Exercise intensity effects on total sweat electrolyte losses and regional vs. whole-body sweat [Na⁺], [Cl^-], and [K⁺]

Purpose: To quantify total sweat electrolyte losses at two relative exercise intensities and determine the effect of workload on the relation between regional (REG) and whole body (WB) sweat electrolyte concentrations.

Methods: Eleven recreational athletes (7 men, 4 women; 71.5 ± 8.4 kg) completed two randomized trials cycling (30 °C, 44% rh) for 90 min at 45% (LOW) and 65% (MOD) of VO_2max in a plastic isolation chamber to determine WB sweat [Na⁺] and [Cl^-] using the washdown technique. REG sweat [Na⁺] and [Cl^-] were measured at 11 REG sites using absorbent patches. Total sweat electrolyte losses were the product of WB sweat loss (WBSL) and WB sweat electrolyte concentrations.

Results: WBSL (0.86 ± 0.15 vs. 1.27 ± 0.24 L), WB sweat [Na⁺] (32.6 ± 14.3 vs. 52.7 ± 14.6 mmol/L), WB sweat [Cl^-] (29.8 ± 13.6 vs. 52.5 ± 15.6 mmol/L), total sweat Na⁺ loss (659 ± 340 vs. 1565 ± 590 mg), and total sweat Cl^- loss (931 ± 494 vs. 2378 ± 853 mg) increased significantly (p < 0.05) from LOW to MOD. REG sweat [Na⁺] and [Cl^-] increased from LOW to MOD at all sites except thigh and calf. Intensity had a significant effect on the regression model predicting WB from REG at the ventral wrist, lower back, thigh, and calf for sweat [Na⁺] and [Cl^-].

Conclusion: Total sweat Na⁺ and Cl^- losses increased by ~ 150% with increased exercise intensity. Regression equations can be used to predict WB sweat [Na⁺] and [Cl^-] from some REG sites (e.g., dorsal forearm) irrespective of intensity (between 45 and 65% VO_2max), but other sites (especially ventral wrist, lower back, thigh, and calf) require separate prediction equations accounting for workload.