Effect of Two Hydration Strategies on CapillarySodium Concentrations in Runners Participating in 170-km Trail Race: The 2015 UltraTrail du Mont-Blanc Experience.

Objective: This study assessed the impact of 2 hydration strategies on capillary sodium concentrations during Ultra-Trail du Mont-Blanc (UTMB) 2015.

Design: Prospective exposed/nonexposed cohort study.

Setting: Ultra-Trail du Mont Blanc 2015 (170 km).

Participants: Thousand five hundred sixty-three registered runners of UTMB 2015 asked to predefine their hydration strategy as either "drinking to thirst" or any other mode ("not drinking to thirst").

Intervention: One hundred "drinking to thirst" participants were randomly selected and paired (sex and age) with 96 "not drinking to thirst" participants. Participant weight and capillary sodium concentrations were measured before and after the race.

Main outcome measures: Variations of capillary sodium concentrations and weight during the trail according to predefine hydration strategy as either "drinking to thirst" or "not drinking to thirst." Adverse events were recorded. P < 0.05 was considered statistically significant.

Results: Among 196 participants, 88 (62 finishers) "drinking to thirst" and 87 (64 finishers) "nondrinking to thirst" participants were analyzed, respectively. There was no difference in the change in capillary sodium concentrations prerace to postrace between the groups (1.5 ± 4.4 vs 1.5 ± 4.7 mEq/L, P = 0.98). The change in participant weight was also not different (P = 0.3877). Hypernatremia or hyponatremia were reported in 19 of 88 (21.6%) and 24 of 87 (27.6%) in "drinking to thirst" and "non-drinking to thirst" participants, respectively (P = 0.20). The incidence of adverse events was not different between the groups.

Conclusion: This study observed no impact of hydration strategy on the change in capillary sodium concentrations before and after UTMB 2015.