PurposeTo examine the dose–response effects of acute glutamine supplementation on markers of gastrointestinal (GI) permeability, damage and, secondary, subjective symptoms of GI discomfort in response to running in the heat.MethodsTen recreationally active males completed a total of four exercise trials; a placebo trial and three glutamine trials at 0.25, 0.5 and 0.9 g kg−1 of fat-free mass (FFM) consumed 2 h before exercise. Each exercise trial consisted of a 60-min treadmill run at 70% of in an environmental chamber set at 30 °C. GI permeability was measured using ratio of lactulose to rhamnose (L:R) in serum. Plasma glutamine and intestinal fatty acid binding protein (I-FABP) concentrations were determined pre and post exercise. Subjective GI symptoms were assessed 45 min and 24 h post-exercise.ResultsRelative to placebo, L:R was likely lower following 0.25 g kg−1 (mean difference: − 0.023; ± 0.021) and 0.5 g kg−1 (− 0.019; ± 0.019) and very likely following 0.9 g kg− 1 (− 0.034; ± 0.024). GI symptoms were typically low and there was no effect of supplementation.DiscussionAcute oral glutamine consumption attenuates GI permeability relative to placebo even at lower doses of 0.25 g kg−1, although larger doses may be more effective. It remains unclear if this will lead to reductions in GI symptoms. Athletes competing in the heat may, therefore, benefit from acute glutamine supplementation prior to exercise in order to maintain gastrointestinal integrity.