Low prevalence of exercise-associated hyponatremia in male 100 km ultra-marathon runners in Switzerland
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We investigated the prevalence of exercise-associated hyponatremia (EAH) in 145 male ultra-marathoners at the ‘100-km ultra-run’ in Biel, Switzerland. Changes in body mass, urinary specific gravity, haemoglobin, haematocrit, plasma [Na+], and plasma volume were determined. Seven runners (4.8%) developed asymptomatic EAH. Body mass, haematocrit and haemoglobin decreased, plasma [Na+] remained unchanged and plasma volume increased. Δ body mass correlated with both post race plasma [Na+] and Δ plasma [Na+]. Δ plasma volume was associated with post race plasma [Na+]. The athletes consumed 0.65 (0.30) L/h; fluid intake correlated significantly and negatively (r = −0.50, p < 0.0001) to race time. Fluid intake was neither associated with post race plasma [Na+] nor with Δ plasma [Na+], but was related to Δ body mass. To conclude, the prevalence of EAH was low at ~5% in these male 100 km ultra-marathoners. EAH was asymptomatic and would not have been detected without the measurement of plasma [Na+].