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

Scotney, Bianca MBBS*; Reid, Steve MBChB

Clinical Journal of Sport Medicine: July 2015 - Volume 25 - Issue 4 - p 341–346
doi: 10.1097/JSM.0000000000000131
Original Research

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.

*Olympic Park Sports Medicine Centre, Melbourne, Australia; and

The Sports Medicine Practice, Hobart, Australia.

Corresponding Author: Bianca Scotney, MBBS, 73 Edinburgh St, Richmond VIC 3121, Australia (biancascotney@bigpond.com).

The authors report no conflicts of interest.

Received December 03, 2013

Accepted May 25, 2014

Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.