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

Abstract

What is the central question of this study? Aerobic fitness modulates heat loss, albeit the heat load at which fitness-related differences occur in young healthy women remains unclear. What is the main finding and its importance? We demonstrate using direct calorimetry that fitness modulates heat loss in a heat-load dependent manner, with differences occurring between young women of low and high fitness and matched physical characteristics when the metabolic heat load is at least 400W in hot, dry conditions. Although fitness has been known for some time to modulate heat loss, our findings define the metabolic heat load at which fitness-related differences occur. 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±3years old) healthy women matched for physical characteristics, but with low (low-fit; 35.8±4.5mlO2 kg-1 min-1 ) or high aerobic fitness (high-fit; 53.1±5.1mlO2 kg-1 min-1 ; both n=8; indexed by peak oxygen consumption), during three 30min bouts of cycling performed at increasing rates of metabolic heat production of 250 (Ex1), 325 (Ex2) and 400W (Ex3), each separated by a 15min 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±16W; high-fit, 231±20W; P>0.05) and Ex2 (low-fit, 278±15W; high-fit, 301±20W; P>0.05), but was lower in the low-fit (316±21W) compared with the high-fit women (359±32W) during Ex3 (P<0.01). Consequently, the low-fit group stored 1.3-fold more heat (429±61kJ) throughout the three exercise bouts relative to the high-fit group (330±113kJ; 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 ∼17mlO2 kg-1 min-1 starting at a metabolic heat load of 400W.