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Exercise-Associated Hyponatremia and Hydration Status in 161-km Ultramarathoners


Medicine & Science in Sports & Exercise: April 2013 - Volume 45 - Issue 4 - p 784–791
doi: 10.1249/MSS.0b013e31827985a8
Applied Sciences

Purpose This work combines and reanalyzes 5 yr of exercise-associated hyponatremia (EAH) research at 161-km ultramarathons in northern California with primary purposes to define the relationship between postrace blood sodium concentration ([Na+]) and change in body weight; to examine the interactions among EAH incidence, ambient temperature, and hydration state; and to explore the effect of hydration status on performance.

Methods Prerace and postrace body weight and finish time data were obtained on 887 finishers, and postrace [Na+] was also obtained on a subset of 669 finishers.

Results EAH incidence was 15.1% overall (range, 4.6%–51.0% by year) and had a significant positive relationship with ambient temperature. Of the runners with EAH, 23.8% were classified as overhydrated (weight change, ≥0), 40.6% were euhydrated (weight change, <0% to –3%), and 35.6% were dehydrated (weight change, <–3%) at the finish. There was a weak significant relationship (r = 0.17, P < 0.0001) between postrace [Na+] and change in body weight such that a lower [Na+] was more common with increased weight loss. Considering all finishers examined, 18.5% were dehydrated and 34.9% were overhydrated at the finish. There was a weak significant relationship (r = 0.092, P = 0.006) between change in body weight and performance in that faster runners tended to lose more weight. Top finishers varied in body weight change from ∼1% gain to ∼6% loss.

Conclusions EAH incidence can be high in 161-km ultramarathons in northern California. In this environment, EAH is more common with dehydration than overhydration and is more common in hotter ambient temperature conditions. Because weight loss >3% does not seem to have an adverse effect on performance, excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst are ill advised.

1Department of Veterans Affairs, Northern California Health Care System, Mather, CA; 2Department of Physical Medicine and Rehabilitation, University of California Davis Medical Center, Sacramento, CA; 3Exercise Science Program, Oakland University, Rochester, MI; and 4Health Sciences Department, Gettysburg College, Gettysburg, PA

Address for correspondence: Martin D. Hoffman, M.D., Department of Physical Medicine and Rehabilitation, Sacramento VA Medical Center, 10535 Hospital Way, Sacramento, CA 95655-1200; E-mail:

Submitted for publication September 2012.

Accepted for publication October 2012.

©2013The American College of Sports Medicine