Effect Of Hydration Status On Acute Changes In Sweat Electrolyte And Cortisol Concentrations During Exercise

PURPOSE: Previous work has shown changes in sweat biomarkers occur with dehydration (DEH). However, few studies have directly compared sweat biomarkers in DEH versus euhydrated (EUH) states. We sought to determine the effect of hydration status on changes in sweat electrolytes and cortisol concentrations during exercise-heat stress. METHODS: Fifteen subjects (6 female, 9 male; 29 ± 9 y; 71 ± 14 kg) completed 90 min of cycling (81% HRmax) in the heat (~33 °C, 42% relative humidity) with (EUH) or without fluid replacement (DEH). Sweat was collected from the forehead (FH), right scapula (SCAP), left (LVFA) and right (RVFA) ventral forearms using absorbent pads at the beginning (0-30 min) and end of exercise (60-90 min). Sweat was analyzed for [Na+], [Cl-], and [K+] using ion chromatography. Due to low sample volume, cortisol concentration was only measured in SCAP samples at both time points by an immunoassay kit. Paired t-tests or Wilcoxon signed-rank tests were used to determine the effect of hydration status on the change in sweat electrolyte concentrations from the beginning to end of exercise. Data are reported as mean ± SD or median ± IQR. RESULTS: DEH led to a 2.4 ± 0.4% body mass loss while EUH resulted in minimal change in body mass (0.1 ± 0.1%). At each site, there were no differences between EUH and DEH in the change in sweat [Na+] (FH: 24.3 ± 21.5 vs. 30.8 ± 22.4 mmol/L, p = 0.07; SCAP: 9.7 ± 6.2 vs. 9.6 ± 8.2 mmol/L, p = 0.95; LVFA: 7.5 ± 6.0 vs. 5.6 ± 5.9 mmol/L, p = 0.29; RVFA: 8.2 ± 8.6 vs. 7.8 ± 5.2 mmol/L, p = 0.84), [Cl-] (FH: 27.7 ± 22.7 vs. 32.8 ± 22.5 mmol/L, p = 0.14; SCAP: 15.9 ± 12.9 vs. 13.8 ± 13.1 mmol/L, p = 0.31; RVFA: 15.3 ± 9.6 vs. 14.0 ± 6.7 mmol/L, p = 0.36; LVFA: 14.9 ± 9.7 vs. 14.0 ± 5.7 mmol/L, p = 0.62), or [K+] (FH: -0.4 ± 1.8 vs -0.2 ± 1.5 mmol/L, p = 0.48; SCAP: -0.7 ± 0.5 vs. -0.8 ± 0.4 mmol/L, p = 0.74; LVFA: -1.0 ± 2.4 vs. -1.1 ± 2.7 mmol/L, p = 0.99; RVFA: -0.7 ± 1.2 vs. -0.7 ± 3.0 mmol/L, p = 0.26). Likewise, there was no difference in the change in sweat cortisol concentration between EUH and DEH (-1.0 ± 1.6 vs. -0.1 ± 0.9 ng/mL, p = 0.16). CONCLUSIONS: While most subjects had an increase in sweat [Na+] and [Cl-] and a decrease in [K+] and cortisol by the end of exercise, these changes did not differ between the EUH and DEH states. These findings suggest sweat electrolyte and cortisol concentrations may not be a good indicator of hydration status. Supported by: This study was funded by PepsiCo, Inc. The views expressed in this abstract are those of the authors and do not necessarily reflect the position or policy of PepsiCo, Inc.