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Hyponatremia Among Runners in the Zurich Marathon

Mettler, Samuel MSc*; Rusch, Carmen MSc†; Frey, Walter O MD‡; Bestmann, Lukas Dr§; Wenk, Caspar MD*; Colombani, Paolo C MD*

Clinical Journal of Sport Medicine:
doi: 10.1097/JSM.0b013e31817e3515
Original Research

Objective: Hyperhydration and exercise-associated hyponatremia (EAH) are critical issues during endurance events. We studied a cohort of marathon runners to examine EAH's prevalence in a marathon with a short time limit and to investigate underlying mechanisms that may be responsible for its development.

Design: Observational cohort study.

Setting: 2006 Zurich Marathon (cool and rainy weather, time limit of 5 hours).

Participants: 167 marathon runners were recruited the month before the race.

Main Outcome Measures: Body mass, plasma sodium, and osmolality were measured just before the start and immediately after the race. Fluid intake during the race was ascertained by a recall questionnaire.

Results: Five subjects (3 %) developed asymptomatic EAH, and no symptomatic EAH was found. Body mass change during the race correlated similarly with postrace sodium levels (r = −0.72, P < 0.0001) and with sodium change during the race (r = −0.66, P < 0.0001). Postrace sodium levels correlated significantly with sodium change during the race (r = 0.74, P < 0.0001). Fluid intake correlated significantly (r = −0.43, P < 0.0001) with plasma sodium change between the start and finish of the race. Postrace sodium levels and postrace osmolality were significantly correlated (r = 0.68, P < 0.0001).

Conclusion: In this study we observed a relatively low incidence of EAH in subjects running the marathon in around 2.5 to 5 hours and in a cool environment. Plasma sodium change during the race and postrace sodium levels correlated with body mass change. There was also a direct correlation between fluid intake and plasma sodium change during the race.

Author Information

From the *Department of Agricultural and Food Sciences, ETH Zurich; †Institute of Human Movement Sciences, ETH Zurich; ‡move>med Swiss Olympic Medical Center; and §Institute of Clinical Chemistry, University Hospital Zurich, Zurich, Switzerland.

Submitted for publication February 5, 2008; accepted April 13, 2008.

There are no conflicts of interest for any of the authors.

Study data have not been previously published and are not currently submitted for publication.

Reprints: Samuel Mettler, MSc, Department of Agricultural and Food Sciences, ETH Zurich, ETH Zentrum-LFH A2, CH-8092 Zurich, Switzerland (e-mail:

© 2008 Lippincott Williams & Wilkins, Inc.