Body Weight, Serum Sodium Levels, and Renal Function in an Ultra-Distance Mountain Run.

OBJECTIVE: To determine body weight and serum [Na] changes in runners completing an 85-km mountain run, particularly with reference to their "in-race" hydration protocols. DESIGN: Prospective observational cohort study. SETTING: Cradle Mountain Run, Tasmania, Australia, February 2011. PARTICIPANTS: Forty-four runners (86% of starters) prospectively enrolled, with 41 runners (80% of starters) eligible for inclusion in final data set. MAIN OUTCOME MEASURES: Body weight change, serum sodium concentration change, and hydration plan (according to thirst vs preplanned fluid consumption). RESULTS: There was 1 case of exercise-associated hyponatremia (EAH) [postrace [Na], 132 mmol/L]. This runner was asymptomatic. There was a strongly significant correlation between the change in serum [Na] and body weight change during the race. There was a significant inverse correlation between serum [Na] and volume of fluid consumed. Change of serum [Na] was not correlated with the proportion of water versus electrolyte drink consumed. Runners drinking to thirst consumed significantly lower average fluid volumes and had higher postrace serum [Na] than those complying with a preplanned hydration protocol (142 mmol/L vs 139 mmol/L). More experienced runners tended to drink to thirst. CONCLUSIONS: There was a 2% incidence of EAH in this study. Serum [Na] change during an 85-km mountain run was inversely correlated with the volume of fluid consumed. The results provide further evidence that EAH is a dilutional hyponatremia caused by excessive consumption of hypotonic fluids. Drinking to thirst represents a safe hydration strategy for runners in a wilderness environment. CLINICAL RELEVANCE: Drinking to thirst during endurance running events should be promoted as a safe hydration practice.