Fitness‐related differences in the rate of whole‐body total heat loss in exercising young healthy women are heat‐load dependent

Abstract

Aerobic fitness has recently been shown to alter heat loss capacity in a heat-load dependent manner in young men. However, given that sex-related differences in heat loss capacity exist, it is unclear whether this response is consistent in women. We therefore assessed whole-body total heat loss in young (21 ± 3 years old) healthy women matched for physical characteristics, but with low (low-fit; 35.8 ± 4.5 ml O kg min ) or high aerobic fitness (high-fit; 53.1 ± 5.1 ml O kg min ; both n = 8; indexed by peak oxygen consumption), during three 30 min bouts of cycling performed at increasing rates of metabolic heat production of 250 (Ex1), 325 (Ex2) and 400 W (Ex3), each separated by a 15 min recovery, in hot, dry conditions (40°C, 11% relative humidity). Whole-body total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat content was measured as the temporal summation of heat production and loss. Total heat loss did not differ during Ex1 (low-fit, 215 ± 16 W; high-fit, 231 ± 20 W; P > 0.05) and Ex2 (low-fit, 278 ± 15 W; high-fit, 301 ± 20 W; P > 0.05), but was lower in the low-fit (316 ± 21 W) compared with the high-fit women (359 ± 32 W) during Ex3 (P < 0.01). Consequently, the low-fit group stored 1.3-fold more heat (429 ± 61 kJ) throughout the three exercise bouts relative to the high-fit group (330 ± 113 kJ; P < 0.05). We show that aerobic fitness independently modulates heat loss capacity during exercise in hot, dry conditions in women separated by a peak oxygen consumption of ∼17 ml O kg min starting at a metabolic heat load of 400 W.

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

“…Therefore, contrary to previous reports, was found to be a significant determinant of evaporative heat loss when assessed among large heterogeneous samples of men and women, albeit during only light‐to‐moderate exercise. This finding is consistent with recent between‐group comparisons of aerobic fitness‐related differences in whole‐body heat exchange (Lamarche et al., , b) and further highlights the importance of examining individual factors at increasing levels of heat stress to determine the true extent to which those factors modulate heat exchange.…”

“…Participants were non‐smokers and did not report a history of cardiovascular, respiratory or metabolic disease. Some of these data ( n = 48) have been reported elsewhere (Lamarche et al., , b) and are analysed retrospectively herein with a larger, unpublished data set collected specifically to facilitate this investigation ( n = 52).…”

“…To prevent continued rises in body temperature during exercise‐induced heat stress, humans rely on sweat secretion and cutaneous vasodilation to facilitate evaporative and dry heat exchange. Those heat exchanges vary greatly among individuals owing, in part, to several inter‐individual factors, including age (Larose et al., ; Larose, Boulay, Sigal, Wright, & Kenny, ; Notley et al., ; Stapleton et al., ), sex (Gagnon & Kenny, , ), body morphology and composition (Adams et al., ; Bar‐Or, Lundegren, & Buskirk, ; Notley, Park, Tagami, Ohnishi, & Taylor, ; Notley, Park, Tagami, Ohnishi, & Taylor, ) and aerobic fitness (Lamarche, Notley, Louie, Poirier, & Kenny, ; Lamarche, Notley, Poirier, & Kenny, ), in addition to relative differences (i.e. per unit body size) in metabolic heat production and the evaporative heat loss requirement (metabolic heat production ± dry heat exchange; Cramer & Jay, ; Gagnon, Jay, & Kenny, ).…”

“…To achieve this, we used direct calorimetry to assess whole‐body evaporative and dry heat exchange and body heat storage in 100 young men ( n = 57) and women ( n = 43) during three bouts of cycling at increasing, fixed rates of metabolic heat production (and therefore heat load) in dry heat. Given that most previous assessments of whole‐body heat exchange at increasing fixed rates of metabolic heat production have evaluated only one individual factor (Gagnon & Kenny, ; Lamarche et al., , b; Larose et al., , ; Notley et al., ; Stapleton et al., ), a hypothesis‐driven experiment was not appropriate. Instead, our analysis focused on identifying the individual factor(s) with the greatest capacity to explain the variation in whole‐body heat exchange and body heat storage among men and women at each exercise‐induced heat load.…”

Aerobic but not thermoregulatory gains following a 10‐day moderate‐intensity training protocol are fitness level dependent: A cross‐adaptation perspective

“…These findings seem in sharp contrast with the long‐held tenet that, following exercise training, humans demonstrate a decreased threshold temperature for the onset of sweating (Roberts et al, ) as well as an increased gain of the sweating response (Nadel et al, ). Evaporative heat loss has recently been shown to be superior in trained individuals only when a certain threshold of net heat load is exceeded (Lamarche, Notley, Poirier, & Kenny, ). Going one step further, it could be speculated that MF participants were training at an intensity high enough to sufficiently stimulate heat loss in each session, whereas LF participants did not reach that threshold.…”