The amino acid glutamine appears to play a role in several aspects of human physiology that might benefit athletes, including their muscle function and immune system.¹ Intense exercise lowers blood levels of glutamine, which can remain persistently low with overtraining.² Glutamine supplementation raises levels of growth hormone at an intake of 2 grams per day,³ an effect of interest to some athletes because of the role of growth hormone in stimulating muscle growth,⁴ and glutamine, given intravenously, was found to be more effective than other amino acids at helping replenish muscle glycogen after exercise.⁵ However, glutamine supplementation (30 mg per 2.2 pounds body weight) has not improved performance of short-term, high-intensity exercise such as weightlifting or sprint cycling by trained athletes,^{6, 7} and no studies on endurance performance or muscle growth have been conducted. Although the effects of glutamine supplementation on immune function after exercise have been inconsistent,^{8, 9} double-blind trials giving athletes glutamine (5 grams after intense, prolonged exercise, then again two hours later) reported 81% having no subsequent infection compared with 49% in the placebo group.¹⁰