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Protein
Chapter 3
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Protein
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A component of every living cell
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Accounts for 20% of adult weight
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Immune to the controversy over optimal intake
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Protein (cont’d)
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Amino acids
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Basic building blocks of protein
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Composed of carbon, hydrogen, oxygen, and nitrogen atoms
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20 common amino acids
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9 are considered essential because the body cannot make them—indispensable;
they must be consumed through food
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11 are considered nonessential because they can be made by the body—
dispensable
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Essential Amino Acids
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Histidine
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Isoleucine
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Leucine
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Lysine
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Methionine
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Phenylalanine
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Threonine
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Tryptophan
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Valine
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Nonessential Amino Acids
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Alanine
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Arginine
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Asparagine
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Aspartic acid
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Cystine (cysteine)
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Glutamic acid
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Glutamine
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Glycine
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Proline
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Serine
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Tyrosine
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Protein (cont’d)
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Protein structure
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Most contain several dozen to several hundred amino acids
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Shape determines function
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Protein (cont’d)
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Functions of protein
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Major structural and functional component of every living cell
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Body structure and framework
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Enzymes
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Other body secretions and fluids
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Acid–base balance
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Transport molecules
Protein (cont’d)
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Functions of protein (cont’d)
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Other compounds
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Some amino acids have specific functions within the body
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Fueling the body
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Protein (cont’d)
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How the body handles protein
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Digestion
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Begins in the stomach
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Hydrochloric acid converts pepsinogen to the active enzyme pepsin
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Small intestine is the principal site of protein digestion
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Enzymes located on the surface of the cells that line the small intestine
complete the digestion
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Protein (cont’d)
How the body handles protein (cont’d)
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Absorption
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Amino acids, and sometimes a few dipeptides or larger peptides, are absorbed
through the mucosa of the small intestine
Metabolism
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Liver acts as a clearinghouse
 Retains amino acids to make liver cells, nonessential amino acids, and
plasma proteins such as heparin, prothrombin, and albumin
 Regulates the release of amino acids into the bloodstream
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Protein (cont’d)
Metabolism (cont’d)
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Liver acts as a clearinghouse (cont’d)
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Removes the nitrogen from amino acids
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Converts protein to fatty acids which form triglycerides for storage in adipose
tissue
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Forms urea from the nitrogenous wastes of protein
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Protein (cont’d)
Protein synthesis
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Complicated but efficient process that quickly assembles amino acids into proteins the
body needs
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Part of what makes every individual unique is the minute differences in body proteins
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Important concepts
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Protein turnover
 Important Concepts
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Protein turnover
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Continuous process
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Body proteins vary in their rate of turnover
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Metabolic pool
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Contains supply of each amino acid
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Consists of recycled amino acids from body proteins that have broken down and also amino
acids from food
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In a constant state of flux
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Important Concepts (cont’d)
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Nitrogen balance
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Reflects the state of balance between protein breakdown and protein synthesis
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Determined by comparing the amount of nitrogen consumed (intake) with the amount
of nitrogen excreted (output)
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Healthy adults are in neutral nitrogen balance
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Important Concepts (cont’d)
Nitrogen balance (cont’d)
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Positive nitrogen balance: when protein synthesis exceeds protein breakdown
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Negative nitrogen balance: an undesirable state that occurs when protein breakdown
exceeds protein synthesis
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Protein (cont’d)
Protein catabolism for energy
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Physiologic and economic waste
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Overtime, loss of lean body tissue occurs
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Loss of 30% of body protein causes:
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Impaired breathing
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Altered immune function
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Altered organ function
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Ultimately death
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Sources of Protein
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Protein quality
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Differs based on content of essential amino acids
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Quality can become a crucial concern
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Sources of Protein (cont’d)
Complete and incomplete proteins
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Complete proteins
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High biologic value
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Provide adequate amounts and proportions of all essential amino acids needed
for protein synthesis necessary to support tissue growth and repair
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Animal proteins and soy protein are complete proteins
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Sources of Protein (cont’d)
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Complete and incomplete proteins (cont’d)
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Incomplete proteins
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Lack adequate amounts of one or more essential amino acids
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Except for soy protein, all plants are sources of incomplete proteins
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Gelatin is also an incomplete protein
Complementary proteins
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2 proteins that when combined provide adequate amounts and proportions of
all essential amino acids needed to support protein synthesis
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Examples of Complementary Plant Proteins
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Black beans and rice
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Bean tacos
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Pea soup with toast
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Lentil and rice curry
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Falafel sandwich (ground chickpeas on pita)
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Peanut butter sandwich
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Pasta e fagioli (pasta with white beans)
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Examples of a Plant Protein Complemented by a Small Amount of an Animal Protein to Form a
Complete Protein
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Bread pudding
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Rice pudding
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Corn pudding
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Cereal and milk
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Macaroni and cheese
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Cheese fondue
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French toast
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Cheese sandwich
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Vegetable quiche
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Dietary Reference Intakes
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RDAs
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For healthy adults is 0.8 g/kg
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Acceptable macronutrient distribution range for protein for adults is 10% to 35% of total
calories
When the RDA does not apply
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Intended for healthy people only
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Dietary Reference Intakes (cont’d)
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Protein deficiency
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Protein-energy malnutrition (PEM)
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Kwashiorkor
 Results mainly from acute critical illnesses
 Aggressive nutritional support is used to restore metabolic balance as
quickly as possible
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Marasmus
 Occurs secondary to chronic diseases
 Nutritional therapy is started slowly and advanced gradually
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Dietary Reference Intakes (cont’d)
Protein excess
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No proven risks from eating an excess of protein
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Conflicting data as to whether high-protein diets increase the risk of:
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Osteoporosis
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Renal stones
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Protein in Health Promotion
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Not addressed in the Dietary Guidelines for Americans
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Vegetarian diets
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Pure vegetarians or vegans
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Eat no animal products
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Eat only plants
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They form the smallest group of vegetarians
Protein in Health Promotion (cont’d)
Vegetarian diets (cont’d)
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Most American vegetarians are:
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Lacto-vegetarians whose diets include milk products
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Lacto-ovo vegetarians, whose diets include milk products and eggs
Nutrients of concern
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Most vegetarian diets meet or exceed the RDA for protein and are nutritionally
adequate across the life cycle
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Iron, zinc, calcium, vitamin D, and alpha-linolenic acid are nutrients of concern
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Protein in Health Promotion (cont’d)
Is vegetarianism for everyone?
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A personal choice, subject to personal interpretation
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Proper planning means paying close attention to the nutrients of concern and using a
vegetarian food guide for planning