Research indicates that levels of glutamine fall after exhaustive exercise. One theory is that a shortage of glutamine is at least partially responsible for the increased frequency of illness observed in athletes during periods of heavy training and after endurance competitions. If feeding the body more glutamine to meet the needs in times of heavy endurance training and competition would strengthen the immune system, frequency of illness could be decreased.
In a study by Dr. Castell and co-workers, the consumption of glutamine immediately and 2 hours after a marathon and ultramarathon reduced the incidence of infections during the week after competition by 32% when compared with a placebo. However, in a follow-up study, the researchers were unable to show a positive effect of glutamine supplementation on immune function in a group of marathon runners. In another study, competitive swimmers taking a glutamine supplement during a season reported fewer symptoms of chest congestion and frequency of coughing. On balance, current evidence indicates that adequate glutamine aids the immune system in times of stress. Whether the glutamine needs to be provided as a supplement or simply provided through an adequate diet remains unanswered.