Characterising the effect of heat strain and exercise intensity on gut permeability in well-trained male runners

Introduction: Given the need to train and compete in hot and humid conditions, it is important to maximise exercise performance while minimising the risk of heat illness. Exercise-induced endotoxemia is a risk of strenuous exercise in the heat, primarily attributed to translocation of lipopolysaccharides (LPS) through the permeable gut into the circulation. This study aimed to quantify changes in physiological, perceptual and gut permeability markers during exercise under thermoneutral conditions and heat strain. Methods: Twelve well-trained male runners (age 37 ± 7 y, body mass 71.5 ± 7.9 kg, VO2max: 61.0 ± 6.8 ml/kg/min; mean ± SD) participated in a randomised cross-over design study. Each participant performed a treadmill running trial of 2 × 15-minute steady-state submaximal (60% and 75% VO2max) efforts, and up to 8 × 1-minute maximal (95% VO2max) intervals, separated by 7-days, in HOT (33 °C, 68%RH) and TEMPERATE (18 °C, 57%RH) conditions, to simulate a typical interval training session. Heart rate (HR), core body (Tcore) and skin (Tskin) temperatures, perceived exertion (RPE) and thermal sensation (TS) were measured. Venous blood samples were analysed for markers of intestinal permeability (zonulin, LPS, LPS binding protein [LBP] and intestinal fatty acid binding protein [iFABP]) after each interval set and 1 h post-exercise. Results: Tcore was significantly higher after exercise at 75% VO2 (1.6 °C, 95%CI [1.1–2.0]; p < 0.001) and 95% VO2 (2.0 °C; 1.5–2.4; p < 0.001), and during exercise in HOT compared to TEMPERATE (p = 0.04). Tskin was 4.9 ± 0.7 °C higher (p = 0.015) during HOT, HR was 12 ± 8 bpm higher in HOT compared to TEMPERATE, and HR increased incrementally with exercise intensity (p < 0.001). LPS concentration was 4.7% higher (0.2 EU/ml, 0.1–0.4; p = 0.011) during 75% exercise compared with baseline, and LBP concentration increased following all exercise intensities (3.4%, 25% and 23% respectively; p < 0.001) in HOT compared with TEMPERATE. RPE was higher following 75% (2.1 units, 1.1–3.7; p < 0.001) and 95% (3.2 units, 1.9–4.6; p < 0.001) exercise in HOT compared with TEMPERATE. The increase in TS was greater in HOT (1.5 ± 1.1, 1.3 ± 0.9 and 1.7 ± 1.2 units respectively; p < 0.001) compared to TEMPERATE across all exercise intensities. Discussion: Submaximal and maximal exercise performance irrespective of ambient temperature increased intestinal permeability at 75% VO2max as represented by increased LPS. Exercise in HOT yielded a greater increase in LBP during all exercise intensities than TEMPERATE condition. The increased concentration of LPS and LBP in the heat is evidence of modest disturbance to intestinal tight junctions in athletes undertaking high intensity interval training. Exercise and heat strain could be a primary driver of gastrointestinal disturbance and symptoms potentially increasing the risk of athlete discomfort and decreased performance.