Glutamine is the most abundant amino acid found in the body. It is predominantly synthesized and stored in the muscles and participates in a number of reactions important to athletes’ health and recovery. It is classified as semiessential, meaning that under normal circumstances the body can synthesize adequate amounts to support physiological demands. Prolonged periods of heavy training as well as ultraendurance competition, including adventure racing, long-course triathlon, and marathon running, are associated with significant drops in plasma (blood) glutamine levels, which is a postulated cause of exercise-induced immune impairment and increased susceptibility to infection (especially upper respiratory infections). Supplementation with glutamine is thought to help offset these drops, thereby providing athletes with immune support. This aminp acid also promotes protein synthesis and thus may help protect athletes against muscle breakdown.