Can Changes in Body Mass and Total Body Water Accurately Predict Hyponatremia After a 161-km Running Race?

OBJECTIVE: To relate changes in body mass, total body water (TBW), extracellular fluid (ECF), and serum sodium concentration ([Na]) from a 161-km ultramarathon to finish time and incidence of hyponatremia. DESIGN: Observational. SETTING: : The 2008 Rio Del Lago 100-Mile (161-km) Endurance Run in Granite Bay, California. PARTICIPANTS: Forty-five runners. MAIN OUTCOME MEASUREMENTS: Pre-race and post-race body mass, TBW, ECF, and serum [Na]. RESULTS: Body mass and serum [Na] significantly decreased 2% to 3% (P < 0.001) from pre-race to post-race, but TBW and ECF were unchanged. Significant relationships were observed between finish time and percentage change in body mass (r = 0.36; P = 0.01), TBW (r = 0.50; P = 0.007), and ECF (r = 0.61; P = 0.003). No associations were found between post-race serum [Na] and percentage change in body mass (r = -0.04; P = 0.94) or finish time (r = 0.5; P = 0.77). Hyponatremia (serum [Na] < 135 mmol/L) was present among 51.2% of finishers. Logistic regression prediction equation including pre-race TBW and percentage changes in TBW and ECF had an 87.5% concordance with the classification of hyponatremia. CONCLUSIONS: Hyponatremia occurred in over half of the 161-km ultramarathon finishers but was not predicted by change in body mass. The combination of pre-race TBW and percentage changes in TBW and ECF explained 87.5% of the variation in the incidence of hyponatremia. CLINICAL SIGNIFICANCE: Exercise-associated hyponatremia can occur simultaneously with dehydration and cannot be predicted by weight checks at races.