Interindividual variability in sweat electrolyte concentration in marathoners

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.

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.

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.

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.