Introduction: The premise of this study was to investigate the effect of acute glutamine supplementation on 20 km time trial cycling performance in the heat, neuromuscular function, inflammation and endotoxemia.

Methods: Twelve cyclists completed two, 20-km time trials (20TT) in 35 °C (50% relative humidity). Participants ingested either glutamine (GLUT; 0.9 g∙kg-1 fat-free mass) or a placebo (CON) 60 min before each 20TT. Physiological and perceptual measures were recorded during each 20TT, and neuromuscular function assessed pre- and post-exercise. Venous blood was analysed for endotoxins, markers of gut damage (inflammatory fatty acid binding protein; I-FABP) and inflammatory cytokines (interleukin-6, IL-6; tumour necrosis factor-alpha, TNF-α). Data were analysed using linear mixed models in a Bayesian framework.

Results: 20TT in the heat increased I-FABP and elevated inflammatory cytokines (IL-6 and TNF-α) compared to pre-exercise values but did not result in endotoxemia. Completion time was not statistically different between conditions (mean difference [95% credible interval] = 11 s [-23, 44]). Relative to CON, GLUT did not alter any physiological or perceptual measures during the 20TT.

Conclusion: Glutamine supplementation does not improve 20TT performance in the heat or preserve neuromuscular function when compared to a placebo. These findings suggest that glutamine is not an ergogenic aid or prophylactic intervention for heat-induced gut damage during short-duration self-paced exercise in hot environments.