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The Incidence, Risk Factors, and Clinical Manifestations of Hyponatremia in Marathon Runners

Hew, Tamara D. DPM*; Chorley, Joseph N. MD†; Cianca, John C. MD‡; Divine, Jon G. MD§

Clinical Journal of Sport Medicine:
Original Research

Objective: To report on the incidence, identify the risk factors, and clarify the clinical manifestations of acute hyponatremia in marathon runners.

Design: An observational and retrospective case-controlled series.

Setting: The medical care area of the 2000 Houston Marathon.

Patients: Marathon finishers treated in medical area receiving intravenous fluids (N=55), including a more detailed analysis of 39 runners completing a retrospective questionnaire.

Main Outcome Measures: Vital signs, serum electrolytes, and finish time were analyzed via ANOVA studies between all non-hyponatremic (NH: N=34)) and hyponatremic (H: N=21)) runners. Fluid intake, training variables, NSAID use, and Symptomatology were further analyzed to delineate all significant differences between groups.

Results: There were no significant differences in vital signs, training variables, or NSAID use between H and NH groups, although there was a trend towards the less experienced runners presenting with lower post-race sodium levels. H runners had lower potassium [K] (p=.04), chloride [Cl] (p<.001), and blood urea nitrogen [BUN] (p=.004) levels than NH runners. There was a significant inverse linear relationship between both finish time versus [Na] (r2 =.51) and total amount of fluid ingested versus [Na] (r2=.39). The total cups of water (p=.004), electrolyte/carbohydrate solution (p=.005) and total amount of fluid ingested (p<.001) were significantly higher in H compared to NH runners and the degree of hyponatremia was related in a dose dependant manner. Vomiting was observed more frequently in H than NH runners (p=.03).

Conclusion: 21 runners presented to the medical area of the Houston Marathon with hyponatremia (.31% of entrants). Excessive fluid consumption and longer finishing times were the primary risk factors for developing this condition. Vomiting was the only clinical sign differentiating hyponatremia from other conditions that induce exercise-associated collapse.

Author Information

*Private Practice; †Department of Pediatrics, Baylor College of Medicine; ‡Department of Physical Medicine and Rehabilitation, Baylor College of Medicine; §Memorial-Hermann Sports and Family Medicine, Houston, Texas.

Address correspondence to Tamara D. Hew, DPM, 6624 Fannin, Suite 2450, Houston, Texas 77030, U.S.A. E-mail:

© 2003 Lippincott Williams & Wilkins, Inc.