Effect of aerobic fitness on the relation between age and whole-body heat exchange during exercise-heat stress: a retrospective analysis

Abstract

New Findings: What is the central question of this study? Does higher aerobic fitness, indexed by peak oxygen uptake ((Formula presented.)), attenuate the age-related decline in thermoregulatory function during exercise in the heat? What is the main finding and its importance? When assessed in aerobically fit and less fit adults ((Formula presented.) : ∼30 vs. ∼50 ml kg−1 min−1) aged 18–66 years, a steeper decline in whole-body total heat loss (evaporative + dry heat exchange) was observed with increasing age in less aerobically fit adults. These outcomes indicate that increased aerobic fitness may attenuate the age-related decline in thermoregulatory function during exercise-heat stress. Abstract: Ageing is associated with decrements in cutaneous vasodilatation and sweating that attenuate whole-body total heat loss (evaporative + dry heat exchange) during exercise-heat stress. However, it remains uncertain whether increased aerobic fitness, as indexed by peak oxygen uptake ((Formula presented.)), slows that age-related decline. To evaluate this possibility, we conducted a retrospective analysis of data from aerobically fit (n = 38; (Formula presented.) : (mean (SD)) 49 (4) ml kg−1 min−1) and less fit (n = 35; (Formula presented.) : 32 (3) ml kg−1 min−1) adults spanning a broad age range (18–65 vs. 18–66 years). Participants performed three, 30 min bouts of cycling at metabolic heat productions of 150, 200 and 250 W m−2, each separated by 15 min recovery, in dry heat (40˚C, ∼15% relative humidity). Metabolic heat production and whole-body total heat loss were measured using indirect and direct calorimetry, respectively. Total heat loss (mean (95% CI)) declined at a rate of 5 (2, 8), 6 (3, 8) and 5 (3, 10) W m−2 per decade during exercise at metabolic heat productions of 150, 200 and 250 W m−2, respectively, in less aerobically fit individuals (all P ≤ 0.002), due primarily to reductions in evaporative heat loss. In contrast, no significant associations between age and total heat loss were observed in aerobically fit individuals (all P ≥ 0.146). As such, the slope of the age-related reduction in total heat loss was steeper in less fit compared to fit individuals across all three exercise bouts (all P ≤ 0.029). These outcomes indicate that increased aerobic fitness attenuates the age-related decline in exercise thermoregulation.