Prediction of sweating rate during heat acclimation in men and women

Background: As temperatures rise due to global warming, understanding sweating rate is imperative to preventing dehydration for those who are physically active in the heat. A recently published predictor of sweating rate during indoor exercise uses height, weight, ambient temperature, wind speed, relative humidity, speed, grade, and duration ( sweatratecalculator.com ). However, this calculator does not account for heat acclimation (HA), which can alter sweating rate. Purpose: To evaluate the accuracy in estimated whole body sweating rate (E-WBSR) relative to measured WBSR (M-WBSR) during a 10-day HA in men and women. Methods: 22 healthy adults (15F; 23 ± 5 years, 167.7 ± 9.4 cm, 70.9 ± 8.3 kg) walked indoors for 120 minutes (1.39 m·s -1 , 2% grade, wind speed: 1.34 m·s -1 ) in the heat (40°C, 40% RH) for 10 days to induce HA. Core temperature (T c ) was measured continuously via a rectal suppository. Whole body sweat rate (WBSR) was calculated from pre- and post-exercise nude mass, which accounted for water intake and urine excreted, over the total duration of exercise heat stress. A repeated measures analysis of variance was utilized to evaluate the differences between M-WBSR and E-WBSR values from sweatratecalculator.com . Results: HA was successful evidenced by lower peak T c during exercise on day 10 (HAD10) than on day 1 (HAD1: 38.8 ± 0.5 °C vs HAD10: 38.4 ± 0.4 °C, p = <0.0001). On HAD1, the estimated sweating rate calculation overestimated measured WBSR by 8% (E-WBSR: 706.5 ± 87.6 mL·hr -1 vs M-WBSR: 653.1 ± 151.8 mL·hr -1 , p = 0.038). On HAD2-HAD6 and HAD8, there were no differences between estimated and measured sweat rate. On HAD7, HAD9, and HAD10, estimated WBSR underestimated measured WBSR by 6-9% (HAD7: M-WBSR: 753.0 ± 165.4 mL·hr -1 vs E-WBSR: 710.0 ± 85.5 mL·hr -1 , p = 0.027; HAD9: M-WBSR: 774.3 ± 168.2 mL·hr -1 vs E-WBSR: 701.1 ± 86.7 mL·hr -1 , p = 0.028; and HAD10: M-WBSR: 775.4 ± 125.9 mL·hr -1 vs E-WBSR: 717.0 ± 78.6 mL·hr -1 , p =0.005). Conclusion: Peak T c was lower after 10 days of walking in the heat, suggesting that HA was successful. The online sweating rate calculator significantly overestimated WBSR on the first day of exercise-heat stress and significantly underestimated WBSR after a period of heat acclimation. These findings show that heat acclimation status modifies the accuracy of the online sweating rate calculator. Importantly, more research is needed to fully understand the influence of HA on this sweat rate calculation. Funded by USAMRDC; author views not representative of the Army/ DoD. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.