Sweat sodium concentration during exercise in the heat in aerobically trained and untrained humans

The purpose of this study was to determine whether sweat sodium concentration ([Na⁺]_sweat) during exercise in the heat differs between aerobically trained and untrained individuals. On three occasions, ten endurance-trained (Tr) and ten untrained (UTr) subjects (\( \dot{V}{\text{O}}_{{2{\text{peak}}}} \) = 4.0 ± 0.8 vs. 3.4 ± 0.7 L min⁻¹, respectively; P < 0.05) cycled in a hot-ventilated environment (36 ± 1°C; 25 ± 2% humidity, airflow 2.5 m s⁻¹) at three workloads (i.e., 40, 60, and 80% \( \dot{V}{\text{O}}_{{2{\text{peak}}}} \)). Whole-body (SR_WB) and back sweat rates (SR_BACK) were measured. At the conclusion of the study, Na⁺ in sweat and blood samples was analyzed to calculate Na⁺ secretion and reabsorption rates. SR_WB and SR_BACK were highly correlated in Tr and UTr (r = 0.74 and 0.79, respectively; P < 0.0001). In both groups, SR_BACK increased with the increases in exercise intensity (P < 0.05). Likewise, [Na⁺]_sweat increased with the exercise intensity in both groups (P < 0.05) and it tended to be higher in Tr than in UTr at 60 and 80% \( \dot{V}{\text{O}}_{{2{\text{peak}}}} \) (~22 mmol L⁻¹ higher; P = 0.06). However, when normalized for SR_BACK, [Na⁺]_sweat was not different between groups. In both groups, Na⁺ secretion and reabsorption rates increased with the increases in SR_BACK (P < 0.05). However, Na⁺ reabsorption rate was lower in the Tr than in the UTr (mean slope = 48 vs. 82 ηmol cm⁻² min⁻¹; P = 0.03). In conclusion, using a cross-sectional study design, our data suggest that aerobic fitness level does not reduce sweat Na⁺ secretion or enhance Na⁺ reabsorption during prolonged exercise in the heat that induced high sweat rates.