Prolonged strenuous exercise is associated with a temporary immunodepression that affects macrophages, neutrophils, and lymphocytes.36-38 The mechanisms involved are not fully established and appear to be multifactorial, including the actions of stress hormones (e.g., catecholamines and cortisol), inhibition of macrophage and T-cell cytokine production, altered heat shock protein expression, increased oxidative stress, and a fall in the plasma concentration of glutamine.38,39 BCAAs are nitrogen donors for glutamine synthesis, and some studies have evaluated the effectiveness of BCAA supplements during exercise to maintain the plasma glutamine concentration and modify immune responses to exercise. One recent study showed that BCAA supplementation (6 g/day) for 2 to 4 weeks and a 3-g dose 30 min before a long-distance run or triathlon race prevented the 24% fall in the plasma glutamine concentration observed in the placebo group and also modified the immune response to exercise.40 These authors reported that BCAA supplementation did not affect the lymphocyte proliferative response to mitogens before exercise, but did prevent the 40% fall in lymphocyte proliferation observed after exercise in the placebo group. Furthermore, blood mononuclear cells obtained from athletes in the placebo group after exercise presented a reduction in the production of several cytokines, including tumor necrosis factor-a (TNF-a), interferon-y (IFN-y), interleukin-1 (IL-1), and IL-4, compared with before exercise. BCAA supplementation restored the production of TNF-a and IL-1 and increased that of IFN-y. However, athletes given BCAA supplements presented an even greater reduction in IL-4 production after exercise. There were, however, flaws in the experimental design and statistical analysis of the data in this study, and the results need to be confirmed in more controlled studies. Since several previous studies have indicated that glutamine supplementation during exercise does not prevent the exercise-induced fall in lymphocyte proliferation,4142 these findings must be viewed with some caution.
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