Sweat electrolytes: Influence of environment, sex and exercise intensity.
Mindy Millard-Stafford FACSM, Michael L. Jones, Teresa K. Snow, and Nicholas W. Shea. School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA
Sweat rate and electrolyte loss are highly variable among individuals; but sources of intra-individual variability due to test conditions remain to be quantified. PURPOSE: To determine the impact of exercise intensity and environment on sweat electrolyte losses in men and women. METHODS: Twenty adult women and men completed two sessions during summer months: 3 x 20 min intermittent cycling beginning at low intensity (50/75 Watts) with 25 W increases in work rate under hot-humid (35^oC, 60%RH) or hot-dry (35^oC, 20%RH) conditions. Whole body sweat rate, regional sweat [Na+] and [K+] were obtained at each work rate. Sweat was acquired via Opsite (Brisson method) on the scapula. Electrolytes were measured using Horiba LAQUAtwin Ion meters. RESULTS: Sweat rate and [Na+] was significantly higher (p<0.001) due to increased RH in the heat and incremental changes in exercise intensity. Compared to low exercise intensity, sweat [Na+] increased by 26 (72%) and 39 mmol (108%) with successive 25W increases, similar in relative magnitude to sweat rate increases of 0.4 l/min (67%) and 0.6 l/min (100%) compared to low exercise intensity. However, [Na+] difference due to greater %RH of environment (60% vs. 20%RH) was only 7.7 mmol (14%) higher for all bouts combined. Sweat [K+] was not different (p=0.4) based on environment, but significantly higher (p=0.003) under low intensity exercise compared to higher work rates (6.9±1.9 > 6.0±1.4 mmol). When work rate was matched (75W) under humid conditions, no differences between men and women were observed in sweat rate (0.8 ± 0.3 l/min) or sweat sodium (49.2 ± 17.2 vs. 52.7 ± 30.0 mmol, respectively). CONCLUSION: At matched low intensity exercise, sex differences in sweat sodium and rate were not observed. Intra-individual variability in sweat sodium is influenced more by modest incremental changes (25 W) in exercise intensity than the ambient humidity in hot conditions. Sweat testing of athletes using field techniques should carefully consider the intensity of the training session to accurately translate results.
Supported by a grant from The Coca-Cola Company