Tamara Hew-Butler coauthors study on exercise-associated hyponatremia

Exercise-associated hyponatremia (EAH), defined as a blood sodium concentration ([Na+]) < 135 mmol L^-1 during or within 24 h after prolonged physical activity, seemed to be an uncommon issue when first described in the scientific literature in 1985. At least nine deaths from EAH have subsequently confirmed the potential seriousness of this condition, and EAH incidences have been reported to be as high as 18% in an Ironman triathlon, 28% in a standard 42-km marathon, 50% in an endurance cycling event, and 30%–51% in 161-km ultramarathon runs.

The Western States Endurance Run (WSER) is the premier 161-km ultramarathon traversing mountain trails. Participants travel from Squaw Valley, California, through the Sierra Nevada to Auburn, California. Although a case report of EAH from the sixth official running of the event in 1982 published in a non–peer-reviewed magazine represented one of the first reports of EAH, there were no formal studies of EAH at northern California ultramarathons until decades later. A stimulus for such work came in 2006, when spectators at the finish of the WSER watched the first runner collapse multiple times in the final 300 m. This runner had reportedly gained 2 kg during the run and was found to be hyponatremic via on-site analysis at the finish. Fortunately, he survived, but the assistance he received in crossing the finish line was grounds for disqualification.

In the initial study of EAH at northern California 161-km ultramarathons, the incidence of EAH was found to be 51%. That study also noted that more than half of the hyponatremic finishers had lost 93% of their body weight, which was in contrast to previous studies from shorter duration competitions in which EAH was relatively rare in subjects who had lost weight. Our research for the subsequent 4 yr demonstrated some interesting differences compared with findings previously reported from investigations at other events including a positive, rather than inverse, relationship between postrace [Na+] and change in body weight.

The present work combines and reanalyzes our data from 5 yr of EAH investigations at northern California 161-km ultramarathons, with the following purposes: 1) to explore the effect of ambient temperature on hydration status at the finish; 2) to gain insight into the level of body weight loss that might impair performance through examining the relationship of change in body weight with performance; 3) to define the relationship of postrace [Na+] with change in body weight across the 5 yr of investigations; 4) to explore the interactions among EAH incidence, ambient temperature, and hydration state; and 5) to examine whether the effect of hydration state on EAH incidence is statistically different for the present work compared with findings previously reported.