Fitness‐related differences in the rate of whole‐body evaporative heat loss in exercising men are heat‐load dependent

Abstract

What is the central question of this study? Aerobic fitness modulates heat loss, but the heat-load threshold at which fitness-related differences in heat loss occur in young healthy men remains unclear. What is the main finding and its importance? We demonstrate using direct calorimetry that aerobic fitness modulates heat loss in a heat-load-dependent manner, with fitness-related differences occurring between young men who have low and high fitness when the heat load is ∼≥500 W. Although aerobic fitness has been known for some time to modulate heat loss, our findings define the precise heat-load threshold at which fitness-related differences occur. The effect of aerobic fitness (defined as rate of peak oxygen consumption) on heat loss during exercise is thought to be related to the level of heat stress. However, it remains unclear at what combined exercise and environmental (net) heat-load threshold these fitness-related differences occur. To identify this, we assessed whole-body heat exchange (dry and evaporative) by direct calorimetry in young (22 ± 3 years) men matched for physical characteristics with low (Low-fit; 39.8 ± 2.5 ml O kg min ), moderate (Mod-fit; 50.9 ± 1.2 ml O kg min ) and high aerobic fitness (High-fit; 62.0 ± 4.4 ml O kg min ; each n = 8), during three 30 min bouts of cycling in dry heat (40°C, 12% relative humidity) at increasing rates of metabolic heat production of 300 (Ex1), 400 (Ex2) and 500 W (Ex3), each followed by a 15 min recovery period. Each group was exposed to a similar net heat load (metabolic plus ∼100 W dry heat gain; P = 0.83) during each exercise bout [∼400 (Ex1), ∼500 (Ex2) and ∼600 W (Ex3); P < 0.01]. Although evaporative heat loss was similar between groups during Ex1 (P = 0.33), evaporative heat loss was greater in the High-fit (Ex2, 466 ± 21 W; Ex3, 557 ± 26 W) compared with the Low-fit group (Ex2, 439 ± 22 W; Ex3, 511 ± 20 W) during Ex2 and Ex3 (P ≤ 0.03). Conversely, evaporative heat loss for the Mod-fit group did not differ from either the High-fit or Low-fit group during all exercise bouts (P ≥ 0.09). We demonstrate that aerobic fitness modulates heat loss in a heat-load-dependent manner, such that young, highly fit men display greater heat-loss capacity only at heat loads ∼≥500 W compared with their lesser trained counterparts.

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

“…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.…”

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

“…Statistical significance was set at P < 0.05, and a Bonferroni adjustment was used for all post hoc comparisons. An a priori power analysis indicated that based on the effect size (Cohen's d = 1.88) for a 32 W difference in total heat loss with a standard deviation of 17 W between low‐ and high‐fit men (Lamarche et al., ), a minimum of six subjects were required to detect between‐group differences of this effect size with at least 80% statistical power (Faul, Erdfelder, Lang, & Buchner, ). Therefore, with the present sample (each group n = 8) these analyses were adequately powered (>80%).…”

“…We recently reported that young men of higher aerobic fitness [peak oxygen consumption () of >60 ml O 2 kg −1 min −1 ] display greater heat loss capacity compared with their lesser trained counterparts ( of <40 ml O 2 kg −1 min −1 ) during exercise eliciting moderate to high (400–500 W), but not low (300 W), fixed metabolic heat loads in hot, dry conditions (Lamarche, Notley, Louie, Poirier, & Kenny, ). Furthermore, given that those individuals had matched physical characteristics, we could confirm that those fitness‐related differences were unrelated to differences in body mass and surface area, which are both known to influence heat transfer (Cramer & Jay, ; Havenith & Van Middendorp, ; Havenith, Luttikholt, & Vrijkotte, ; Jay, Bain, Deren, Sacheli, & Cramer, ; Notley, Park, Tagami, Ohnishi, & Taylor, ).…”