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. © 2019 The Authors. Experimental Physiology © 2019 The Physiological Society