When protein is consumed from food sources, the protein is digested into its individual amino acids through the enzymatic action of proteases. Individual amino acids then become part of the amino acid pool as do the amino acids from the catabolism of body tissue. Since the body cannot store amino acids or proteins, there is constant protein turnover in the body. Some proteins are continually being synthesized, while others are being digested. If the rate of synthesis equals the rate of catabolism, there is protein balance. If the rate of protein breakdown exceeds the rate of protein synthesis, one is in negative protein balance, or a state of catabolism. This happens in periods of illness, inadequate physical activity, or when dietary intake of protein is insufficient.
Protein’s Role in Exercise
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Many athletes are seeking to achieve muscle hypertrophy. For muscle mass to increase, a positive protein balance must be achieved whereby the rate of muscle protein synthesis exceeds that of muscle protein breakdown. While dietary consumption of protein can stimulate muscle protein synthesis, to actually have a positive net balance that results in muscle hypertrophy, there also needs to be a loading stimulus to skeletal muscle. This requires resistance exercise. Thus, consuming protein in the diet alone is insufficient to increase muscle mass. Similarly, engaging in resistance exercise in the absence of dietary protein intake will not only be insufficient to increase muscle mass, but may also result in a negative muscle protein balance, that is, the muscle is in a catabolic state (Tipton and Sharp 2005). Therefore, resistance exercise in the presence of hyper-aminoacidemia, or having excess amino acids in the blood, is required to stimulate muscle protein synthesis. The timing of protein intake is also important, and this will be discussed in the following section on “Protein Recommendations.”
Another misconception is that protein is a good source of energy for exercise. In reality, the body prefers to use carbohydrate and fat as an energy source because they are more efficiently utilized by muscle cells. Using protein as an energy source is very inefficient, in that the process of converting protein into energy actually consumes energy before energy can be produced. This is because the amino group must be removed from the amino acid through deamination or transamination and a then carbon skeleton, or a keto acid, is formed. These carbon skeletons can then be metabolized to produce energy, that is, glucose via gluconeogenesis, ketone bodies, or other substances. However, these pathways require the input of energy before energy can be produced. The net yield of glucose from protein metabolism is significantly less than is the amount of glucose produced from carbohydrates via glycolysis. The result is that protein is a very inefficient energy source.
Additionally, the body prefers to utilize amino acids for anabolic purposes. Think of the many functions of protein in the body. Amino acids resulting from protein metabolism are needed for muscle and tissue synthesis, for bone turnover, and for the production of other body proteins, among other functions. Thus, adequate carbohydrate and, to some degree, fat help to “spare” protein from being metabolized for energy production and to maintain energy balance. Under normal circumstances, protein contributes only ~5 percent of energy; under periods of starvation or during prolonged periods of exercise, where there is insufficient carbohydrate availability, this amount may increase up to 18 percent of energy contribution. This is not a desired nutritional status and dietary strategies can help prevent excessive protein oxidation.
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