The index of maximum sweat ion reabsorption rates of sweat glands does not correlate to whole body sweat sodium concentration in exercising young healthy men

Factors explaining individual variations in whole body sweat sodium ion concentration ([Na⁺]) during exercise are not fully understood. Galvanic skin conductance (GSC) reflects the electrical properties of the skin influenced by sweat rate (SR) and the presence of ions. Initiation of increases in this response to elevating sweating may reflect exceeding the maximal capacity of sweat ion reabsorption in sweat glands. We investigated whether the SR threshold for increasing GSC, an indirect measure of maximum ion reabsorption rates of sweat glands, explains the variations in whole body sweat [Na⁺]. Thirty young healthy males cycled for 90 min at incremental exercise intensities of 30, 45, and 60% peak oxygen uptake (30 min each) in the heat (32°C, 50% relative humidity). Whole body sweat [Na⁺] was measured using a whole body washdown technique. The SR threshold for increasing GSC was determined from the relationship between the local SR (ventilated capsule) and GSC on the forearm and chest. The average whole body sweat [Na⁺] was 42.8 ± 18.9 (range: 14.4-81.0) mmol L^-1, and the SR threshold for increasing GSC was 0.29 ± 0.20 (range: 0.02-0.62) and 0.35 ± 0.30 (range: 0.01-1.40) mg cm^-2 min^-1 for the forearm and chest, respectively. Whole body sweat [Na⁺] was not correlated with the SR threshold for increasing GSC in the forearm or chest (<i>r</i>² ≤ 0.001, <i>P</i> ≥ 0.921). We conclude that the SR threshold for increasing GSC at the forearm and chest does not explain the individual variation in whole body sweat [Na⁺] during exercise in the heat.<b>NEW & NOTEWORTHY</b> Galvanic skin conductance is influenced by sweat rate and sweat ions, and the sweat rate at which this response begins to increase may reflect the exceeding capacity of sweat ion reabsorption in sweat glands. However, we show that this indirect measure of the sweat gland's capacity of ion regulation on the forearm and chest does not correlate with whole body sweat sodium concentration during exercise, excluding its role as a determinant of systemic sweat sodium loss.