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

Objectives A simple method to evaluate clinically the sweating rate and the salt loss rate via sweating would enable the early detection of heat-related illness in workers. We measured K+ concentration in sweat and calculated the Na+/K+ ratio to evaluate its utility for estimating sweating rate and salt loss rate. We hypothesized that compared with single electrolyte concentrations, the 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 the Na+/K+ ratio were significantly lower among habitual exercisers. Sweating rate was more strongly associated with the Na+/K+ ratio than with Na+ concentration. Similarly, the calculated NaCl loss rate was more strongly correlated with the Na+/K+ ratio than with Na+ concentration. The open sweat collection method reliably demonstrated individual variation in the Na+/K+ ratio. Conclusions The 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. The Na+/K+ ratio is not influenced by sweat evaporation, and samples can be practically obtained by briefly applying absorbent paper to the skin surface.