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Protein and Amino Acids in Sports Nutrition
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Functions of Proteins
 Enzymes
– All biological enzymes are made of protein
• For example, the digestive enzymes trypsin and amylase
 Hormones (some)
– Insulin and glucagon
– Not all hormones are proteins
• Testosterone is a steroid
 Structural
– Actin & myosin (muscle)
– Collagen (skin)
Lesk AM. Introduction to Protein Science: Architecture, Function, and Genomics. 2nd ed. New York, NY: Oxford University Press USA.; 2010.
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Functions of Proteins (cont’d)
 Immunologic
– All antibodies
 Transport and storage
– Carriers of fatty acids, oxygen (hemoglobin), iron, vitamin A, copper,
and other nutrients
– Cholesterol and triglycerides carried by lipoproteins
 pH buffering
– In blood, muscle, essentially everywhere
 Energy source
– When carbohydrates are limited (gluconeogenesis)
Lesk AM. Introduction to Protein Science: Architecture, Function, and Genomics. 2nd ed. New York, NY: Oxford University Press USA; 2010.
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Structure of Proteins
 Proteins are made of individual “building block” units called amino acids
(AAs) that are linked together
– A chain of AAs is called a peptide
– Long peptides are proteins
 The order of the AAs allows different proteins to have
different functions
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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What Are Amino Acids?
 Building blocks of proteins
– Body uses 20 different amino acids to make proteins
 9 of the 20 amino acids must be consumed in the diet (essential
amino acids; EAA)
– Body cannot make them on its own
Leucine
Isoleucine
Tryptophan
Threonine
Methionine
Lysine
Valine
Histidine
Phenylalanine
 Other 11 amino acids are not essential (NEAA)
– Can be made from other amino acids in the diet
 Some NEAAs can become EAAs under certain conditions
– Infants have different needs for growth
– Defects in amino acid metabolism
• Tyrosine can become essential in individuals with phenylketonuria (PKU), an inborn error
of phenylalanine metabolism
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Basic Structure of an Amino Acid
 Central carbon atom (alpha carbon [Cα]) linked to
–
–
–
–
Amino group (positive)
Carboxylic acid group (negative)
Hydrogen
Distinctive side chain (R)
• Makes each AA different
–
+
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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R Groups for Some Amino Acids
H-
Glycine
CH3-
Alanine
H3C
HO-CH2CH3-CH-
Serine
Threonine
OH
CH-
Valine
H3C
CH2-
H3C
SH
CH-CH2-
Cysteine
Leucine
H3C
CH2-CH2Methionine
S-CH3
CH2
CH2
CH-
Isoleucine
-OOC-CH 2
Aspartic acid
CH3
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Protein Digestion and Absorption
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Protein Digestion
Gastric phase (stomach)
 Hydrochloric acid (HCl) from cells in stomach unfolds protein
 Pepsinogen (chief cells)
 Pepsin digests proteins
HCl
Pepsin (enzyme)
Large peptide fragments
Small intestine phase
 Cholecystokinin (hormone released in upper small intestine)
triggers pancreas to secrete digestive enzymes once digestion
products leave the stomach
 Digestive enzymes are activated and continue to break down
peptides into di-/tripeptides and free amino acids, which are
taken up by intestinal cells
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Amino Acid and Peptide Absorption
 Intestinal cells have various transport proteins at both the luminal
side and the blood side to give AAs and peptides access
to the circulation
Lumen
Transporters
Intestinal cell
Blood
Amino acids
Peptidases
Oligopeptides
Tripeptides
Dipeptides
Cellular membrane
 Transporters can carry
– Free AAs
– Di- and tripeptides
• For example, can transport carnosine, a dipeptide
– Nonprotein AAs
• For example, can transport creatine from intestine to blood
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Fast Versus Slow Digestion of Proteins
 Proteins are digested at different rates
– Similar to glycemic index concept for carbohydrates
 Whey versus casein
– Whey is rapidly digested and results in quick rise in plasma AAs (fast protein)
– Casein forms a curd and takes longer to empty from the stomach (slow protein)
• Less dramatic, but more sustained rise in plasma AAs after consumption
 Soy protein
– Digested faster than whole milk protein, which contains both whey + casein
• Overall, more like a fast protein, but slower than whey
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
Boirie Y, et al. Proc Natl Acad Sci U S A. 1997;94(26):14930-14935.
Rossi AL, et al. J Nutraceuticals, Functional and Medical Foods. 2000;3(1):33-44.
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Nutrition and Cellular Synthesis of Protein
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Transcription of DNA for Making Cellular Proteins
 Transcription of DNA
– Makes the template (mRNA) for synthesizing proteins
– Sequence of mRNA bases (A, C, G, & U) arranged in units of 3 that “code” for
specific amino acids
Nucleus
DNA
Rough ER
mRNA
A, adenine; C, cytosine; G, guanine; U, uracil; ER, endoplasmic reticulum.
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Translation of DNA for Making Cellular Proteins
 Translation of mRNA
– Actual building of new proteins by the ribosome
– tRNA brings a specific AA to the ribosome, which matches the mRNA
template and adds the AA to a growing peptide chain
Amino acid
tRNAs
Rough ER
Peptide bond
Met …….. Cys
U AC
A CA
A U G
U GU
Tyr
Phe
Gln
A UA
AAA
GUU
U AU
U UU
C AA
mRNA at ribosome
Initiator codon
Ribosome
Direction of ribosome movement
Base pairing of mRNA is always A with U and G with C.
A, adenine; C, cytosine; Cys, cysteine; ER, endoplasmic reticulum; G, guanine; Gln, glutamine; Met, methionine;
Phe, phenylalanine; Tyr, tyrosine; U, uracil.
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
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Protein Turnover
 There is a constant flux between making new muscle protein and
breaking down old muscle protein
– Known as “protein turnover”
 Goal for increasing muscle size is for muscle protein synthesis to
exceed breakdown
Muscle
protein
Muscle
breakdown
Muscle
synthesis
Amino
acids
Blood
Phillips SM, et al. J Am Coll Nutr. 2009;28(4):343-354.
Amino acids
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Fast- Versus Slow-Digested Proteins in Protein Turnover
 In general, milk proteins are superior to soy for stimulating
protein synthesis
– Whey (fast protein) stimulates protein synthesis to a greater extent
than casein (slow protein) and soy (moderate-fast protein)
– Casein reduces muscle protein breakdown better
than whey protein
 Mixed protein sources provide benefits of all
Phillips SM, et al. J Am Coll Nutr. 2009;28(4):343-354.
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Branched Chain Amino Acids (BCAAs)
 BCAAs are
H3C
CH-CH2- C
H3C
CH2
H
COO-
NH3+
Leucine
H
CH2
CH- C
CH3
H
COO-
NH3+
Isoleucine
H3C
H3C
CH- C
COO-
NH3+
Valine
 Unlike other amino acids, most BCAA metabolism occurs in
skeletal muscle
– Liver lacks first 2 enzymes in the pathway that break down BCAAs
 Leucine is also unique among AAs in its ability to stimulate
synthesis of new muscle proteins
Tom A, et al. J Nutr. 2006;136(suppl 1): 324S-330S.
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BCAAs and β-hydroxy-β-methylbutyrate (HMB)
 What is HMB?
– Derived from breakdown of leucine
– Hypothesized to inhibit muscle protein breakdown and increase muscle
synthesis, especially in combination with resistance training
– Some evidence in humans that doses of 3 g/day may be effective
• Additive benefits with creatine
– Some studies suggest that HMB may provide greater benefit to untrained
people who start weight training compared with previously conditioned
athletes
Nemet D, et al. Isr Med Assoc J. 2005;7(5):328-332.
Nissen S, et al. J Appl Physiol. 1996;81(5):2095-2104.
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Protein Quality
 Complete protein
– Contains all the essential AAs in amounts that meet or exceed the amounts
needed by humans
• Animal proteins (except gelatin)
• Soy protein
 Incomplete protein
– Too low in one or more of the essential AAs to support human growth
and development
• Cannot serve as a sole source of protein in the diet
• Most plant proteins are incomplete proteins (except soy)
McDonald L, et al. The Protein Book: A Complete Guide for the Coach and Athlete. 1st ed. Salt Lake City, UT: Lyle McDonald Publishing; 2007.
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Protein Quality (cont’d)
 Complementary proteins
– Combinations of incomplete proteins that, when added together, result
in a complete protein
• Legumes: methionine,  lysine
• Grains: methionine, lysine
• Example: rice and beans
– Usually, combining a complete protein with an incomplete protein is
considered complementary
• Exception is milk and legumes
– Although milk has a greater amount of sulfur-containing AAs (eg,
methionine and cysteine) per gram compared with legumes,
• Not enough sulfur-containing AAs are present for an ideal AA profile
when the 2 foods are consumed together
– Combining complementary proteins at each meal for vegetarians
is not necessary
• What matters is total intake of complementary proteins spread over
the course of the day
McDonald L. The Protein Book: A Complete Guide for the Coach and Athlete. 1st ed. Salt Lake City, UT: Lyle McDonald Publishing; 2007.
Gropper SS, et al. Advanced Nutrition and Human Metabolism. 5th ed. Belmont, CA: Wadsworth Publishing; 2009:237.
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Determining Protein Recommendations
for Athletes
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Protein Requirements
 Current RDA for protein is 0.8 g/kg body weight per day
– ~65 g/day for a 180-lb (82-kg) individual
– ~47 g/day for a 130-lb (59-kg) individual
 The RDA for protein is set to prevent deficiency (ie, maintain
protein balance) in healthy adults
 Does not consider potential benefits that might be obtained from
amounts beyond RDA
– For example, the optimal protein intake for muscle function
and athletic performance
 Most Americans appear to consume adequate protein
by this definition
USDA National Agricultural Library Food and Nutrition Information Center. Available at:
http://fnic.nal.usda.gov/nal_display/index.php?info_center=4&tax_level=3&tax_subject=256&topic_id=1342&level3_id=5140.
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Protein Intake Recommendations
 American College of Sports Medicine (ACSM) / American Dietetic
Association (ADA)
– Endurance athletes, 1.2 to 1.4 g/kg per day
• Accounts for greater use of protein as fuel for energy
– Strength athletes, 1.2 to 1.7 g/kg per day
• To support muscle growth, particularly during early training phase when gains are
greatest and protein utilization is less efficient
 Clinical studies suggest there is no apparent benefit at intakes
above 2.0 g/kg per day
American Dietetic Association, et al. Med Sci Sports Exerc. 2009;41(3):709-731.
Tarnopolsky MA, et al. J Appl Physiol. 1992;73(5):1986-1995.
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Vegetarian Diets
 Most vegetarian athletes meet the RDA for protein of
0.8 g/kg per day
– Protein quality of non-animal / non-dairy sources is lower
 ACSM/ADA recommends 1.3 to 1.8 g/kg per day for
vegetarian athletes
American Dietetic Association, et al. Med Sci Sports Exerc. 2009;41(3):709-731.
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How Much Protein Are Athletes Eating?
 Many athletes may already meet or exceed protein
recommendations
– Definition of “high protein” can be the absolute amount of protein, % of
total energy (calories), or protein ingested per kg of body weight
 Strength athletes in particular may believe that much higher
protein intakes are needed for muscle building
– Intakes at 4- to 6-g/kg range are not uncommon
– Unlikely to provide benefit beyond 2.0 g/kg
– It is possible that this much protein intake could adversely affect the nutrient
quality of the overall diet
Protein intake below 2 g/kg per day is safe in healthy individuals
Protein intake above 2 g/kg per day is not recommended due to
lack of proven benefit and potential for adverse health effects
Tipton KD. Proc Nutr Soc. 2011;70(2):205-214.
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Potential Downside to “High-Protein” Diets
 Hydration status
– Nitrogen that is obtained from consuming protein must be excreted in the
urine as urea
– Increased urinary output due to high protein load may increase chances
of dehydration
 Diets very high in protein may lack appropriate amounts of
carbohydrate, fiber, and some vitamins/minerals
– Could impair exercise performance
– Could increase long-term risk of diseases such as colon cancer
• Possibly due to lack of fiber or increased intake of red meat
 Excessively fatty protein sources could increase risk
of cardiovascular disease
– Choose mostly lean protein sources
• For example, salmon is more desirable than a ribeye steak
Tipton KD. Proc Nutr Soc. 2011;70(2):205-214.
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Potential Downside to “High-Protein” Diets (cont’d)
 Kidney disease
– No good evidence of damage in individuals with healthy kidneys
– Protein-rich diets are high in phosphorus, which can be harmful to
individuals with kidney disease
• Primarily a concern with elderly or sick individuals, as opposed to healthy athletes
 Bone health
– Higher protein diets may increase calcium loss in urine
• However, gut absorption of calcium is likely improved, so there may be
no net difference
– Elevated protein diets appear to have either no or a slightly beneficial effect
on skeletal health
Tipton KD. Proc Nutr Soc. 2011;70(2):205-214.
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Summary of Protein Recommendations
 Daily Recommended Intake is 0.8 g/kg per day (2002)
– No recommendation for increase in athletes
 American College of Sports Medicine
– Strength athletes: 1.2 to 1.7 g/kg per day
– Endurance: 1.2 to 1.4 g/kg per day
 Vegetarians may have higher dietary supplementation protein needs
than omnivores
 Protein intakes up to 2.0 g/kg per day are generally safe in healthy adults
and may be beneficial
– Many athletes may already unconsciously eat this amount of protein
 Few convincing data show that > 2 g/kg per day is helpful
– May actually increase risk of adverse events
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What About Amino Acid Supplements?
 As long as the complete protein requirements are met, the individual
AA requirements will be met as well
– No need for additional AA supplements to prevent deficiency
 Because vegetarians eat few complete sources of protein, they
should be cognizant of complementary protein sources throughout
the day to prevent deficiency of particular AAs
 Branched chain amino acids are popular as a supplement
among athletes
– Claims mainly center on decreasing muscle soreness and improving either
performance or recovery from exercise
– Doses can range from 2 to 7 g/day to more than 20 g/day
 There are potential risks associated with AA supplements
– Large doses of single AAs can prevent the absorption of other AAs, which
may lead to diarrhea
– Can indirectly cause deficiency of other AAs as a result
American Dietetic Association, et al. Med Sci Sports Exerc. 2009;41(3):709-731.
Sharp CPM, et al. J Strength Cond Res. 2010;24(4):1125-1130.
Jackman SR, et al. Med Sci Sports Exerc. 2010;42(5):962-970.
Harper AE, et al. Physiol Rev. 1970;50(3):428-558.
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Example: Quantity of Amino Acids in Food
 1 cup of low-fat cottage cheese (2%) has 31 g of protein
– Translates to 31,000 mg of AAs
– The branched chain amino acid content of the cottage cheese
(leucine + isoleucine + valine) is 6,942 mg (6.9 g)
 Whole protein sources are best (may be less expensive)
Pennington JAT. Bowes and Church’s Food Values of Portions Commonly Used. 17th ed. Philadelphia PA: Lippincott Williams & Wilkins; 1998:30,318.
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Is Protein Used for Energy in Athletes?
 In general, the body prefers to spare its own protein stores
from use for energy production
– Skeletal muscle, in particular, will be protected in order to allow crucial
fight or flight response
– Only in conditions of starvation or extreme energy requirements
(eg, ultramarathons) will the body break down muscle for energy
 However, dietary protein is commonly used as a fuel source in certain
situations (eg, during endurance exercise, which can deplete carbohydrate
stores in muscle)
– 6 of the 20 amino acids can be metabolized for energy in resting muscle
(frequent during exercise)
Berg JM, et al. Biochemistry. 5th ed. New York, NY: WH Freeman & Co.; 2002.
Wagenmakers AJ. Exerc Sport Sci Rev. 1998;26:287-314.
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Timing of Ingestion and Macronutrient Content of Meals
 There is increasing agreement that immediate post-exercise ingestion
of protein and/or carbohydrate has beneficial effects on
– Muscle glycogen replenishment (particularly carbohydrate)
– Muscle protein synthesis (particularly protein)
 A combination of both protein and carbohydrate seems to work better
than either carbohydrate or protein alone
– Proportions of carbohydrate/protein vary based on individual needs
• Endurance athletes prioritize carbohydrate intake for glycogen
replenishment
• Bodybuilders prioritize protein intake for muscle growth
Zawadzki KM, et al. J Appl Physiol. 1992;72(5):1854-1859.
Ivy JL, et al. J Appl Physiol. 2002;93(4):1337-1344.
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Putting a Meal Plan Together
 Example: 70-kg athlete requiring 4,000 kcal/day who is exercising 120 min/day,
4 to 6 times/week
 Macronutrient target recommendations
– Grams/kg body weight/day
• Carbohydrate
7-10 g/kg (490-700 g/day)
• Protein
1.5-2.0 g/kg (105-140 g/day)
• Fat
Typically use percentage of energy
– Percentage of energy
• Carbohydrate
55-65% of energy (550-650 g/day)
• Protein
10-15% of energy (100-150 g/day)
• Fat
20-30% of energy (88-133 g/day)
 Target recommendations for this athlete
− Carbohydrate 600 g/day (60% of energy)
− Protein
130 g/day (13% of energy)
− Fat
120 g/day (27% of energy)
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A Potential Distribution of Macronutrients
Over the Course of 6 Meals/Day
Meal
Time
Carbohydrate, g
Protein, g
Fat, g
Breakfast
7:00 AM
90
15
15
Mid-morning snack
10:00 AM
25
10
5
Noon
75
20
20
Pre-exercise meal
1:30 - 2:00 PM
90
10
5
During exercise
3:00 - 5:00 PM
100
0
0
Post-exercise meal
5:00 PM
75
30
25
Dinner
6:30 PM
120
30
35
Evening snack
9:00 PM
25
15
15
600
130
120
Lunch
TOTALS
34
Protein Content of Various Foods
1 egg, 2 egg whites, or 1/4 cup egg substitute
1 cup of milk
¼ cup cottage cheese
1 cup of yogurt
1 oz. of chicken, fish, pork, or beefa
1 oz. of cheese (except cream cheese)
1 slice of bread or ½ bagel
1 cup of cereal
2 tablespoons peanut butter
1/2 to 2/3 cup of dried beans or lentils
1 cup miso
4 oz. raw, firm tofu
½ cup peas or corn
½ cup of non-starchy vegetables
8 oz. soy milk
Protein drinks and powders/serving
a3-ounce
Protein Content, g
6-7
8-10
7
8-13
7
7
3
3-6
7
8
8
9
3
2
5-6
10-45
portion (21 g protein) is the size of a deck of cards.
Pennington JAT. Bowes and Church’s Food Values of Portions Commonly Used. 17th ed. Philadelphia PA: Lippincott Williams & Wilkins; 1998.
35
Summary
 Adequate protein intake is critical for athletic performance
and good health
 For most athletes, protein intakes of 1.5 to 1.8 g/kg/day
(0.68-0.81 g/lb/day) will meet protein requirements
 High-quality protein sources (eg, dairy products, meats, fish, chicken,
soy, eggs) should be included in the diet
 Eating a combination of carbohydrate and protein soon after
exercise can help with muscle recovery and muscle building
 There are potential disadvantages to excessive protein intake above
2 g/kg/day
– In general, no additional benefit for strength or muscle building
– Increased water loss from the body that may lead to dehydration
– High-protein intake may replace carbohydrates and other vital nutrients
for athletic performance and good health
• Decreased overall diet quality
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