The speculation that glutamine supplementation can enhance performance during endurance activities is largely based on the acute immune system suppression observed during strenuous exercise. Since glutamine is utilized as a fuel source for immune system cells, the assumption is that supplementary glutamine will attenuate the mobilization of glutamine from the skeletal muscle. Prolonged endurance exercise, such as marathon running, may reduce plasma glutamine concentration. Plasma glutamine concentrations seem to significantly decrease in overtrained athletes compared to control, nonovertrained athletes.36 Physical stress such as illness or increased physical activity can induce hypercatabolic states, thereby decreasing the body's endogenous rate of glutamine synthesis, making it a conditionally essential amino acid. Early studies suggested that glutamine may be beneficial to endurance athletes because of its impact on muscle glycogen synthesis, by serving as a substrate for gluconeogenesis in the liver, thereby decreasing amino acid release from muscle during extended exercise and decreasing muscle protein degradation. Mourtzakis and Graham37 observed that during prolonged exercise, carbohydrate oxidation peaked at 30 min of exercise and decreased for the remainder of the exercise bout, while pyruvate production was greatest at 1 h of exercise and was closely linked to glutamate concentrations, which was the predominant amino acid taken up during exercise and recovery. Alanine and glutamate were associated with pyruvate metabolism and comprised ~68% of the total amino acids released during exercise and recovery, thus implying that a reduced supply of these amino acids can limit endurance exercise performance.
Glutamine supplementation has also been shown to decrease incidence of infection secondary to overtraining and to improve the response of cells of the immune system, thereby enabling athletes to maintain training at a greater frequency and intensity.11 However, other studies have not found any beneficial effects of glutamine supplementation in athletes.171831
Strenuous exercise can cause significant immunosuppression along with a reduction in plasma glutamine levels. Following intense exercise of greater than 1 h, lymphocyte count, natural killer cell activity, and lymphokine-activated killer cell activity were observed to decline.411 Additionally, the lymphocyte proliferative response to T-cell mitogens decreased during exercise. Concomitant decline in plasma glutamine concentrations may play a role in impaired immune function after sustained physical activity. It has been shown that glutaminase activity is increased during immunologic challenges, which in rapidly dividing cells is a source of nitrogen and carbon, which can serve as precursors for macromolecules and energy.38 Thus, under life stresses and disease states, several dispensable amino acids can become conditionally essential because their utilization within the body exceeds their endogenous production.
While oral or parenteral glutamine supplementation has helped maintain muscular glutamine concentrations, improve nitrogen balance, and increase protein syn-thesis,39 the benefits for athletes are not well established. Castell et al.19 investigated the effects of consumption of glutamine-containing drinks immediately after heavy exercise and 2 h after exhaustive exercise in middle-distance, marathon, and ultramarathon runners and in elite rowers during training and competition. The glutamine-supplemented group reported a reduced instance of infections compared to the placebo group. Castell et al.40 later observed that glutamine supplementation did not appear to have an effect on immune function (as assessed by lymphocyte distribution) following completion of the Brussels marathon. Similarly, Rohde et al.1718 examined the impact of glutamine supplementation on exercise-induced immune change after 30, 45, and 60 min at 75% VO2max. Arterial glutamine concentration decreased by 20% after the last exercise bout in the placebo trial, whereas the glutamine concentration was maintained at a level above rest at all times in the glutamine-supple-mented group (900 mg/kg body weight). However, these differences between the two groups were not significant. The concentration of leukocytes increased during and after each exercise bout, which was attributed to an increase in neutrophils, lymphocytes, and monocytes (during) and neutrophils and monocytes (after); however, no differences were observed between the placebo and glutamine groups. Thus, the post-exercise immune changes did not appear to be caused by decreased plasma glutamine concentrations.1718 Kargotich et al.41 observed glutamine responses to strenuous interval exercise before and after 6 weeks of endurance training. Prior to training, glutamine concentrations progressively decreased (16 to 18%) post-exercise. However, with training glutamine concentrations increased by 14%, suggesting that training-induced changes in glutamine may be able to prevent the decline in glutamine levels following strenuous exercise below a threshold where immune function might be acutely compromised. These results further suggest that oral glutamine supplements may prevent the post-exercise reduction in plasma glutamine concentrations without influencing the immune system, but the ergogenic benefits of glutamine supplementation need further examination.
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