Objective: To determine the incidence of and risk factors for exercise-associated hyponatremia (EAH) in cyclists completing a long-distance bike ride and to assess whether postexercise serum NT-proBNP concentration (brain natriuretic protein precursor) differed between riders with and without EAH.
Design: Observational study.
Setting: “Around the Bay in a Day” cycle event, October 2010.
Participants: One hundred thirty-nine cyclists prospectively enrolled, with 90 completing 210 or 250 km.
Main Outcome Measures: Body weight change and fluid intake during the event, and postevent serum sodium concentration ([Na+]) and NT-proBNP concentration ([NT-proBNP]).
Results: Four riders (4.5%) were hyponatremic ([Na+] < 135 mmol/L). The lowest postride [Na+] was 126 mmol/L. Hyponatremia was associated with a mean weight gain of 3.4 kg (3.9% of total body weight). Significant negative correlations were found between postride [Na+] and change in weight (r = −0.34; P < 0.01) and fluid intake when expressed as total volume (r = −0.35; P < 0.01), mL/kg body weight (r = 0.33; P < 0.01), mL·kg−1·h−1 (r = −0.27; P < 0.01), or mL/h (r = −0.29; P < 0.01). NT-proBNP concentrations levels in 3 of the 4 hyponatremic subjects were markedly elevated compared with eunatremic subjects matched for age, sex, distance ridden, training, and medical history.
Conclusions: Exercise-associated hyponatremia was found to occur in 4.5% of the study group and was associated with weight gain during a prolonged bike ride. Postride [Na+] varied inversely with weight change and with fluid intake. Three of 4 hyponatremic riders had significant elevations of [NT-proBNP]. These results support the hypothesis that overconsumption of hypotonic fluids in this setting is the most important cause of EAH.
*MP Sports Physicians, Victoria, Australia
†Sports Medicine Practice, Hobart, Tasmania, Australia
‡Chemical Pathology, Melbourne Pathology, Melbourne, Australia
§Centre for Health, Exercise and Sports Medicine, University of Melbourne, Melbourne, Australia
¶Australian Centre for Research into Sports Injury and Its Prevention, Monash University, Clayton, Australia.
Corresponding Author: Gregory Harris, MBBS(Hons), MP Sports Physicians, Suite 1, 20 Clarendon St, Frankston, Victoria 3199, Australia (email@example.com).
G. Harris has received payment from Bicycle Victoria for authorship of magazine articles on topics unrelated to this research.
K. Sikaris is a salaried chemical pathologist with Melbourne Pathology. Melbourne Pathology provided laboratory analysis for this research free of charge.
P. McCrory has received grants, royalties, and travel expenses from bodies unrelated to this current research, both directly and through the Centre for Health, Exercise and Sports Medicine. Neither he nor CHESM have received any payment or support in kind for any aspect of the submitted work.
S. Reid reports no conflicts of interest.
Received September 7, 2011
Accepted March 27, 2012