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Can Changes in Body Mass and Total Body Water Accurately Predict Hyponatremia After a 161-km Running Race?

Lebus, Daniel K MS*; Casazza, Gretchen A PhD*; Hoffman, Martin D MD†‡; Van Loan, Marta D PhD§

Clinical Journal of Sport Medicine: May 2010 - Volume 20 - Issue 3 - p 193-199
doi: 10.1097/JSM.0b013e3181da53ea
Original Research

Objective: To relate changes in body mass, total body water (TBW), extracellular fluid (ECF), and serum sodium concentration ([Na+]) from a 161-km ultramarathon to finish time and incidence of hyponatremia.

Design: Observational.

Setting: The 2008 Rio Del Lago 100-Mile (161-km) Endurance Run in Granite Bay, California.

Participants: Forty-five runners.

Main Outcome Measurements: Pre-race and post-race body mass, TBW, ECF, and serum [Na+].

Results: Body mass and serum [Na+] significantly decreased 2% to 3% (P < 0.001) from pre-race to post-race, but TBW and ECF were unchanged. Significant relationships were observed between finish time and percentage change in body mass (r = 0.36; P = 0.01), TBW (r = 0.50; P = 0.007), and ECF (r = 0.61; P = 0.003). No associations were found between post-race serum [Na+] and percentage change in body mass (r = −0.04; P = 0.94) or finish time (r = 0.5; P = 0.77). Hyponatremia (serum [Na+] < 135 mmol/L) was present among 51.2% of finishers. Logistic regression prediction equation including pre-race TBW and percentage changes in TBW and ECF had an 87.5% concordance with the classification of hyponatremia.

Conclusions: Hyponatremia occurred in over half of the 161-km ultramarathon finishers but was not predicted by change in body mass. The combination of pre-race TBW and percentage changes in TBW and ECF explained 87.5% of the variation in the incidence of hyponatremia.

Clinical Significance: Exercise-associated hyponatremia can occur simultaneously with dehydration and cannot be predicted by weight checks at races.

From the *Sports Medicine Program; and †Department of Physical Medicine and Rehabilitation, University of California Davis Medical Center, Sacramento, California; ‡VA Northern California Health Care System, Sacramento, California; and §USDA, ARS, Western Human Nutrition Research Center, Davis, California.

Submitted for publication August 27, 2009; accepted January 30, 2010.

Supported by the Western States Endurance Run Foundation.

Technical support and equipment were provided by ImpediMed, Inc, who may benefit from the results presented in this article.

The authors state that they have no financial interest in the products mentioned within this article.

Reprints: Daniel K. Lebus, MS, 7489 Rollingdell Dr, Cupertino, CA 95014 (e-mail:

© 2010 Lippincott Williams & Wilkins, Inc.