Amino-acid supplements have been marketed to athletes for the last 2 decades. The claims that the amino acids they contain are superior to those found in food are not well supported, nor has it been shown that they achieve a superior functional outcome. Although there is some evidence that taking amino-acid supplements after exercise might promote better net muscle-protein retention, it is equally likely that this effect can be achieved from eating food containing amino acids. In fact, coingestion of carbohydrate is important in maximizing the effect. There is a theory that "central fatigue" can occur during prolonged exercise as a result of changes in plasma ratios of free tryptophan to branched-chain amino acids (BCAAs), leading to an increase in serotonin production. Although it has been proposed that supplementation with BCAAs might attenuate the development of central fatigue, studies have not shown a clear benefit in prolonged exercise. In fact, ingesting carbohydrate during exercise will also produce a metabolic profile attenuating the development of central fatigue.
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