While BC supplementation may protect against enterocyte damage during exercise in thermonuetral environments, our data suggest that BC supplementation may not be an effective technique for preventing enterocyte damage during exercise when core temperature exceeds 39 °C.

New Findings What is the central question of this study?What is the effect of hypobaric hypoxia on markers of exercise‐induced intestinal injury and symptoms of gastrointestinal (GI) distress? What is the main finding and its importance?Exercise performed at 4300 m of simulated altitude increased intestinal fatty acid binding protein (I‐FABP), claudin‐3 (CLDN‐3) and lipopolysaccharide binding protein (LBP), which together suggest that exercise‐induced intestinal injury may be aggravated by concurrent hypoxic exposure. Increases in I‐FABP, LBP and CLDN‐3 were correlated to exercise‐induced GI symptoms, providing some evidence of a link between intestinal barrier injury and symptoms of GI distress. Abstract We sought to determine the effect of exercise in hypobaric hypoxia on markers of intestinal injury and gastrointestinal (GI) symptoms. Using a randomized and counterbalanced design, nine males completed two experimental trials: one at local altitude of 1585 m (NORM) and one at 4300 m of simulated hypobaric hypoxia (HYP). Participants performed 60 min of cycling at a workload that elicited 65% of their NORM trueV̇normalO2max${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{max}}}}$. GI symptoms were assessed before and every 15 min during exercise. Pre‐ and post‐exercise blood samples were assessed for intestinal fatty acid binding protein (I‐FABP), claudin‐3 (CLDN‐3) and lipopolysaccharide binding protein (LBP). All participants reported at least one GI symptom in HYP compared to just one participant in NORM. I‐FABP significantly increased from pre‐ to post‐exercise in HYP (708 ± 191 to 1215 ± 518 pg ml−1; P = 0.011, d = 1.10) but not NORM (759 ± 224 to 828 ± 288 pg ml−1; P > 0.99, d = 0.27). CLDN‐3 significantly increased from pre‐ to post‐exercise in HYP (13.8 ± 0.9 to 15.3 ± 1.2 ng ml−1; P = 0.003, d = 1.19) but not NORM (13.7 ± 1.8 to 14.2 ± 1.6 ng ml−1; P = 0.435, d = 0.45). LBP significantly increased from pre‐ to post‐exercise in HYP (10.8 ± 1.2 to 13.9 ± 2.8 μg ml−1; P = 0.006, d = 1.12) but not NORM (11.3 ± 1.1 to 11.7 ± 0.9 μg ml−1; P > 0.99, d = 0.32). I‐FABP (d = 0.85), CLDN‐3 (d = 0.95) and LBP (d = 0.69) were all significantly higher post‐exercise in HYP compared to NORM (P ≤ 0.05). Overall GI discomfort was significantly correlated to ΔI‐FABP (r = 0.71), ΔCLDN‐3 (r = 0.70) and ΔLBP (r = 0.86). These data indicate that cycling exercise performed in hypobaric hypoxia can cause intestinal injury, which might cause some commonly reported GI symptoms.

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