HLTH 213
Proteins
Chapter 5
Proteins
Chapter Objectives
1. Introduce the importance of proteins in the diet.
2. Explain the basic structure and various classifications of proteins.
3. Discuss the main functions of proteins in the body.
4. Explain the concept of "nitrogen balance."
5. Discuss how much protein should be consumed and the various factors that can affect the protein needs of athletes.
6. Discuss the various sources of dietary proteins.
7. Discuss protein supplementation and its place in sport nutrition.
8. Explain the importance of protein intake for the athlete involved in daily training.
9. Discuss the type, quantity, and timing of protein consumption before exercise.
10. Discuss the type, quantity, and timing of protein consumption during exercise.
11. Discuss the type, quantity, and timing of protein consumption after exercise.
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
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.
Author Note: John is an aspiring bodybuilder who currently trains 6 days per week. He weighs 180 pounds and consumes about 3,800 calories per day. Have a class discussion on how much protein John should be taking in based on current recommendations. Have the class calculate the total grams of protein John should be ingesting each day and then calculate the percentage of his total daily calories. Is it still 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.
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.
Author Note: Ask the class for instances in which they think protein supplementation would make sense or be warranted.
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 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.
You Are the Nutrition Coach
Jamar is a 17-year-old high school junior who has a chance of starting as a linebacker on the football team his senior year. He is 6 feet tall and weighs 175 pounds. His coach has
recommended that he gain 10 to 15 pounds over the next 8 months, but not at the sacrifice of his speed and quickness. Jamar eats home-cooked, well-balanced meals for breakfast and supper. At breakfast, he also drinks a mega-protein supplement that supplies 56 grams of protein. For school, he packs his own lunches, which usually include two tuna or chicken sandwiches with potato chips and milk. He also has a midmorning and mid afternoon snack, which typically consist of a protein bar (24 grams of protein per bar). He works out in the high school weight room two to three times per week in the late afternoon. When he arrives home, he studies until the family eats supper around 7 p.m. His final snack of the day occurs just before bedtime when he consumes a mega-protein supplement that supplies 56 grams of protein.
1. Is Jamar consuming enough dietary protein to achieve his goals?
2. What are the recommended protein intake guidelines for an athlete who wants to gain weight?
3. Is it detrimental to consume too much protein?
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