Glutamine

Main functions

• Enhances immune function.
• Prevents overtraining syndrome.

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Theoretical foundations

Glutamine is the most common amino acid found in human plasma and muscles. Skeletal muscles synthesize, accumulate and release glutamine at a high rate. It participates in protein synthesis, is a nitrogen donor for nucleotide synthesis, transports nitrogen between different tissues, and is a substrate for urine formation. Glutamine is a powerful source of nutrition for intestinal enterocytes and immune system cells.

Glutamine appears to be conditionally essential during times of high metabolic stress or critical illness. Skeletal and plasma glutamine levels are reduced by infections, surgery, trauma, acidosis, and burns. Long-term endurance exercise, such as marathons, can also reduce plasma glutamine concentrations. Furthermore, plasma glutamine concentrations are significantly lower in overtrained athletes than in control athletes.

Because glutamine is essential for optimal immune system function, decreased plasma glutamine levels may impair immune function and increase the risk of infection. Glutamine supplements may enhance immune function, reduce the risk of infection, and help prevent overtraining syndrome.

Research results

The benefits of glutamine supplementation in hospitalized patients during periods of major physiological stress are well established. Oral or parenteral glutamine supplementation after major trauma or surgery helps maintain muscle glutamine concentrations, improve 3-methylhistidine excretion (a marker of muscle catabolism), prevent intestinal atrophy, increase body weight, and shorten hospital stays.

However, the benefits of glutamine supplementation in athletes during periods of intense training have not yet been established. Castell et al. studied the effects of glutamine supplementation in middle-distance, marathon, and ultramarathon runners and elite rowers. Observations were made during training sessions and competitions. Immediately after exercise to exhaustion, 72 athletes were given a glutamine-containing drink and 79 were given a placebo. The athletes completed questionnaires about the occurrence of infections for seven days after the exercise. The proportion of athletes reporting no infections was significantly higher in the glutamine-supplemented group (81%) than in the placebo group (49%). Infections were lowest in the middle-distance runners and highest in the marathon and ultramarathon runners and elite rowers after intense training. In a later study, Castell et al. reported that glutamine supplementation did not appear to affect immune system function (assessed by lymphocyte distribution).

Rohde et al. studied the effects of glutamine supplementation and repeated exercise on immune cell status in a randomized, placebo-controlled, crossover experiment. Eight athletes cycled for 30, 45, and 60 min at 75% V02max and rested for 2 h between rides. Although plasma glutamine levels were maintained in the glutamine supplemented group and decreased in the placebo group, lymphocyte and phytohemagglutinin-stimulated lymphocyte counts decreased 2 h after each ride in both groups. Thus, postexercise immune changes are independent of decreased plasma glutamine concentrations.

Recommendations

Plasma glutamine concentrations may decrease after intense exercise, leading to glutamine depletion. However, adequate daily carbohydrate intake may help prevent muscle glycogen depletion and overtraining, and maintain normal glutamine status. While some preliminary data suggest that glutamine supplementation may reduce the incidence of respiratory infections in athletes, further studies are needed to confirm these findings.