Diurnal Variation in Physiological and Immune Responses to Endurance Sport in Highly Trained Runners in a Hot and Humid Environment

PURPOSE: The purpose of this study was to investigate the physiological and immunological response of highly trained runners to an intense bout of exercise performed at two different times of day, in a hot, humid environment. METHODS: Using a crossover randomized design, 13 highly trained runners (range [Formula: see text] 64–79 ml·kg(−1) min(−1)) performed a 10 km time trial run in hot (28°C) and humid conditions (70%), at 2 different times of day (09:00 hs and 18:00 hs). Venous blood samples were taken to determine WBCs (white blood cells), IL-6 (interleukin-6), CC16 (club cell protein 16), and HSP70 (heat shock protein-70) concentrations. Upper respiratory tract inflammation was additionally assessed using a nasal lavage procedure. RESULTS: A significant diurnal difference (p < 0.05) was found for core body temperature, total WBC, and neutrophil and lymphocyte concentrations with higher values at 18:00 hs. A phase response in IL-6, HSP70, WBC, neutrophil, lymphocyte, and CC16 was noted, being more pronounced at 18:00 hs, whilst core body temperature and HR phase responses were more pronounced at 09:00 hs. CONCLUSION: In hot and humid conditions, athletes may wish to consider, when possible, racing and particularly training in the morning where the least homeostatic perturbation occurs.