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

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What is the central question of this study? Does higher aerobic fitness, indexed by peak oxygen uptake ( ), 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 ( : ∼30vs. ∼50mlkg-1 min-1 ) aged 18-66years, 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. 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 ( ), slows that age-related decline. To evaluate this possibility, we conducted a retrospective analysis of data from aerobically fit (n=38; : (mean (SD)) 49 (4) mlkg-1 min-1 ) and less fit (n=35; : 32 (3) mlkg-1 min-1 ) adults spanning a broad age range (18-65vs. 18-66years). Participants performed three, 30min bouts of cycling at metabolic heat productions of 150, 200 and 250Wm-2 , each separated by 15min 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) Wm-2 per decade during exercise at metabolic heat productions of 150, 200 and 250Wm-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.