Impact of moderate environmental heat stress during running exercise on circulating markers of gastrointestinal integrity in endurance athletes

In the present study, we aimed to determine the effect of moderate ambient heat stress on exercise-provoked patterns of "leaky gut" biomarkers and stress markers in well-trained athletes. Eleven triathletes performed a strenuous 1-h treadmill run, both under normal ambient conditions (N, 18-21°C) as well as under moderate heat environmental conditions (H, 28-30°C). Core body temperature (Tc), heart rate (HR), and rating of perceived exertion (RPE) significantly increased under both conditions, with significantly higher values during and after the H run. We observed a significant main effect of acute exercise on circulating leukocyte numbers, release of cell-free human DNA (cfDNA) but not bacterial DNA (bacDNA), and on plasma levels of intestinal fatty-acid binding protein (I-FABP), lipopolysaccharide-binding protein (LBP), endotoxin (LPS), and D-lactate. Exercising under H conditions accelerated the mobilization of circulating neutrophils and lymphocytes, and significantly affected the release of cfDNA, D-lactate, I-FABP, creatinine, and blood potassium levels. Multiple correlation analysis revealed a significant association between Tc, max and exercise-provoked release of cfDNA (r = 0.583, p = 0.012) as well as with I-FABP (r = 0.554, p = 0.026). Our data indicate that acute exercising and heat stress may not only affect paracellular but also transcellular intestinal permeability.