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 degrees 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.