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

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 ( V ̇ O 2 peak ), slows that age-related decline. To evaluate this possibility, we conducted a retrospective analysis of data from aerobically fit (n = 38; V ̇ O 2 peak : (mean (SD)) 49 (4) ml kg-1  min-1 ) and less fit (n = 35; V ̇ O 2 peak : 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.