Sweat sodium loss influences serum sodium concentration in a marathon.

Abstract

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; <30mmol/L of sweat Na+ concentration), typical sweaters (n=20; ≥30 and <60mmol/L of sweat Na+ concentration), and salty sweaters (n=10; ≥60mmol/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±0.21L/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±16.6mmol), and fluid intake (1.3±0.8, 1.2±0.8, 1.2±0.6L) during the race. However, salty sweaters presented lower post-race serum Na+ concentration (140.8±1.3 vs 142.5±1.1, 142.4±1.4mmol/L; P<0.01) and serum osmolality (297±6 vs 299±5, 301±6mOsm/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.