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.