Background: To date, no study has examined the hydration status of runners competing in a 24-h winter race under extremely cold environmental conditions. Therefore, the aim was to examine the effect of a 24-h race under an average temperature of -14.3°C on hydration status. Methods: Blood and urine parameters and body mass (BM) were assessed in 20 finishers (women, n = 6; men, n = 14) pre- and post-race. Results: Five (25%) ultra-runners had lower pre-race plasma sodium [Na+] and 11 (52%) had higher pre-race plasma potassium [K+] values than the reference ranges. Post-race plasma [Na+], plasma osmolality, urine osmolality and urine specific gravity remained stable (p > 0.05). The estimated fluid intake did not differ (p > 0.05) between women (0.30 ± 0.06 L/h) and men (0.46 ± 0.21 L/h). Runners with a higher number of completed ultra-marathons (r = -0.50, p = 0.024) and higher number of training kilometers (r = -0.68, p = 0.001) drank less than those with lower running experience. Pre-race and post-race plasma [Na+] were related to plasma osmolality (r = 0.65, p = 0.002, r = 0.69, p < 0.001, respectively) post-race, but not to fluid intake (p > 0.05). BM significantly decreased post-race (p = 0.002) and was not related to plasma [Na+] or fluid intake (p > 0.05). Post-race hematocrit and plasma [K+] decreased (p < 0.001) and transtubular potassium gradient increased (p = 0.008). Higher pre-race plasma [K+] was related to higher plasma [K+] loss post-race (p = -0.85, p < 0.001). Conclusion: Hydration status remained stable despite the extremely cold winter weather conditions. Overall fluid intake was probably sufficient to replenish the hydration needs of 24-h runners. Current recommendations may be too high for athletes competing in extremely cold conditions.

Centre of Sports Activities Brno University of Technology Brno Czechia

Exercise Physiology Laboratory Nikaia Greece

Faculty of Mechanical Engineering Brno University of Technology Brno Czechia

Institute of Primary Care University of Zurich Zurich Switzerland

Medbase St Gallen Am Vadianplatz St Gallen Switzerland

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