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Fitness‐related differences in the rate of whole‐body total heat loss in exercising young healthy women are heat‐load dependent
Author(s) -
Lamarche Dallon T.,
Notley Sean R.,
Poirier Martin P.,
Kenny Glen P.
Publication year - 2018
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1113/ep086752
Subject(s) - aerobic capacity , vo2 max , physical fitness , heat load , zoology , thermoregulation , chemistry , biology , medicine , physical therapy , thermodynamics , heart rate , physics , blood pressure
New FindingsWhat 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 400 W 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.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 2 kg −1 min −1 ) or high aerobic fitness (high‐fit; 53.1 ± 5.1 ml O 2 kg −1 min −1 ; 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 2 kg −1 min −1 starting at a metabolic heat load of 400 W.