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To determine the incidence of exercise-associated hyponatremia (EAH), the associated biochemical measurements and risk factors for EAH, and whether there is an association between postrace blood sodium concentration ([Na+]) and changes in body mass among participants in the 2009 Western States Endurance Run, a 161-km mountain trail run.


Change in body mass, postrace [Na+], and blood creatine phosphokinase (CPK) concentration, and selected runner characteristics were evaluated among consenting competitors.


Of the 47 study participants, 14 (30%) had EAH as defined by a postrace [Na+] <135 mmol/L. Postrace [Na+] and percent change in body mass were directly related (r = .30, P = .044), and 50% of those with EAH had body mass losses of 3–6%. EAH was unrelated to age, sex, finish time, or use of nonsteroidal anti-inflammatory drugs during the run, but those with EAH had completed a smaller (P = .03) number of 161-km ultramarathons. The relationship of CPK levels to postrace [Na+] did not reach statistical significance (r = –.25, P = .097).


EAH was common (30%) among finishers of this 161-km ultramarathon and it was not unusual for those with EAH to be dehydrated. As such, changes in body mass should not be relied upon in the assessment for EAH during 161-km ultramarathons.

Martin D. Hoffman is with the Department of Physical Medicine & Rehabilitation, Department of Veterans Affairs, Northern California Health Care System, and the University of California Davis Medical Center, Sacramento, CA. Kristin J. Stuempfle is with the Health Sciences Department, Gettysburg College, Gettysburg, PA. Ian R. Rogers is with the Department of Emergency Medicine, St. John of God Murdoch Hospital and University of Notre Dame, Perth, WA, Australia. Louise B. Weschler is an independent researcher, Colts Neck, NJ. Tamara Hew-Butler is with the Exercise Science Program, Oakland University, Rochester, MI.