A comparison of thermoregulatory responses to exercise between mass-matched groups with large differences in body fat

Abstract

We sought to determine 1) the influence of adiposity on thermoregulatory responses independently of the confounding biophysical factors of body mass and metabolic heat production (Hprod); and 2) whether differences in adiposity should be accounted for by prescribing an exercise intensity eliciting a fixed Hprod per kilogram of lean body mass (LBM). Nine low (LO-BF) and nine high (HI-BF) body fat males matched in pairs for total body mass (TBM; LO-BF: 88.7 ± 8.4 kg, HI-BF: 90.1 ± 7.9 kg; P = 0.72), but with distinctly different percentage body fat (%BF; LO-BF: 10.8 ± 3.6%; HI-BF: 32.0 ± 5.6%; P < 0.001), cycled for 60 min at 28.1 ± 0.2 °C, 26 ± 8% relative humidity (RH), at a target Hprod of 1) 550 W (FHP trial) and 2) 7.5 W/kg LBM (LBM trial). Changes in rectal temperature (ΔTre) and local sweat rate (LSR) were measured continuously while whole body sweat loss (WBSL) and net heat loss (Hloss) were estimated over 60 min. In the FHP trial, ΔTre (LO-BF: 0.66 ± 0.21 °C, HI-BF: 0.87 ± 0.18 °C; P = 0.02) was greater in HI-BF, whereas mean LSR (LO-BF 0.52 ± 0.19, HI-BF 0.43 ± 0.15 mg·cm(-2)·min(-1); P = 0.19), WBSL (LO-BF 586 ± 82 ml, HI-BF 559 ± 75 ml; P = 0.47) and Hloss (LO-BF 1,867 ± 208 kJ, HI-BF 1,826 ± 224 kJ; P = 0.69) were all similar. In the LBM trial, ΔTre (LO-BF 0.82 ± 0.18 °C, HI-BF 0.54 ± 0.19 °C; P < 0.001), mean LSR (LO-BF 0.59 ± 0.20, HI-BF 0.38 ± 0.12 mg·cm(-2)·min(-1); P = 0.04), WBSL (LO-BF 580 ± 106 ml, HI-BF 381 ± 68 ml; P < 0.001), and Hloss (LO-BF 1,884 ± 277 kJ, HI-BF 1,341 ± 184 kJ; P < 0.001) were all greater at end-exercise in LO-BF. In conclusion, high %BF individuals demonstrate a greater ΔTre independently of differences in mass and Hprod, possibly due to a lower mean specific heat capacity or impaired sudomotor control. However, thermoregulatory responses of groups with different adiposity levels should not be compared using a fixed Hprod in watts per kilogram lean body mass.

Screening criteria for increased susceptibility to heat stress during work or leisure in hot environments in healthy individuals aged 31–70 years

“…Although our measurements refer to acute heat stress, it is interesting to note that this mirrors epidemiological data for morbidity and mortality during extreme heat events. 42,43 The array of indicators [age, sex, anthropometry (i.e., body composition), and cardiorespiratory fitness] used in our study have been very well studied and rigorously identified as risk factors for SHS 5,[15][16][17][18] as well as morbidity and mortality during extreme heat events. 6,42,43,50,51 In this study, we used these same risk factors in a more controlled environment and showed, for the first time, that they can be used to reliably detect SHS.…”

“…6 To date, while some indices do consider human factors (e.g., activity level, clothing, posture), 5 this aspect has received much less attention. More importantly, additional intrinsic factors for heat injury prevention that are known to increase an individual's susceptibility to heat stress (SHS)such as sex, 15 age, 16 body composition, 17 and cardiorespiratory fitness 18 have not been considered. The aim of the present study was to define and validate sex-specific screening criteria for SHS during work and leisure activities in hot environments in individuals aged 31-70 years using age, anthropometry, and cardiorespiratory fitness.…”

Increasing age is a major risk factor for susceptibility to heat stress during physical activity

“…Moreover, the effects of age occurred independently from other factors such as body morphology and cardiorespiratory fitness. While body morphology has been shown to directly impact heat dissipation (Dervis et al 2016), it must be considered that the authors tested 2 extremes of body fat percentage (11% vs 32% body fat). In fact, our findings are consistent with their results such that we only found a difference in WBHL between our groups with the lowest (<20%) and highest (>30%) body fat percentage.…”

“…To date, research has demonstrated several factors -primarily age, cardiorespiratory fitness, and body fat -to influence wholebody heat loss (WBHL) and therefore alter susceptibility to heat stress in otherwise healthy adults (Larose et al 2013b;Stapleton et al 2015;Dervis et al 2016). Specifically, recent reports depict impairments in WBHL in physically active adults as young as 40 years of age (Larose et al 2013a(Larose et al , 2013b, although these studies isolated aging from other factors (i.e., participants were matched for body morphology and fitness).…”

“…Cardiorespiratory fitness has yielded contradictory evidence in smaller studies (Jay 2014); however, it was found to have a critical role in attenuating the agerelated decrements in WBHL (Stapleton et al 2015). Furthermore, increased body fat has been shown to independently diminish heat dissipation (Dervis et al 2016). The primary limitation of the aforementioned studies is the small sample and/or the nature in which the participants were specifically selected (i.e., to match physical characteristics and fitness).…”