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Fluid overload is considered a main risk factor for exercise-associated hyponatremia (EAH). The aim of this study was to investigate the incidence of EAH in ultra-runners at the 100 km ultra-run in Biel, Switzerland.
Pre- and postrace, body mass, urinary specific gravity, hemoglobin, hematocrit, plasma [Na+], and plasma volume were determined.
Of the 145 finishers, seven runners (4.8%) developed asymptomatic EAH. While running, the athletes consumed a total of (median and interquartile ranges) 6.9 (5.1-8.8) L over the 100 km distance, equal to 0.58 (0.41-0.79) L/h. Fluid intake correlated negatively and significantly with race time (r = -.50, P < .0001). Body mass decreased, plasma [Na+] remained unchanged, hematocrit and hemoglobin decreased, and urinary specific gravity increased. Plasma volume increased by 4.6 (-2.3 to 12.8) %. Change in body mass correlated with both postrace plasma [Na+] and Δ plasma [Na+]. Postrace plasma [Na+] correlated to Δ plasma [Na+]. Fluid intake was associated neither with postrace plasma [Na+] nor with Δ plasma [Na+]. Fluid intake was related to Δ body mass (r = .21, P = .012), but not to postrace body mass. Fluid intake showed no correlation to Δ plasma volume. Change in plasma volume was associated with postrace [Na+].
Incidences of EAH in 100 km ultra-marathoners were lower compared with reports on marathoners. Body mass decreased, plasma volume increased, and plasma [Na+] was maintained. Since fluid intake was related neither to Δ plasma volume nor to Δ plasma [Na+], we assume that factors other than fluid intake maintained body fluid homeostasis.
Beat Knechtle is with the Gesundheitszentrum St. Gallen, St. Gallen, Switzerland, and the Institute of General Practice and Health Services Research, University of Zurich, Zurich, Switzerland. Patrizia Knechtle is with the Gesundheitszentrum St. Gallen, St. Gallen, Switzerland. Thomas Rosemann is with the Institute of General Practice and Health Services Research, University of Zurich, Zurich, Switzerland.