Aerobic Fitness Modulates Whole‐body Heat Loss Responses in Young Adult Males during Exercise in Hot Dry Conditions
Authors: Louie Jeff C. , Lamarche Dallon T. , Notley Sean R. , Poirier Martin P. , Zhang Sarah Y. , Lee Yuhan , Kenny Glen P. 
Published: November 2023

It is well established that aerobic fitness (as defined by peak oxygen uptake) improves the body's ability to dissipate heat during exercise in hot conditions. However, it currently remains unclear at what level of heat stress differences in aerobic fitness may alter the body's physiological capacity to dissipate heat. Therefore, in this pilot work, we examined whole‐body heat loss using direct calorimetry in endurance (n=4, 63.5±1.5 mlO2/kg/min) and non‐endurance trained (n=4; 37.3±0.3 mlO2/kg/min, P=0.002) healthy young adult males (22±4 years) matched for body surface area (endurance: 1.89±0.14; non‐endurance: 1.87±0.16 m2, P=0.104). Participants performed three 30‐min bouts of semi‐recumbent cycling at fixed rates of metabolic heat production equal to 300 (Ex1), 400 (Ex2), and 500 W (Ex3) (an equivalent external workload of approximately 45, 75 and 100 W, respectively), each separated by a 15‐min recovery period in dry heat (40°C, 14% relative humidity). These fixed rates of heat production were employed to ensure a similar stimulus for whole‐body sweating. The rates of whole‐body heat loss (evaporative heat loss and dry heat exchange) and metabolic heat production were measured by direct and indirect calorimetry, respectively. Whole‐body heat content was measured as the cumulative summation of heat production and heat loss. We show that the rate of dry heat gain (i.e., from the environment), and therefore the net heat load (i.e., metabolic heat production plus dry heat gain), was similar between groups throughout the intermittent exercise protocol (Ex1, Ex2, Ex3, all P≥0.207). Although not statistically significant at all exercise bouts, the rate of whole‐body evaporative heat loss tended to be lower in the non‐endurance males during Ex1 (non‐endurance: 357±17 vs. endurance: 378±9 W, P=0.047), Ex2 (non‐endurance: 438±17 vs. endurance: 481±18 W, P=0.059), and Ex3 (non‐endurance: 513±29 vs. endurance: 573±26 W, P=0.113) compared to their endurance trained counterparts. Consequently, whole‐body heat content trended towards being greater in the non‐endurance group during Ex1 (non‐endurance: 127±50 vs. 112±36 kJ, P=0.163), Ex2 (non‐endurance: 138±38 vs. 86±25 kJ, P=0.038), and Ex3 (non‐endurance: 227±47 vs. 157±34 kJ, P=0.122) relative to the endurance trained group. Our early findings demonstrate that aerobic fitness may modulate the body's physiological capacity to dissipate heat at metabolic heat loads ≥300 W in young adult males differing in peak oxygen uptake by as much as ~26 mlO2/kg/min. Therefore, our data supports that maintaining a high level of aerobic fitness may facilitate an enhanced capacity to dissipate heat and thereby minimize the incidence of experiencing heat related illness during exercise in the heat.Support or Funding InformationThis study was supported by grants from the Natural Sciences and Engineering Research Council of Canada (Discover grant, RGPIN‐06313‐2014; Discovery Grants Program ‐ Accelerator Supplement, RGPAS‐462252‐2014; funds held by Dr. Glen P. Kenny).