Prolonged endurance exercise performed in hot ambient conditions is associated with an increased prevalence of performance-limiting gastrointestinal perturbations. This study aimed to examine the associations between biomarkers of exercise-induced gastrointestinal syndrome (EIGS) and exercise-associated gastrointestinal symptoms (Ex-GIS) under exertional heat stress (EHS). Fifty-six non-heat acclimated endurance-trained individuals completed 2 h of steady state running at 60% maximal oxygen uptake (V.O2max) with an ambient temperature of 35.1 °C and relative humidity 29.4%. Venous blood samples were collected immediately pre- and post-exercise to quantify plasma concentrations of gastrointestinal epithelial injury and systemic inflammatory biomarkers, whilst gastrointestinal symptoms were recorded at regular intervals throughout the exercise protocol. Spearman's rank correlation identified moderately significant relationships between interleukin-6 (IL-6) with defecation-bloody stools; interleukin-10 (IL-10) with upper abdominal pain; and IL-10, IL-1 receptor antagonist (IL-1ra), and systemic inflammatory response (SIR) profile with flatulence. Simple linear regression demonstrated that IL-6 explained a small but significant proportion of the variance defecation-bloody stool (adjusted R2 = 0.094, p = 0.024); whilst variance in flatulence was independently explained by IL-10 (adjusted R2 = 0.138, p = 0.025), IL-1ra (adjusted R2 = 0.122, p = 0.033), and SIR-Profile (adjusted R2 = 0.112, p = 0.040). These findings suggest that Ex-GIS development is multifactorial in aetiology and pathophysiology, and that symptom reporting alone likely underestimates perturbations to the gastrointestinal tract during EHS.
Keywords: bacterial endotoxin; exertional-heat stress; intestinal epithelium; permeability; systemic inflammation; ultra-endurance.