Interindividual variability in sweat electrolyte concentration in marathoners

Background: sodium (na+) intake during exercise aims to replace the na+ lost by sweat to avoid electrolyte imbalances,especially in endurance disciplines. however,na+ needs can be very different among individuals because of the great inter-individual variability in sweat electrolyte concentration. the aim of this investigation was to determine sweat electrolyte concentration in a large group of marathoners. methods: a total of 157 experienced runners (141 men and 16 women) completed a marathon race (24.4 ± 3.6 °c and 27.7 ± 4.8 % of humidity). during the race,sweat samples were collected by using sweat patches placed on the runners' forearms. sweat electrolyte concentration was measured by using photoelectric flame photometry. results: as a group,sweat na+ concentration was 42.9 ± 18.7 mmol l-1 (minimal-maximal value = 7.0-95.5 mmol l-1),sweat cl- concentration was 32.2 ± 15.6 mmol l-1 (7.3-90.6 mmol l-1) and sweat k+ concentration was 6.0 ± 0.9 mmol l-1 (3.1-8.0 mmol l-1). women presented lower sweat na+ (33.9 ± 12.1 vs 44.0 ± 19.1 mmol l-1; p = 0.04) and sweat cl- concentrations (22.9 ± 10.5 vs 33.2 ± 15.8 mmol l-1; p = 0.01) than men. a 20 % of individuals presented a sweat na+ concentration higher than 60 mmol l-1 while this threshold was not surpassed by any female marathoner. sweat electrolyte concentration did not correlate to sweat rate,age,body characteristics,experience or training. although there was a significant correlation between sweat na+ concentration and running pace (r = 0.18; p = 0.03),this association was weak to interpret that sweat na+ concentration increased with running pace. conclusions: the inter-individual variability in sweat electrolyte concentration was not explained by any individual characteristics except for individual running pace and sex. an important portion (20 %) of marathoners might need special sodium intake recommendations due to their high sweat salt losses. © 2016 the author(s).