This is adapted from the book, Eat Smart, Play Hard, by RW USA Nutrition Editor Liz Applegate.
Found abundantly in your muscles and blood plasma, the nonessential amino acid glutamine serves as fuel for your immune cells. Because physical stress and heavy exercise deplete glutamine from your plasma, some researchers suspect that supplementing with this amino acid may ward off colds and muscle soreness induced by endurance exercise.
While few studies have been done on this supplement, research does suggest that supplemental glutamine improves your immune function and response to infections. A study published in The Journal of Applied Physiology found that the addition of glutamine to a carbohydrate beverage (such as a sports drink) could help you recover faster. Another study, published in the journal Nutrition, found that a total of 10 milligrams of the supplement taken after exhaustive exercise such as a marathon could cut the risk of catching a cold in half.
My recommendation: Take 5 to 20 grams after endurance exercise such as a 4-hour bike ride or 2-hour run. This supplement is no substitute for rest and a proper recovery diet, however.