Sweat Na+/K+ ratio as a practical biological index for estimating sweating rate and salt loss rate

Objectives: A simple method to clinically evaluate the sweating rate and the salt loss rate via sweating will enable the early detection of heat-related illness in workers. We measured K+ concentration in sweat and calculated Na+/K+ ratio to evaluate its utility for estimating sweating rate and salt loss rate. We hypothesized that compared with single electrolyte concentrations, Na+/K+ ratio would be less affected by the rapid evaporation of sweat. Methods: Thirteen healthy males exercised on a bicycle ergometer in an artificial climate room, and their sweating rates and Na+ and K+ concentrations in sweat were measured. We also compared sealed and open methods of sweat collection, which respectively prevent or permit evaporation of sweat from the collection area. Results: When sweating rate increased, Na+ concentration increased, and K+ concentration decreased. Na+ concentration and Na+/K+ ratio was significantly lower among habitual exercisers. Sweating rate was more strongly associated with Na+/K+ ratio than with Na+ concentration. Similarly, the calculated NaCl loss rate was more strongly correlated with Na+/K+ ratio than with Na+ concentration. The open sweat collection method reliably demonstrated individual variation in Na+/K+ ratio. Conclusions: Na+/K+ ratio is more accurate than Na+ concentration for predicting sweating rate and the calculated NaCl loss rate because it accounts for individual sweat variation. Na+/K+ ratio is not influenced by sweat evaporation, and samples can be practically obtained by briefly applying absorbent paper to the skin surface.