Amino acids are small molecules that are the building blocks of proteins. Amino acids are also involved in numerous metabolic pathways. Exercise affects the metabolic pathways of individual amino acids, as well as both the rate of protein breakdown (degradation) into individual amino acids and the rate of protein production (synthesis) from amino acids.
Amino acids, carbohydrates (sugars), and lipids (fats) form the basis of cellular metabolism. The 20 common amino acids are joined together in various combinations by peptide bonds to form linear chains that make up all the proteins in the body, including the structural components of cells and the enzymes that carry out metabolic reactions. The proteins actin and myosin are the major components of muscle fibers and, together with energy from fuel burning, enable muscles to contract.
The human body can synthesize 11 of the common amino acids (arginine, glutamine, tyrosine, cysteine, glycine, proline, serine, ornithine, alanine, asparagine and aspartate) via carbohydrate metabolism. These are known as nonessential amino acids because they do not have to be supplied in the diet. Some nonessential amino acids are often termed conditionally essential as they are needed in the diets of children because, while they are young, they are unable to make enough to meet their needs. Particular amino acids can become conditionally essential for adults during times of severe trauma or disease or during times of intense training.
The other nine common amino acids—the essential amino acids (EAAs)—must be obtained from food (valine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and histidine). During digestion, proteins in food are broken down into their constituent amino acids, which are absorbed by the bloodstream and delivered to the cells of the body for the synthesis of new proteins.
Exercise requires metabolic adjustments throughout the body. These adjustments depend on the type, intensity, and duration of the exercise. Changes in amino acid metabolism during exercise are most pronounced in skeletal muscle, and some evidence indicates that exercise may improve the efficiency of amino acid and protein metabolism; however, because of the complexity of metabolic processes involving amino acids, quantifying the metabolism of individual amino acids during exercise has proven difficult.
Increasing muscle strength and mass is a common goal of exercise, because strength is dependent, to a large degree, on muscle mass. Muscle mass is dependent on the amount of muscle protein and the balance between protein degradation and protein synthesis in muscle, known as protein turnover. Protein turnover is dependent on amino acid metabolism.
Muscle growth (hypertrophy) occurs when protein synthesis exceeds protein degradation. In general, exercise appears to stimulate muscle protein synthesis for as long as 48 hours after exercise. Although protein synthesis may be reduced immediately following exercise, the intake of food restimulates synthesis. Therefore over time, muscle hypertrophy results from net protein synthesis that is stimulated by exercise followed by food intake, especially intake of amino acids and protein. Studies have found that there is an optimum dose of 20–25 g protein to maximally stimulate muscle growth after exercise. A lower amount results in suboptimal rates of protein synthesis, whereas much larger doses stimulate protein breakdown, negating the effects of the simultaneous protein synthesis.
Dietary amino acids and protein appear to be particularly important for increasing muscle mass and strength following resistance exercise. Age-related muscle loss appears to be associated with changes in amino acid metabolism and with a decrease in protein synthesis. Resistance exercise increases muscle protein synthesis in older adults, but to a lesser degree than in younger people.
In addition to their role in protein turnover, amino acids can be burned as fuel. Although their energy contribution during short-term intense exercise is negligible, during prolonged exercise amino acids provide 3%–6% of the body's energy consumption. When carbohydrate availability is limited, amino acids may provide as much as 10% of the body's fuel. Skeletal muscle, which is composed largely of protein, is a major source of amino acids for fuel. Therefore, dietary amino acids and protein are essential for minimizing the degradation of muscle protein to supply amino acids for energy.
The three essential BCAAs account for more than 20% of the amino acids obtained from food, and they appear to be particularly important during exercise. Their concentrations in blood and tissues change rapidly in response to exercise, dietary intake, and metabolic stress, and they are the only amino acids that are primarily broken down in muscle.
During exercise, BCAAs move from the circulating blood into muscle, where they undergo oxidative degradation in proportion to exercise duration and intensity. For example, levels of BCAAs in blood have been reported to fall after 90 minutes of intense cycling, and their levels continue to fall during recovery from exercise. However, this oxidative breakdown of BCAAs in muscle probably contributes only minimally to muscle energy. Rather, BCAA breakdown is required for other metabolic processes.
Leucine is especially affected by exercise. It accounts for about 9% of all amino acids in skeletal muscle protein. During high-intensity exercise, large amounts of leucine are broken down. Leucine also functions as a signaling molecule, working in consort with insulin to stimulate muscle protein synthesis.
Other amino acids may have important roles in muscle metabolism during exercise. For example, exercise increases the production of alanine and glutamine, and glutamine may increase leucine levels in muscle. Glutamine is a basic amino acid and, if ingested before exercise, may help counteract muscle acidity that causes fatigue. Glutamine also may increase growth-hormone levels and metabolic and fat-burning rates, both during exercise and while at rest.
Carnosine is produced in muscle from the amino acid histidine and the unusual amino acid beta-alanine. It has been claimed that carnosine can increase muscle mass and strength and reduce body fat.
Diets, especially meat-based diets that are high in protein have been associated with athletic performance since ancient times, perhaps because early athletes attempted to emulate the strength, speed, and endurance of meat-eating predators. Amino acids from high-quality protein are certainly important for the production and maintenance of muscle. They also may be indirectly involved in the regulation of blood glucose levels during exercise. Yet, the amount of dietary protein required for muscle growth remains unclear. The recommended dietary allowance (RDA) of daily protein is 56 g (1.98 oz.) for adult males and 46 g (1.6 oz) for adult females, or 10%–35% of total calories. Most Americans ingest almost twice this amount of protein. Guidelines for increasing aerobic capacity and building muscle mass and strength generally suggest no more than twice the RDA for resistance athletes and less than twice for endurance athletes.
Eggs and milk contain some of the best-quality protein. Milk protein may be more effective than hydrolyzed soy protein at stimulating uptake of amino acids and increasing muscle protein following resistance exercise. The major casein protein in milk is digested by the stomach over a period of up to seven hours, providing a slow, steady supply of amino acids to build muscle and inhibit muscle breakdown. Whey protein, in the watery fraction of milk, is digested very rapidly and may promote muscle growth when taken before and after workouts. Whey protein is high in BCAAs, and fragments of whey protein may increase blood flow to muscles.
There is little or no evidence that a high-protein diet that supplies excess amino acids can improve either muscle mass or strength. Bodybuilders and other athletes usually obtain the small amount of extra protein they need through additional food. Excess dietary protein is stored as body fat and can cause dehydration and loss of calcium. People on high-protein diets should consume adequate fluids to prevent dehydration and fruits and vegetables to counter calcium loss. Furthermore, eating extra protein often reduces carbohydrate intake, which is more important as a source of energy during exercise.
Amino acids are one of the most popular sports supplements and are aggressively marketed to athletes and other physically active people. Although many such supplements are considered safe when used at the recommended dosages, most people obtain more than enough amino acids from their diet.
Although evidence indicates that amino acid supplementation during exercise has little or no effect on performance, some evidence suggests that supplementation soon after exercise may enhance synthesis of muscle protein. During recovery from resistance exercise, the breakdown of muscle protein into amino acids generally exceeds the synthesis of new muscle protein. Ingestion or infusion of amino acids during the recovery period stimulates muscle protein synthesis: 0.2 oz. (6 g) of EAAs can effectively stimulate protein synthesis following strenuous resistance exercise and can reduce protein degradation. Dietary leucine appears to play a critical role in muscle recovery from both resistance and endurance exercise. Ingestion of carbohydrate increases insulin that promotes the movement of amino acids from the bloodstream into muscle. Supplemental creatine may benefit athletes engaged in intense training. Therefore, it is possible that postexercise energy and amino-acid bars or drinks may be beneficial.
Although amino acid metabolism during exercise is primarily of concern to bodybuilders and other athletes trying to increase their muscle mass and strength, amino acid metabolism also can be important during recovery and rehabilitation from diseases and disorders. Some evidence indicates that arginine supplementation, by stimulating the production of NO, may improve exercise capacity in heart transplant patients and those with congestive heart failure, pulmonary hypertension, and other cardiopulmonary disorders. Amino acid supplementation may also help improve the exercise capacity of elderly patients with chronic heart failure and patients with chronic diseases such as diabetes.
Periods of bed rest and/or limb immobilization lead to loss of muscle mass. This appears to be due primarily to a reduction in synthesis of new muscle protein, in addition to a transient increase in the breakdown of existing muscle protein. Rehabilitation exercise is required to restore muscle mass and function and appears to work, at least in part, by stimulating muscle protein synthesis. Under such circumstances, amino acid supplementation may be of some benefit.
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Margaret Alic, PhD