Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions

Abstract

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38°C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (Tes), arm (LSRarm), and back (LSRback) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30% RH) and 60-min uncompensable (37°C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post–heat acclimation (POST-HA). For compensable heat stress trials, resting Tes was lower POST-TRN (36.74 ± 0.27°C, P = 0.05) and POST-HA (36.60 ± 0.27°C, P = 0.001) compared with PRE-TRN (36.99 ± 0.19°C); however, ΔTes was similar in all trials (PRE-TRN:0.40 ± 0.23°C; POST-TRN:0.42 ± 0.20°C; POST-HA:0.43 ± 0.12°C, P = 0.97). While LSRback was unaltered by HA ( P = 0.94), end-exercise LSRarm was higher POST-TRN (0.70 ± 0.14 mg·cm−2·min−1, P < 0.001) and POST-HA (0.75 ± 0.16 mg·cm−2·min−1, P < 0.001) compared with PRE-TRN (0.61 ± 0.15 mg·cm−2·min−1). Despite matched evaporative heat balance requirements, steady-state WBSR (31st–45th min) was greater POST-TRN (12.7 ± 1.0 g/min, P = 0.02) and POST-HA (12.9 ± 0.8 g/min, P = 0.004), compared with PRE-TRN (11.7 ± 0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77 ± 0.22°C, P = 0.05) and POST-HA (36.62 ± 0.15°C, P = 0.03) compared with PRE-TRN (36.86 ± 0.24°C). But ΔTes was smaller POST-TRN (0.77 ± 0.19°C, P = 0.05) and POST-HA (0.75 ± 0.15°C, P = 0.04) compared with PRE-TRN (1.10 ± 0.32°C). LSRback and LSRarm increased with HA ( P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 ± 2.4 g/min, P < 0.001; POST-HA:16.8 ± 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 ± 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat. NEW & NOTEWORTHY We demonstrate that neither partial nor complete heat acclimation alters the change in core temperature during compensable heat stress compared with an unacclimated state, despite a marginally greater whole body sweat rate. However, the greater local and whole body sweat rate with partial and complete heat acclimation reduced the rise in core temperature during 60 min of uncompensable heat stress compared with an unacclimated state, suggesting the improvements in heat dissipation associated with heat acclimation are best observed when the upper physiological limits for evaporative heat loss are challenged.

Improved neural control of body temperature following heat acclimation in humans

“…2016; Ravanelli et al . 2019), although this is not a consistent finding (Nadel et al . 1974; Roberts et al .…”

“…1997; Ravanelli et al . 2019). These divergent observations could be related to the characteristics of the heat acclimation protocols employed (duration, passive vs .…”

“…Taken together, these observations suggest that greater thermoeffector output following short-term, controlled hyperthermia heat acclimation is attributable to an earlier activation, but not gain, of the thermoeffector loops. It should be noted that some studies have observed a greater thermosensitivity of heat loss thermoeffectors following heat acclimation (Wyndham, 1967;Takeno et al 2001;Patterson et al 2004;Poirier et al 2016;Ravanelli et al 2019), although this is not a consistent finding (Nadel et al 1974;Roberts et al 1977;Hessemer et al 1986;Armstrong & Kenney, 1993;Cotter et al 1997;Ravanelli et al 2019). These divergent observations could be related to the characteristics of the heat acclimation protocols employed (duration, passive vs. active, controlled hyperthermia vs. traditional) and/or the nature of the pre/post acclimation visits (passive heating vs. exercise, compensable vs. uncompensable).…”

A retrospective analysis to determine if exercise training‐induced thermoregulatory adaptations are mediated by increased fitness or heat acclimation

“…WhileV O 2 max does not alter the thermoregulatory responses during exercise in compensable conditions (Jay, Bain, Deren, Sacheli, & Cramer, 2011), it is difficult to separate the influence of a higheṙ V O 2 max vs. repeated thermal stress on training-induced improvements in thermoregulatory capacity during uncompensable heat stress. For example, we recently demonstrated that 8 weeks of aerobic training increases maximum skin wettedness (Ravanelli, Coombs, Imbeault, & Jay, 2018), and increases local and whole-body sweat rates and reduces the change in core temperature by ∼0.2 • C during 60 min of uncompensable heat stress (Ravanelli, Coombs, Imbeault, & Jay, 2019).…”

Methods for improving thermal tolerance in military personnel prior to deployment

“…Despite inconsistencies around changes in skin blood flow, mechanistic studies have found HA to increase skin blood flow at a given core temperature in fixedintensity trials, helping increase the rate of heat dissipation [29,125,[134][135][136]. When wearing military clothing any HA-mediated increase in skin blood flow will likely be less effective in promoting heat loss than current research would assume but may still contribute to total heat loss and slow the rise in core temperature.…”