The Sweating and Core Temperature Response to Compensable and Uncompensable Heat Stress Following Heat Acclimation

Abstract

While a greater maximal sweat rate following heat acclimation has been suggested to mitigate the rise in core temperature during uncompensable heat stress, it remains unclear whether heat acclimation will alter the core temperature response to compensable heat stress wherein heat balance is attainable. Thus, the primary aim of the present study was to evaluate the influence of complete heat acclimation on sweating and core temperature response to exercise in a compensable and uncompensable environment. A total of 8 (6 males, 2 females) unacclimated individuals were recruited. On separate days, participants exercised at a fixed rate of heat production (450 W) for 45‐mins of compensable heat stress (CHS; 37°C, 30% RH) and 60‐min of uncompressible heat stress (UCHS; 37°C, 60% RH) before and after heat acclimation. Core temperature [esophageal (Tes) and rectal (Tre)], local sweat rate of the arm (LSRarm) and back (LSRback), and whole body sweat loss (WBSL) were measured during experimental trials. Heat acclimation included an 8‐wk aerobic training intervention followed by 10 consecutive days of up to 90 minutes of treadmill walking in hot and humid conditions (38°C, 65% RH). Prior to CHS or UCHS, resting absolute Tes and Tre were lower following heat acclimation (Tes: 36.6±0.2°C; Tre: 36.8±0.1°C) relative to unacclimated (Tes: 36.9±0.2°C, P=0.002; Tre: 37.1±0.2°C, P=0.003). During CHS, the change in Tes and Tre were similar following heat acclimation (ΔTes: 0.4±0.1°C; ΔTre: 0.7±0.1°C) compared to unacclimated (ΔTes: 0.4±0.2°C, P=0.72; ΔTre: 0.7±0.1°C, P=0.61). Cumulative WBSL during CHS was marginally greater with acclimation (557±40 g) relative to unacclimated (494±59 g, P=0.01). Despite no difference in LSRback following 45‐min of CHS with or without heat acclimation (P=0.94), stead‐state LSRarm was slightly higher with acclimation (0.75±0.16 mg/cm2/min) compared to unacclimated (0.61±0.15 mg/cm2/min, P<0.001). In contrast, the change in Tes and Tre during UCHS was significantly smaller following heat acclimation (ΔTes: 0.7±0.2°C; ΔTre: 0.9±0.2°C) relative to an unacclimated state (ΔTes: 1.1±0.3°C, P=0.04; ΔTre: 1.1±0.2°C, P<0.001). WBSL was far greater during UCHS post‐heat acclimation (913±126 g) in comparison to prior to acclimation (671±83 g, P<0.001). Further, end‐exercise LSRback and LSRarm were higher post‐heat acclimation (LSRback: 1.48±0.28 mg/cm2/min; LSRarm: 1.20±0.33 mg/cm2/min) relative to pre‐acclimation (LSRback: 1.21±0.26, P<0.001; LSRarm: 0.91±0.26 mg/cm2/min, P<0.001) during UCHS. Taken together, the core temperature response to compensable heat stress is similar irrespective of acclimation status, despite marginal greater whole‐body sweat rates. However, the large increases local and whole body sweat rate following heat acclimation clearly mitigated the rise in core temperature during uncompensable heat stress.Support or Funding InformationThis research was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada (#386143‐2010, held by O.J.). N.R. was supported by an NSERC Postgraduate Scholarship‐Doctoral and a University of Ottawa Excellence Scholarship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.