Revisiting the influence of individual factors on heat exchange during exercise in dry heat using direct calorimetry

Abstract

New Findings What is the central question of this study?The aim was to identify the greatest contributor(s) to the variation in whole‐body heat exchange, as assessed using direct calorimetry, among young men and women with heterogeneous characteristics during exercise at increasing metabolic heat production rates in dry heat. What is the main finding and its importance?The evaporative heat loss requirement, body morphology and aerobic fitness made the greatest contributions to the individual variation in evaporative and dry heat exchange, with the variance explained being exercise intensity dependent. These findings provide a foundation on which to build our ability to explain the individual variation in heat exchange during exercise‐induced heat stress. Abstract Numerous individual factors (e.g. fitness, sex, body morphology) are known to independently modulate heat exchange during exercise in the heat. However, in our view, the individual factor(s) making the greatest contribution to the variation in heat exchange among men and women remains poorly understood, despite several studies. We therefore sought to revisit this question by assessing whole‐body dry and evaporative heat exchange using direct calorimetry in a heterogeneous sample of 100 young men (n = 57) and women (n = 43). Participants performed three 30 min bouts of cycling at very light (men/women; 300/250 W), light (400/325 W) and moderate (500/400 W) metabolic heat production rates, separated by a 15 min recovery, in dry heat (40°C, ∼12% relative humidity). Positive associations were observed between the evaporative heat loss requirement (metabolic heat production ± dry heat exchange) and evaporative heat loss (all P < 0.01), especially during moderate exercise (men, r = 0.62; women, r = 0.82), which explained 19–67% of individual variation. Peak aerobic power (in millilitres per kilogram per minute) was also positively related to evaporative heat loss in both sexes, albeit only during light and moderate exercise (r = 0.33–0.43; all P < 0.05), explaining a further 5–9% of individual variation. Dry heat exchange shared negative associations with body mass and surface area during all exercise bouts in both sexes (r = −0.29 to −0.55; all P < 0.05), explaining 9–30% of individual variation. We therefore demonstrate that the evaporative heat loss requirement, peak aerobic power and body morphology are the greatest contributors to the variation in whole‐body heat exchange among young men and women exercising in dry heat, with the strength of those relationships being heat‐load dependent.

Individual Responses to Heat Stress: Implications for Hyperthermia and Physical Work Capacity

“…whole body heat storage. Using direct calorimetry, differences in whole body heat storage help to identify inter-individual differences in heat exchange pathways (Larose et al, 2013;Stapleton et al, 2013Stapleton et al, , 2015Carter et al, 2014;Kenny et al, 2015;Poirier et al, 2015;Flouris et al, 2018b;Notley et al, 2019b). Due to reasons previously described, the device is primarily limited to cycling exercise in hot dry environments, and with high air flow (to minimize sweat drippage) (Cramer and Jay, 2019).…”

Effect of aerobic fitness on the relation between age and whole‐body heat exchange during exercise‐heat stress: a retrospective analysis

“…Body core temperature was measured within the lower gut using a capsule thermometer (Vital Sense; Mini Mitter, Bend, OR, USA) ingested ∼1 h prior to data collection ( n = 3), or using a thermocouple probe (Mon‐a‐therm General Purpose Temperature Probe, Mallinckrodt Medical, St Louis, MO, USA) inserted 40 cm past the nostril entrance (oesophageal temperature; n = 25) or ∼12 cm past the anal sphincter (rectal temperature; n = 45). These differences occurred due to the selection of different indices of core temperature within the previous experiments comprising this analysis (D'Souza et al., 2020; Notley et al., 2019b; Notley et al., 2019c; Stapleton et al., 2015a). While the temporal profile of rectal and ingestible capsule temperature are similar, oesophageal temperature displays a lower absolute temperature and faster response to thermal transients (Taylor, Tipton, & Kenny, 2014).…”

“…Data from seventy‐three adults (Table 1) spanning a broad age range were extracted from our previous work (D'Souza et al., 2020; Notley et al., 2019b, 2019c; Stapleton et al., 2015a) based on their area‐specific metabolic heat production during exercise (150 ± 10, 200 ± 10 and 250 ± 10 W m −2 ) and mass‐specific (ml kg −1 min −1 ), and analysed retrospectively. Participants were assigned to the aerobically fit group if they possessed a of ≥45 ml kg −1 min −1 ( n = 38; 16 women, 22 men), while participants were included in the less aerobically fit group if their was ≤35 ml kg −1 min −1 ( n = 35; 20 women, 15 men).…”

“…Aerobic fitness, as indexed by peak oxygen uptake (), also declines progressively with increasing age (Hawkins & Wiswell, 2003). Further, aerobic fitness is an important determinant of individual variation in heat loss (Havenith & van Middendorp, 1990; Notley, Lamarche, Meade, Flouris, & Kenny, 2019b), with recent studies demonstrating that adults with a 10–20 ml kg −1 min −1 higher than their less fit counterparts display thermoregulatory adaptations that augment heat loss during moderate‐to‐vigorous exercise in the heat (Lamarche, Notley, Louie, Poirier, & Kenny, 2018a; Lamarche, Notley, Poirier, & Kenny, 2018b; Stapleton et al., 2015a). It is possible, therefore, that the rate of thermoregulatory decline with increasing age may be slower in more aerobically fit individuals.…”

“…The purpose of this work was therefore to assess the interplay between the effects of age and aerobic fitness on thermoregulatory function during exercise‐heat stress. To achieve this, we conducted a retrospective analysis of data from aerobically fit ( n = 38; : ∼50 ml kg −1 min −1 ) and less aerobically fit adults ( n = 35; : ∼30 ml kg −1 min −1 ) spanning a broad age range (18–66 years) from our previous work (D'Souza et al., 2020; Notley et al., 2019b; Notley et al., 2019c; Stapleton et al., 2015a). Participants performed exercise at increasing, fixed rates of metabolic heat production of 150, 200, and 250 W m −2 in dry heat (40°C, ∼15% relative humidity), while whole‐body total heat loss (evaporative + dry heat loss) was measured continuously using direct calorimetry.…”