Effects of age and acclimation on responses to passive heat exposure

To examine the effect of chronological age on thermoregulation during passive heat exposure, six older (0, 61 ± 1 yr) and six young (Y, 26 ± 2 yr) men sat at rest during a 30-min baseline period (dry-bulb temperature = 28°C), a 60-min thermal transient (28-46°C by 2°C steps every 5 min), and 30 min at 46°C dry-bulb temperature. Subjects were matched for maximal O₂ consumption, anthropometry, and body composition. Testing was repeated after a 9-day active heat acclimation protocol. There were no age differences in rectal (T(re)), mean skin (T̄(sk)), or mean body temperature (T̄(b) = 0.8T(re) + 0.2T̄(sk)) before or after acclimation, but heart rate was lower (P < 0.01) in the O group in both acclimation states. Heat acclimation resulted in a significantly lower baseline T(re) and T̄(b) in both groups, which remained lower throughout the passive heat stress (P < 0.05). To examine the effects of age and acclimation on thermoregulatory effector function, forearm blood flow (by venous occlusion plethysmography) and chest sweating rate (SR(ch), by dew-point hygrometry) were plotted against T̄(b). The slope of the forearm blood flow-T̄(b) relationship was significantly (P < 0.05) lower in the O group before and after acclimation. A lower maximal SR(ch) (P < 0.05) was achieved by the O group, but neither the slope of SR(ch)-T̄(b) relationship nor the T̄(b) threshold for sweating was affected by age. Predictably, acclimation resulted in a lower T̄(b) threshold for the onset of sweating and skin vasodilation. These results indicate that, during passive heat stress, older men respond with a lower skin blood flow at a given T̄(b) and a lower maximal SR(ch) than matched young subjects. These relatively decreased effector responses are still evident after rigorous heat acclimation, yet they do not result in poorer heat tolerance or greater heat storage during passive heat stress of this magnitude.