Regulation of Electrolyte and Fluid Metabolism in Multi-stage Ultra-Marathoners

The purposes of this study were (i) to determine the prevalence of exercise-associated hyponatremia (EAH) in multi-stage ultra-marathoners and (ii) to gain more insight into fluid and electrolyte regulation during a multi-stage race. Body mass, sodium concentration ([Na+]), potassium concentration ([K+]), creatinine, urea, specific gravity, and osmolality in urine were measured in 25 male ultra-marathoners in the 'Swiss Jura Marathon' 2008 with 11 000 m gain of altitude over 7 stages covering 350 km, before and after each stage. Haemoglobin, haematocrit, creatinine, urea, [Na+], [K+], and osmolality were measured in plasma before stage 1 and after stages 1, 3, 5, and 7. Two athletes (8 %) showed plasma [Na+] < 135 mmol/l. Body mass, plasma [Na+], and plasma [K+] remained unchanged (p > 0.05). Urine specific gravity (p < 0.001) and osmolality in both plasma (p < 0.01) and urine (p < 0.001) were increased and haematocrit (p < 0.0001), haemoglobin (p < 0.0001) and plasma albumin were decreased (p < 0.001). Plasma volume (p < 0.01) and plasma urea (p < 0.001) were increased. The K+/Na+ ratio in urine increased > 1.0 after each stage and returned to < 1.0 the morning of the next stage (p < 0.001). To summarize, more sodium than potassium was excreted during rest. The increased urinary sodium losses during rest are compatible with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) or the cerebral salt-wasting syndrome (CSWS). Further studies are needed to determine the antidiuretic hormone (ADH) and both the atrial natriuretic peptide (ANP) and the brain natriuretic peptide (BNP) during multi-stage races.