Impact of race pace on development of hyponatraemia in full- and half-marathoners

Objective. Prior studies of full-marathon participants have demonstrated a higher incidence of hyponatraemia in runners with completion times of 4 hours or more. Our primary aim was to determine if slower pace is associated with increased prevalence of hyponatraemia. Secondly, we evaluated the prevalence of hyponatraemia in full-marathoners v. halfmarathoners. Methods. This observational, cross-sectional study comprised consenting runners in the 26.2 With Donna, The National Marathon to Finish Breast Cancer, in Jacksonville Beach, Florida, February 2008. On race day, participants completed a questionnaire, provided finger-stick blood samples, and were weighed both pre- and post-race. Results. A significant negative association was found between pace and post-race sodium level (p<0.001). A negative correlation was found between finishing time and post-race sodium level (p<0.001). The prevalence of post-race hyponatraemia was 4% (4/106) among half-marathoners and 13% (12/89) among full-marathoners (P=0.02). An inverse correlation was found between sodium change and weight change, significant in fullmarathoners (r=-0.55, p<0.001) but not half-marathoners (r=- 0.23, p=0.042). Conclusions. Slower race pace and longer finishing times were associated with lower post-race sodium levels. Full-marathoners had a significantly higher prevalence of hyponatraemia. The development of hyponatraemia was associated with weight gain. Our data indicate that the relationship between post-race sodium concentration and pace differs according to the distance of the event. We can extrapolate from this data that longer race distance with increased availability of fluid stations combined with a slower pace may increase the risk of developing exerciseinduced hyponatraemia.