Abstract
Objective
Previous work has demonstrated that exercise-associated hyponatremia (EAH) rarely occurs in association with dehydration and that there is a significant negative relationship between post-race serum sodium concentration (Na+) and weight change. In other words, EAH has been found to occur more often when there is weight gain rather than weight loss during the event. This has been in contrast to our observations from 161 km ultramarathon races in northern California. The present work consolidates our findings from studies at three 161 km ultramarathons in northern California.
Methods
Data were compiled from studies performed at the 2008 Rio Del Lago, and the 2009 and 2010 Western States Endurance Run, all 161 km trail footraces in northern California. Consenting runners underwent prerace and postrace measurement of body weight on the same calibrated scale. Sodium concentration was measured on-site immediately postrace using an iSTAT (Abbott, NJ) analyzer.
Results
An EAH incidence of 30% to 51% was observed at the 3 events. Overall, EAH was present in 83 (33.3%) of 249 observations. In each case of EAH, the runner appeared neurologically intact. Of the 223 observations in which prerace and postrace body weights were also obtained, 41 (18.4%) were overhydrated (weight change ≥ 0%), 99 (44.4%) were euhydrated (weight change −3% to 0%), and 83 (37.2%) were dehydrated (weight change < −3%). Biochemical EAH (Na+ 129 to < 135 mmol/L) was present among 65 (29.1%) and “clinically significant” EAH (Na+ < 129 mmol/L) was present among 13 (5.8%). Considering the 78 with EAH, 35 (44.9%) were dehydrated. Of the 13 with “clinically significant” EAH, 4 (30.8%) were dehydrated. Spearman correlation between postrace Na+ and percentage change in body weight was significant (r = 0.14, P = .037).
Conclusions
The ≥ 30% incidence of EAH among participants in 161 km ultramarathons in northern California is among the highest rates reported from any event. Unlike the findings from previous work, EAH was commonly observed in association with dehydration. As such, in this group of ultramarathoners, significant weight loss cannot be used to exclude the presence of EAH.