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 racedistance with increased availability of fluid stations combined with a slower pace may increase the risk of developing exerciseinduced hyponatraemia