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Proteins
Proteins
Chapter Outline
I. Why is protein important to athletes?
A. Proteins are second only to water on the body's ingredient list.
B. Most body tissues are comprised of proteins, especially muscle tissue.
C. Because proteins are constantly being turned over in the body, the sports nutrition
professional must be able to ensure that athletes are ingesting adequate amounts of
dietary protein.
II. What are proteins?
A. Proteins are composed of amino acids linked together via peptide bonds (Figure
5.2).
B. Amino acids are molecules composed of carbon, hydrogen, oxygen, and nitrogen.
C. Amino acids all have the same basic structure. They all have a central carbon atom
that is bound to the following (Figure 5.1):
1. A single hydrogen atom
2. A carboxylic acid group (-COOH)
3. An amino group (-NH2)
4. A carbon side chain. Differences in the structure of the carbon side chain are
what differentiate the 20 amino acids from one another.
D. Of the 20 amino acids, 9 are essential and 11 are nonessential. Essential amino acids
must be obtained through the diet; the body can make nonessential amino acids from
other compounds in the body.
E. Under certain conditions, some amino acids may become "conditionally essential."
F. Proteins are classified based on the number of amino acids linked together:
1. Dipeptide: 2 amino acids
2. Tripeptide: 3 amino acids
3. Oligopeptide: 4 to 10 amino acids
4. Polypeptide: 11 or more amino acids
G. Proteins have a three-dimensional structure (Figure 5.3) that is determined by the
sequence of amino acids (primary structure). Hydrogen bonding within or between amino
acids causes bending or coiling (secondary structure). Interactions between the side
groups of the amino acids determine the three-dimensional shape of the protein (tertiary
structure). Finally, the quaternary structure of a protein is the final three-dimensional
structure formed by all of the polypeptide chains making up the protein.
H. Dietary proteins can be classified in several ways
1. Complete and incomplete
a. Complete proteins supply all of the essential amino acids.
b. Incomplete proteins do not contain the full complement of essential
amino acids or may provide only a short supply of an essential amino
acid. The amino acid in short supply is known as the "limiting amino
acid."
c. "Complementing" incomplete proteins can be combined to provide all
of the essential amino acids (Figure 5.4).
2. High- and low-quality proteins
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a. High-quality proteins provide the full complement of essential
amino acids, have extra amino acids available for nonessential amino
acid synthesis, and have good digestibility.
b. Low-quality proteins are lacking in one or more of the
aforementioned criteria for high-quality proteins.
c. Most animal sources of protein are considered high quality; plant
sources are of lower quality.
III. What are the main functions of proteins in the body?
A. They provide structure to muscles and other tissues (e.g., fingernails, hair, connective
tissues, etc.)
B. They regulate cell functions (e.g., hormones, enzymes).
C. They assist in maintaining fluid and acid-base balance in the body (e.g., albumin and
other blood borne proteins).
D. They help transport substances throughout the body (e.g., hemoglobin).
E. They can serve as an energy source for the body.
IV. What is nitrogen balance?
A. Nitrogen balance is achieved when dietary protein intake equals protein loss. For
most athletes, the goal is to stay in a state of positive nitrogen balance. A positive
nitrogen balance suggests that proteins are being assimilated at a greater rate than they
are being broken down. In other words, the body is in an anabolic state.
V. How much protein should athletes consume daily?
A. The Recommended Dietary Allowance (RDA) for protein is 0.8 grams of protein
per kilogram of bodyweight per day.
B. The Acceptable Macronutrient Distribution Range (AMDR) for protein is 10 to 35%
of total daily calories.
C. Most research supports the fact that athletes may need more than the RDA.
D. Current recommendations for protein intake for athletes range from 1.2 to 2.0
grams of protein per kilogram of bodyweight. Protein intakes at these levels will
account for about 15 to 20% of total daily calories, which is well within the AMDR.
E. Various dietary and training factors can affect protein recommendations.
1. Total energy intake
2. Desire to lose or gain weight
3. Carbohydrate availability
4. Exercise intensity and duration
5. Training state/fitness level
6. Dietary protein quality
7. Age
F. Can too much protein be harmful?
1. Based on the AMDR, protein intake should not exceed 35% of total daily
calories.
2. Excessive protein consumption can increase the risk for dehydration, lead
to increased fat deposition, increase calcium loss from the bones, and put
extra strain on the kidneys, which must excrete large amounts of nitrogenous
waste (i.e., urea). If the protein is coming from high-fat animal sources, then
there is also increased risk for cardiovascular disease due to increased fat intake.
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VI. Which foods contain proteins? (Table 5.4)
A. Grains are considered an incomplete source of protein because they contain low levels
of lysine. On average, they provide about 2 to 4 grams of protein per serving.
B. Fruits contain minimal amounts of protein; vegetables provide only 1 to 2 grams of
protein per serving.
C. Foods in the milk/alterative group provide an excellent source of protein. Most
dairy and soy products provide 6 to 8 grams of protein per serving. Rice and other grain
milks have lower protein contents (i.e., 2 to 3 grams/serving).
D. Meat, fish, poultry, eggs, and soy products are excellent sources of complete protein,
providing approximately 15 to 30 grams of protein per serving.
E. Oils and sweets provide little protein.
VII. Are protein supplements beneficial?
A. Most diets provide adequate levels of protein, even when trying to achieve protein
intakes of 1.2 to 2 grams/kilogram of bodyweight.
B. However, protein supplementation can be practiced in some instances.
C. If protein supplementation is being practiced, several factors should be taken into
consideration:
1. What is the quantity of protein or amino acids provided per serving? Many
food sources of protein provide as much protein as supplements. Also megadosing
will not lead to increased muscle mass, but will contribute to increased fat mass.
2. What is the supplement's cost? Many food sources provide similar amounts of
protein at half or less than the cost of supplements.
3. Will the supplement enhance performance? There is no research basis that
engineered proteins will increase sport performance beyond that of proteins
provided by a normal diet. However, in some instances athletes may find it
challenging to meet their protein needs through only whole foods in the diet. As a
result, supplementation may be warranted and thus beneficial.
4. Are there any risks associated with taking the protein supplement? Pay
attention to ingredients lists because many protein supplements may also include
artificial flavors, sweeteners, or colorings that may cause allergic reactions in
some individuals. Some protein supplements provide high levels of one amino
acid or may add other supplements such a creatine or, even worse, substances that
are banned by sports organizations. Finally, companies may use poor
manufacturing practices that can result in contamination from other products,
which could lead to "inadvertent doping" and disqualification of an athlete.
VIII. Why is protein essential for daily training?
A. Sufficient dietary protein is required to maximize protein synthesis in the body in
response to daily training. Protein is not only important for helping in recovery and
rebuilding tissues damaged during training, but it is also needed to help the body adapt to
training.
IX. What type, how much, and when should proteins be consumed before exercise?
A. Similar to the daily guidelines for daily protein intake, athletes should choose lean
protein sources of high-quality protein within the 24 hours leading into a competition or
exercise session. Legumes or other high-fiber sources of protein can lead to
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gastrointestinal discomfort unless the athlete is used to consuming these foods. Protein
intake should be not greater than normal.
B. Because it takes longer to digest proteins, proteins should be consumed about 3
hours prior to competition.
X. What type, how much, and when should protein be consumed during exercise?
A. Proteins provide relatively little energy compared to fats and carbohydrates during
exercise. Although some evidence suggests that branched chain amino acids may offer
some benefits to endurance athletes, the evidence is not strong enough to warrant any
specific recommendations at this time. Any snacks or drinks during exercise should focus
more on carbohydrates.
XI. What type, how much, and when should protein be consumed after exercise?
A. Protein is a critical nutrient for the post-exercise recovery process.
B. The goal is to achieve a positive nitrogen balance. The success of doing so seems to
depend on the amino acid composition of the food ingested, the amino acid concentration
in the blood supplying the muscle, and the timing of the feeding.
C. After exercise, it is important to consume high-quality or complementing protein
sources that provide all of the essential amino acids.
D. Ingestion of 6 to 20 grams of protein in the post-exercise meal is recommended.
E. Timing of protein consumption is important. Protein ingestion should begin as
soon as possible after competition or exercise, and certainly within 3 hours.
F. Combining some carbohydrate with protein may enhance the recovery process by
stimulating an insulin response, which appears to increase both carbohydrate and protein
uptake.
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