Three weeks of daily L-glutamine supplementation at 0.3 g/kg body weight in combat-sport athletes significantly increases salivary immunoglobulin A and nitric oxide concentrations and reduces the incidence of upper respiratory tract infections, indicating an enhancement of mucosal immune defense during periods of intense training.

L-glutamine is absorbed from the gut and turned into arginine, which the body uses to make more nitric oxide. Nitric oxide kills bacteria and helps immune cells attack invaders in the mouth and throat. At the same time, L-glutamine feeds immune cells that produce immunoglobulin A, a protein that coats the mucous membranes and blocks germs from entering. Together, more nitric oxide and more immunoglobulin A create a stronger barrier that prevents colds and other infections.