Carbohydrates
Chapter 5
What are Dietary Carbohydrates?
• Organic compounds containing
– Carbon
– Oxygen
– Hydrogen
– CH2O
• Formed naturally in nature
• Synthesized
• ~4 kcal/gram
Types of Carbohydrates
• Simple CHO
– Monosaccharides
• Glucose (dextrose, grape sugar)
• Fructose (levulose, fruit sugar)
• Galactose (milk sugar)
– Disaccharide
• Maltose (malt sugar, glucose & glucose)
• Lactose (milk sugar, glucose & galactose)
• Sucrose (cane or table sugar, glucose & fructose)
CHO Types
• Complex CHO
– 3 or more glucose molecules
– Polysaccharide (>2 molecules)
• Plant starches
• Animal starch (glycogen)
• glucose polymer (10-20 molecules)
– Maltodextrin
– polycose
– Fiber
Plant Starches
• Contains many (up to several thousand)
monosaccharides
• Storage form of CHO in plants
• Types
– Amylopectin – branched-chain molecule that is rapidly
digested and absorbed
– Amylose – long chain molecule that is more slowly
digested and absorbed
• Most starches contain combinations of both
Concentration Units
• Mole = gram molecular weight
• A mole is the weight in grams of a
particular substance, like glucose
• Example
– Glucose is C6H12O6
– Atomic weight of C is 12, H is 1, O is 16
– Multiply the atomic weight X the number of
that element in the molecule and sum it up.
- 1 mole glucose is 180 grams
Muscle Glycogen Units
• Muscle glycogen/glucose concentration
are expressed per kg of dry weight (d.w.)
or per kg of wet weight (w.w.)
• In muscle, w.w. x 4.5 = d.w.
• Normal Muscle Concentration
– 12-16 g/kg w.w or 1.7 g/100 g muscle
– 65-90 mmole/kg w.w
– Can be ~ doubled by carbo loading
Liver Glycogen Units
• Normal liver glycogen is 50-75 g/kg w.w.
• Liver glycogen reduced by 50% after 1
hour of exercise at 75% VO2max.
• At rest, glucose output is 150 mg/min
– 60% from liver glycogen
– 40% from gluconeogenesis
• At >75% VO2max
– Glucose output is 1 g/min
– 90% from glycogen
TABLE 4.7 Major hormones involved in regulation of blood glucose levels
Hormone
Gland
Stimulus
Action
Insulin
Pancreas Increase in blood
glucose
Helps transport glucose into
cells; decreases blood glucose
levels.
Glucagon
Pancreas Decrease in blood
glucose; Exercise
stress
Promotes gluconeogenesis in
liver; helps increase blood
glucose levels.
Epinephrine Adrenal
Exercise stress;
decrease in blood
glucose
Promotes glycogen breakdown
and glucose release from the
liver: helps increase blood
glucose levels
Cortisol
Exercise stress;
decrease in blood
glucose
Promotes breakdown of protein
and resultant gluconeogenesis;
helps increase blood glucose
levels
Adrenal
Causes of Muscular Fatigue
Related to CHO Use
• Muscle Glycogen Depletion
– Fatigue begins to occur at approx 30-40
mmole/kg ww.
– Short duration high intensity (<60 sec) not
affected until glycogen drops below 20
mmole/kg
– Normal glycogen levels – 60-90 mM/kg
– Max levels – 200 mM/kg
Causes of Fatigue Continued
• Liver Glycogen Depletion
– Normal glycogen range: 250-300 mM/Kg
– At rest, glucose from liver is 150 mg/min
• 60% from liver glycogen
• 40% from gluconeogenesis
– During exercise at 75% VO2max, output is 1 gm/min
with 90% from glycogen.
– Normoglycemia: 60-100 mg/dl
– Hyperglycemia: >140 mg/dl
– Hypoglycemia: <45 mg/dl
• Reactive Hypoglycemia
Glycemic Index (GI)
The GI reflects the rate of digestion and
absorption of CHO
Blood glucose area after test food
GI =
X 100
Blood glucose area after reference
food
Glycemic Load
• Glycemic index relative to the serving size
• Some CHO have high GI but are
consumed in small quantities per serving
• GL = (GI x CHO/serving)/100
• Ratings of glycemic loads
– High GL = >20
– Medium GL = 11-19
– Low GL = <11
Use Of GI In Sports Nutrition
• Before Exercise: A low-GL CHO should be
eaten, particularly before prolonged
exercise, to promote sustained CHO
availability
• During Exercise: Moderate to High-GL
CHO foods or drinks are most appropriate
• After Exercise: High-GL CHO for glycogen
resynthesis
Major Factors Influencing Skeletal
Muscle CHO Metabolism During Exercise
•
•
•
•
Exercise Intensity
Exercise Duration
Training
Diet
Effect of Intensity of Exercise on
CHO Utilization
Exogenous CHO Oxidation &
Intensity of Exercise
• Exo CHO oxidation increases with
increasing intensity up to 60% VO2max.
• Peaks at 1.0-1.1 g/min
• Above 60% VO2max, increases in CHO
oxidation are due to increases in muscle
glycogen oxidation up to 4 g/min
• Limitation is due to rate of digestion,
absorption and transport of glucose into
systemic circulation
Effect of Exercise Duration
on CHO Utilization
Effect of Training
on CHO Utilization
Effect of Diet on CHO Utilization
Mixed Diet
 Low CHO
 High CHO
Daily CHO Reqirements
• Minimum of 100 g/day for nervous system
• Moderate duration/low intensity training of
1-3 hours – 5-7 g/kg
• Moderate to heavy endurance training of
1-2 hours – 7-12 g/kg
• Extreme exercise of 4-6 hours – 10-12
g/kg
General Considerations
• Must get adequate overall nutrition in
addition to the CHO
– Vitamins and minerals
– Adequate protein
– Total calories
• Glycemic Index needs to be considered
Consumption 3-4 hours
Before Exercise (Pre-Competition Meal)
•
•
•
•
Up to 2-3 g/kg of CHO (200-300 gm)
Moderate to high glycemic index CHO
Minimal fat and protein
Increases muscle and liver glycogen
Less Than 1 Hour Before Exercise
• Individuals prone to reactive hypoglycemia
should avoid CHO, especially high
glycemic CHO
– May increase glycogen use
• Up to 100 g low to moderate GI if no
feeding during exercise
• If feeding during exercise, then nothing up
to 1 hour before.
During Exercise
• Maximal use of exogenous CHO is ~ 70 grams
per hour
• Timing of feedings seem unimportant
• 5-10% solution of 15-20 grams every 15-20 min
– 8 oz of Gatorade contains approx. 15 grams of CHO
– High Fructose may cause stomach upset
• Multiple CHO types may increase use
• Always test feeding prior to competition
After Exercise
• Glycogen resynthesis rate is about 5-7%
per hour
• 2 hour window following exercise for
maximal rate of resynthesis
• High glycemic foods
• Combination of CHO and protein is best in
a 3:1 ratio of CHO to protein. However, if
total CHO intake is sufficient, PRO doesn’t
matter.
When Recovery Time is <8 hrs
• 1.2 gm/kg immediately and each hour for 4-6
hours
• Can consume in one bolus each hour or smaller
quantities every 30 minutes
• If lesser amounts of CHO are available,
consuming protein with CHO in 3:1 ratio is best
(may also increase protein anabolism)
• CHO in fluid solution will also replace fluids
When Recovery Time is >8 hours
• Most important factor is meeting total CHO
requirement if the intake is spread
throughout the next 24 hours.
• However, you can’t wait until the last
minute as total resynthesis takes
approximately 20 hours
Factors Affecting Glycogen
Resynthesis
• Trained have higher rates than untrained
• A lower starting concentration will increase
the rate of synthesis
• No difference in fiber types???
• Eccentric exercise has lower rates than
concentric exercise after 18-72 hours, but
not up to 6 hours
• No difference in males and females
Table 4.8 CHO Loading
Original Classic Method
Day 1
Depletion exercise
Day 2
High-protein/fat, 15-20% CHO
Day 3
High-protein/fat, 15-20% CHO
Day 4
High-protein/fat, 15-20% CHO
Day 5
High CHO (70-80%)
Day 6
High CHO
Day 7
High CHO
Day 8
Competition
Table 4.8 CHO Loading
Contemporary Recommended Method
Day 1
Depletion exercise (optional)
Day 2
Mixed diet (50-55% CHO)
Day 3
Mixed diet
Day 4
Mixed diet
Day 5
High CHO (70-80%)
Day 6
High CHO
Day 7
High CHO
Day 8
Competition
Most Recent Loading Technique
• 3 min of supramaximal exercise
• 10-12 gm/kg CHO for 24 hrs.
Alcohol As An Energy Substrate
• 7 kcal/gram
• By-products of alcohol metabolism
released in blood appear to be of little
importance to exercising muscle
• Alcohol consumed prior to exercise may
contribute 5% of energy over 90 min of
exercise
• Alcohol requires more O2 for metabolism
than CHO or fat
• Alcohol may interfere with glucose
metabolism
• Reduced aerobic endurance at 80-85% of
VO2max
• May reduce rate of gluconeogenesis
• Typically represents non-nutritive excess
calories contributing to fat storage