Introduction. Endurance exercise poses many challenges to an athlete, with one of the most important being maintenance of proper hydration. Inadequate fluid intake may result in dehydration, whereas overconsumption can cause hyponatremia, a condition of plasma electrolyte dilution that may lead to impaired physiological function. Change in body mass is frequently used to assess an athlete's hydration status during race events, though few studies have examined change in body mass relative to body water and electrolyte levels. Furthermore, a lack of data exists in regard to these variables for ultra-endurance races lasting longer than ∼12.5 hours. Such is the reasoning for conducting the following study.

Purpose. To examine the relationships among electrolyte concentrations, changes in body mass and changes in total body water (TBW) during a 161-km ultramarathon run, and relate these to finish time and incidence of hyponatremia. Methods. Subjects were recruited from the 161-km 2008 Rio Del Lago Endurance Race. Body mass, TBW, and serum electrolyte concentrations were measured approximately 12 hours pre-race and immediately post-race.

Results. Body mass (p < 0.0001) and serum sodium concentration ([Na⁺]) (p < 0.001) decreased significantly from pre- to post-race, but TBW was unchanged. A significant relationship was observed between finish time and percent change in body mass (r=0.36; p=0.01), as well as between finish time and change in TBW (r=0.51; p=0.0003). Change in body mass was strongly related to percent change in TBW (r=0.79; p=0.0001). No association was found between post-race serum [Na⁺] and change in body mass (r=0.03, p=0.83), nor between post-race serum [Nat ⁺] and finish time (r=0.03, p=0.72). Based on post-race serum [Na ⁺ < 134.9 mmol/l, ∼50% of finishers were classified as having either biochemical or clinical hyponatremia.

Conclusions. Post-race serum [Na ⁺] was not a good predictor of race performance, though faster runners tended to lose more body water and body mass than slower runners. TBW did not change, but serum [Na⁺ decreased by 2.9% (p < 0.001); inadequate sodium intake was thus probably the largest contributor of the reduction in serum [Na⁺].