Glutamine is considered an essential amino acid during stress and critical illness which means that the human body cannot synthesize enough of glutamine to satisfy the needs of rapidly proliferating cells of the immune system, especially in the gut.
Even in otherwise healthy people in situations of extreme stress, such as heavy exercise, viral or bacterial infections, the concentration of glutamine in the blood diminishes. Glutamine and arginine: immune nutrients for improved health. Med Sci Sports Exerc. 2000;32(7S): S377-S388). Since glutamine is used as a fuel by rapidly proliferating cells of the immune system, oral intake of glutamine has been seen to have a beneficial effect on gut function and on some aspects of immune cell function in clinical studies.
Athletes and otherwise healthy subjects under stress, clearly benefit from the provision of glutamine due to stimulated immune system and enhanced functionality of immune cells in the gut.