular contraction. The amount of phosphocreatine in skeletal muscle partially determines the length of time in which maximum muscle work can be done. Once the phosphocreatine is gone, ATP must be regenerated through the metabolism of substrates such as glycogen, glucose, fatty acids, ketones, and amino acids. While the building blocks of ATP can be recycled, creatine cannot be due to the reactivity of the phosphoguanidine group in which the carboxyl group displaces the phosphate. The resultant cyclic compound is creatinine which is excreted in the urine. A dramatic increase in muscle use as in high-intensity training for bodybuilding results in an increase in creatine phosphate breakdown, an increase in creatinine excretion, and an increased synthesis of creatine from the amino acids arginine, glycine, and methionine. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 230 29.10.2007 7:57pm Compositor Name: JGanesan 230 Amino Acids and Proteins for the Athlete: The Anabolic Edge | | NH2 C NH CH2 COO | || || ⊕ H2N | NH2 | In kidney C NH2 ⊕ | NH | CH2 Glycine | Ornithine CH2 | CH2 ⊕ | CH NH3 | COO Arginine Guanidoacetate S-adenosyl methionine | In liver S-adenosylhomocysteine | ATP | | O O P Creatine phosphokinase O | | | Pi + H2O NH HN | | Creatinine C | N CH2 | COO | | 3HC ADP || CH2 | N Nonenzymatic || || | C | | || O C | HN | NH2 C NH CH2 COO | CH3 Creatine | H | N || ⊕ H2N CH3 Creatine phosphate FIGURE 7.2 Creatine and phosphocreatine dynamics. (From http:==isu.indstate.edu=mwking) The phosphate of ATP is transferred to creatine, generating creatine phosphate, through the action of creatine phosphokinase. The reaction is reversible such that when energy demand is high (e.g., during muscle exertion) creatine phosphate donates its phosphate to ADP to yield ATP. Figure 7.2 shows this reaction. It is believed that of the many causes of fatigue, one of the more important is the decrease in phosphocreatine in muscle. In the past, creatine supplements have been used by some athletes to try to increase phosphocreatine in muscle and thus increase intensity and the length of muscular contraction during short-term, high-intensity work. Lately, use of creatine monohydrate (CM) has risen dramatically as have the advertised claims for it. The observation that increased muscular activity leads to muscle hypertrophy is well known and many studies have attempted to identify the biochemical and physiological mechanisms by which this occurs. Some of these studies looked into the possibility that creatine may be the chemical signal coupling increased muscular activity and increased contractile mass. In the past, two muscle models have been used in experimental tests of this hypothesis: differentiating skeletal muscle cells in culture and the fetal mouse heart in organ culture. Using these culture models, it is possible to alter the intracellular creatine concentration and to measure the effect of increased creatine concentrations on the rates of synthesis and accumulation of both muscle-specific and nonspecific proteins. The results show that muscle-specific protein synthesis in both skeletal and cardiac muscles is selectively stimulated by creatine.275 CREATINE SUPPLEMENTATION A century ago, studies with creatine feeding concluded that some of the ingested creatine was retained in the body.276 Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 231 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 231 A number of studies done in the 1990s laid the groundwork on the effectiveness of CM supplementation on exercise performance, particularly anaerobic exercise, 277–285 and in diseases such as mitochondrial myopathies and neuromuscular disease.286,287 During that time, research found that oral creatine supplements not only increase creatine content in muscle (the increase is greatest in exercised muscles)288 but delays fatigue,289 improves recovery (by increasing the rate of phosphocreatine resynthesis in muscle),290 and increases muscle torque during repeated bouts of maximal voluntary exercise.291 Other studies showed that oral creatine supplements increase both power output and the total amount of short-term work,292 and enhances single and repeated short-sprint performance.293 Creatine may also independently result in increased body mass,294 although much of this increase seen in the first few weeks may be due to increased water retention. Also studies investigated the effect of carbohydrate ingestion on skeletal muscle creatine accumulation during creatine supplementation in humans.295,296 Creatine supplementation had no effect on serum insulin concentration, but creatine and carbohydrate ingestion dramatically elevated insulin concentration. These findings demonstrate that CHO ingestion substantially augments muscle-creatine accumulation during creatine feeding in humans, which appears to be insulin mediated. Another study concluded that in competitive rowers, a 5-day period of creatine supplementation was effective in raising whole-body creatine stores, the magnitude of which provided a positive, though statistically nonsignificant relationship with 1000 m rowing performance.297 This study investigated the change in 1000 m simulated rowing performance in two matched groups of 19 competitive rowers following a 5-day period of supplementation with placebo (CON group) or creatine at a dose equivalent to 0.25 g CM=kg of body mass (EXP group). Creatine uptake was calculated from the difference between the amount fed and the amount recovered in urine during each 24 h period of supplementation. After supplementation with placebo, the CON group showed no change in 1000 m rowing performance. Of these subjects, 7 decreased and 10 increased their performance times. By contrast, 16 of the 19 subjects in the EXP group improved their performance times. The mean improvement in rowing performance for the EXP group was just over 1%. The results of another study suggest improvements in performance secondary to CM use were mediated via improved ATP resynthesis as a consequence of increased phosphocreatine (PCr) availability in type II fibers.298 In this study, nine male subjects performed two bouts of 30 s maximal, isokinetic cycling, before and after ingestion of 20 g CM=day, for 5 days. Creatine ingestion produced a 23.1+4.7 mmol=kg dry matter increase in the muscle total creatine (TCr) concentration. Total work production during bouts 1 and 2 increased by 4% and the cumulative increases in both peak and total work production over the two exercise bouts were positively correlated with the increase in muscle TCr. Cumulative loss of ATP was 30.7+12.2% less after creatine ingestion, despite the increase in work production. Resting PCr increased in type I and II fibers. Changes in PCr prior to exercise bouts 1 and 2 in type II fibers were positively correlated with changes in PCr degradation during exercise in this fiber type and changes in total work production. On the other hand, some studies during that time did not show any ergogenic effects from creatine supplementation.299–304 A study concluded that a 4-day period of creatine supplementation failed to significantly raise resting muscle creatine, or that multiple-sprint performance was not enhanced by increases in resting muscle creatine.305 In this study, the influence of oral CM supplementation on repeated 10 s cycle ergometer sprint performance was examined. Seventeen recreationally active males participated in the 16-day experiment. All the subjects initially completed a VO2 peak test and were then administered glucose (4 3 10 g=day) in a single blind fashion for 4 days, after which they completed the first series of multiple sprints (7 3 10 s). For the following 4 days, diets were supplemented with either creatine (4 3 70 mg=kg body mass=day mixed with 5 g glucose) or glucose (4 3 10 g=day); supplementation Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 232 29.10.2007 7:57pm Compositor Name: JGanesan 232 Amino Acids and Proteins for the Athlete: The Anabolic Edge during this phase was double-blind. Subjects then repeated the multiple sprint and VO2 peak tests. Measures of peak power output (PPO), mean power output (MPO), end-power output (EPO), and percent power decline were recorded during the sprints. Analysis of variance revealed that 4 days of creatine supplementation did not influence multiple-sprint performance, plasma lactate, blood pH, and excess post-sprint oxygen consumption. Furthermore, VO2 peak was unchanged following creatine supplementation. There were dozens of studies done from 2000 to 2007, many of them simply repeating the results of the previous studies—both for 306–315 and against.316–319 The one difference was more studies showing significant effects of creatine on body composition,320,321 and on aerobic performance.322–324 There are both—responders, those in which creatine has a positive effect on performance, and nonresponders, those in which creatine does not have a positive effect on performance. Physiologic profiles of nonresponders appear to be different and may limit their ability to uptake creatine.325 For example, certain groups of people, such as vegetarians, may benefit more from creatine supplementation.326 Variables such as initial levels of creatine plus PCr, a greater percentage of type II fibers, and greater preload muscle fiber cross-sectional area (CSA) and fat-free mass may help to partially explain the reported equivocal performance findings in the creatine supplementation literature.327 CREATINE SUPPLEMENTATION COMBINED WITH OTHER INGREDIENTS There are several studies that have looked at the ergogenic and body composition effects of combining CM with one or more ingredients, including chromium, b-alanine, arginine, glutamine, ribose, taurine, and pyruvate.328–336 In general, these studies all found a beneficial effect on body composition and both power- and endurance-exercise performance when CM supplementation is combined with various other ingredients. For example, one study looked at the effects of a formula combining several ingredients, including niacin-bound chromium (III), extract of Withania somnifera, D-ribose, caffeine, and selected amino acids such as phenylalanine, taurine, glutamine.334 From the results of the study, the authors concluded that the supplement formula was safe over the long term and resulted in increased energy production as demonstrated by increased ATP, creatine phosphate (CP), and p-AMPK levels both in male and female rats. Furthermore, the formula showed significant cardioprotective effect during ischemia and reperfusion. Other studies, however, found that combining certain ingredients with creatine did not increase muscle strength and body composition. For example, one study found that combination of effervescent creatine, ribose, and glutamine did not enhance muscular strength, muscular endurance, or body composition in resistance-trained men.337 CREATINE AND CAFFEINE In an early study, the combination of creatine and caffeine was reported as being counterproductive.338 As such, my advice in the first edition was not to use caffeine if you use CM, since this study was showing that the use of caffeine negates the ergogenic effects of CM. At the time, I thought that this may explain why creatine supplementation does not seem to work for some athletes. Studies have shown that caffeine can enhance power- and endurance-exercise performance.339–341 A study looking at the effects of creatine on relaxation time (RT) found that part of creatine’s ergogenic effects is due to its effects on shortening RT during intermittent maximal isometric muscle contractions.342 Corroborating the earlier study on creatine and caffeine, a study published in 2002 found that caffeine and creatine have opposing effects on muscle RT.343 The authors concluded that the impact of caffeine on RT counteracts the beneficial effect of creatine supplementation on muscle RT. Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 233 29.10.2007 7:57pm Compositor Name: JGanesan Essential Amino Acids 233 However, a study published later that same year found that combining creatine and caffeine was ergogenic.344 Based on the previous studies on the counterproductive effects of using creatine and caffeine together, the study looked at an alternative strategy to optimize the independent effects of creatine and caffeine and to minimize the interference of combining these nutritional interventions. The authors felt that a way to do this would be to supplement with creatine while abstaining from caffeine for the duration of the loading period. Ingestion of a single dose of caffeine would then be taken before exercise (i.e., as is the case in most caffeine studies). This study, however, demonstrates that acute ingestion of caffeine has ergogenic effects on short-term, high-intensity exercise regardless of whether or not creatine supplementation precedes the caffeine ingestion. The results also suggest that the ergogenic effects of caffeine are possibly related to alterations in the perceptual response and the energetics of short-term, high-intensity exercise. It is obvious that future studies are needed to solve the puzzle of the interaction between caffeine, creatine, and their combined effects on exercise performance. PHENYLALANINE Phenylalanine is an essential a-amino acid. It exists in two forms, D and L forms, which are enantiomers (mirror-image molecules) of each other. L-Phenylalanine can also be converted into L-tyrosine, one of the 20 protein-forming amino acids. While the body can convert phenylalanine to tyrosine, it cannot convert tyrosine to phenylalanine. Tyrosine is the parent compound for the manufacture of L-dopa, which is further converted into dopamine, norepinephrine, and epinephrine. Tyrosine is also converted into thyroxine and triiodothyronine by the thyroid gland. Also, the pigment melanin which occurs in the skin, hair, and choroid lining of the eye forms from the enzymatic conversion of tyrosine. Those individuals not able to convert phenylalanine to tyrosine, due to a hereditary lack of phenylalanine hydroxylase, suffer from the inborn error of metabolism known as phenylketonuria, a human genetic disorder that if left untreated can cause brain damage and progressive mental retardation.345 Phenylalanine suppresses appetite by increasing cholecystokinin (CCK) secretion and results in a reduction in food intake.346 Phenylalanine and tyrosine (which is formed from phenylalanine) are also precursors for the neurotransmitters dopamine, epinephrine, and norepinephrine and as such can have potentiating effects on weight and fat loss. THREONINE Threonine is generally low in vegetarians. Lack of threonine in high-protein diets may compromise protein synthesis (see earlier discussion). TRYPTOPHAN Tryptophan is a precursor for serotonin (a neurotransmitter), melatonin (a neurohormone), and niacin. The plasma tryptophan ratio has been shown to determine brain tryptophan levels and, thereby, to affect the synthesis and release of the neurotransmitter serotonin. In a review on the effects of tryptophan on serotonin,347 the authors summarized evidence showing that (1) the synthesis of serotonin in the brain depends directly on the amount of tryptophan available to it from the circulation, (2) tryptophan uptake into brain depends on the blood levels not only of tryptophan, but also of other aromatic amino acids and BCAAs that compete with tryptophan for a common transport carrier into brain, and (3) dietary factors that influence the blood levels of tryptophan and the other amino acids can modify tryptophan uptake into brain, and consequently the rate of serotonin formation. Additionally, data were reviewed in an attempt to Di Pasquale/Amino Acids and Proteins for the Athlete: The Anabolic Edge 43803_C007 Final Proof page 234 29.10.2007 7:57pm Compositor Name: JGanesan 234 Amino Acids and Proteins for the Athlete: The Anabolic Edge show that appetite for protein or carbohydrates is dependent on the relationship between food intake, plasma amino acid pattern, brain tryptophan uptake, and serotonin synthesis. As we have seen, the essential amino acid tryptophan is the precursor for serotonin, which is in turn an intermediary product in the synthesis of melatonin.348,349 A dose-dependent rise of circulating melatonin has been observed after L-tryptophan infusions.350 Studies have also shown that reduced plasma tryptophan levels, and presumably brain serotonin concentrations, decrease nocturnal melatonin secretion in humans.351,352 Niacin is the generic term for nicotinic acid (pyridine-3-carboxylic acid) and derivatives that exhibit the nutritional activity of nicotinic acid. In one sense, niacin is not a vitamin, since it can be formed from the essential amino acid tryptophan. In the humans, an average of about 1 mg of niacin is formed from 60 mg of dietary tryptophan. 353 L-tryptophan has a small but significant effect on GH secretion and may significantly increase 354 athletic performance. In 1988, Segura and Ventura reported that 1.2 g of L-tryptophan supplementation, because of its effect on the endogenous opioid systems and pain tolerance, increased total exercise time by 49.4% when the subjects were running at 80% of maximal oxygen uptake (VO2 max). A more recent study failed to repeat these results and found that oral L-tryptophan supplementation does not enhance running performance.355 In the more recent study, 49 well-trained male runners, aged 18–44 participated in a randomized double-blind placebo study. Each subject underwent four trials on the treadmill. The first two served as learning experience, including measurement of VO2 max and anaerobic threshold. During the last two trials the subjects ran until exhaustion at a speed corresponding to 100% of their VO2 max—first an initial trial and then after receiving a total of 1.2 g L-tryptophan or placebo over a 24 h period prior to the run. No significant difference between the improv