Study: Exercise Induced Hyponatremia More Common Among Dehydrated that Overhydrated Ultramarathoners

Somewhat counter-intuitive finding in the study below - hyponatremia more common in dehydrated than overhydrated runners:

Published on-line in Medicine & Science in Sport & Exercise

Exercise-Associated Hyponatremia and Hydration Status in 161-km Ultramarathoners
Hoffman, Martin D.; Hew-Butler, Tamara; Stuempfle, Kristin J.
Medicine & Science in Sports & Exercise:
POST ACCEPTANCE, 6 November 2012

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

Methods: Pre-race and post-race body weight and finish time data were obtained on 887 finishers, and post-race [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 post-race [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.

Conclusion: 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 appear to have an adverse effect on performance, excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst are ill-advised.

(C)2012The American College of Sports Medicine
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