Amino Acids
Both muscle and milk contain all the naturally occurring amino acids, and thus meat and dairy products are valuable foods (both have high biological value, and dairy sources have higher values). The most abundant amino acids in muscle are the three branched-chain amino acids (BCAAs), leucine, valine, and isoleucine, which together account for 20% of the total amino acids found in muscle protein. Both meat and dairy protein have high BCAA content.
Athletes at Risk of Insufficient Protein Intake
People with extremely low protein intakes may suffer from protein deficiency, which can compromise function and ultimately lead to loss of body protein (atrophy). Certain groups of athletes are primarily recognized as being at risk from protein and energy deficiency: female runners, male wrestlers, boxers and other athletes in weight category sports, ski jumpers, male and female gymnasts, and female dancers. Although protein intake for these groups may be adequate on average, certain people within these groups may have protein intakes well below the RDA due to low energy intake.
Another group that has been suggested to be at risk is vegetarian athletes. Plant food
sources typically contain lower-quality proteins that have low levels of one or more
essential amino acids .
In addition, the digestibility of plant protein can be low compared with animal protein. Although some concern exists that vegetarian athletes may struggle to meet the protein requirements, the evidence for this is lacking, and adequate protein intake seems possible through a balanced vegetarian diet.
Amino Acid Metabolism
The metabolism of most amino acids is linked to the metabolism of other amino acids, and some amino acids can be synthesized from other amino acids. This feature is especially important in conditions of limited dietary protein intake or when metabolic requirements increase. Some amino acids are essential and are not synthesized in the body, whereas others can be synthesized in the body (nonessential amino acids) .
Amino acids are involved in a variety of biochemical and physiological processes, some of which are common to all and some of which are highly specific to certain amino acids.
Amino acids are constantly incorporated into proteins (protein synthesis), and proteins are constantly broken down (protein degradation or breakdown). The vast majority of the amino acids in the body are
incorporated into tissue proteins, but a small pool of free amino acids also exists (about 120g of free amino acids are present in the skeletal muscle of an adult). Amino acids are constantly extracted from the free amino acid pool for synthesis of various proteins, and breakdown of protein makes amino acids available for the free amino acid pool.
Timing of Protein Intake
The timing of food intake after exercise is important to the balance between protein
synthesis and protein degradation. Studies have investigated protein ingestion immediately after exercise, 1 or 3 hours after exercise, or before exercise. In one study by Tipton et al. (2001), volunteers ingested 6 g of essential amino acids plus 35 g of carbohydrate immediately before and immediately after completion of an intense leg resistance exercise bout. Amino acid uptake seemed to be greater when the nutrients were ingested before the exercise bout than immediately afterward, but the anabolic response was similar in magnitude for both preexercise and postexercise feeding. This study suggested that the anabolic response to exercise and amino acid and carbohydrate ingestion is greater with preexercise ingestion versus immediately postexercise, and it was suggested that the observed differences were likely related to the delivery of amino acids to the muscle. Free amino acids ingested before exercise may result in increased amino acid delivery (because of increased blood flow to active muscles during the exercise session) and lead to superior amino acid uptake compared with amino acids ingested after exercise. However, this study was performed with essential amino acids. When some of these studies were repeated with whey protein (and no carbohydrate), the difference between feeding before a bout of resistance exercise and after was not evident. The current consensus is that ingesting protein soon after exercise (within approximately 1 hour after exercise),
which is more practical than preexercise protein consumption, is the best way to increase net protein synthesis postexercise.