Sweat sodium loss influences serum sodium concentration in a marathon

The aim of this investigation was to determine the influence of sweat electrolyte concentration on body water and electrolyte homeostasis during a marathon. Fifty‐one runners completed a marathon race in a warm and dry environment (24.4 ± 3.6 °C). Runners were classified as low‐salt sweaters ( n  = 21; <30 mmol/L of sweat Na + concentration), typical sweaters ( n  = 20; ≥30 and <60 mmol/L of sweat Na + concentration), and salty sweaters ( n  = 10; ≥60 mmol/L of sweat Na + concentration). Before and after the race, body mass and a sample of venous blood were obtained. During the race, sweat samples were collected by using sweat patches, and fluid and electrolyte intake were recorded by using self‐reported questionnaires. Low‐salt, typical and salty sweaters presented similar sweat rates (0.93 ± 0.2, 0.92 ± 0.29, 0.99 ± 0.21 L/h, respectively), body mass changes (−3.0 ± 1.0, −3.3 ± 1.0, −3.2 ± 0.8%), total Na + intake (12.7 ± 8.1, 11.5 ± 9.7, 14.5 ± 16.6 mmol), and fluid intake (1.3 ± 0.8, 1.2 ± 0.8, 1.2 ± 0.6 L) during the race. However, salty sweaters presented lower post‐race serum Na + concentration (140.8 ± 1.3 vs 142.5 ± 1.1, 142.4 ± 1.4 mmol/L; P  <   0.01) and serum osmolality (297 ± 6 vs 299 ± 5, 301 ± 6 mOsm/kg; P  <   0.05) than low‐salt and typical sweaters. Sweat electrolyte concentration could influence post‐race serum electrolyte concentration in the marathon. However, even the saltiest sweaters did not develop exercise‐associated hyponatremia or associated symptoms.