Impaired defense of core temperature in aged humans during mild cold stress

Aged humans often exhibit an impaired defense of core temperature during cold stress. However, research documenting this response has typically used small subject samples and strong cold stimuli. The purpose of this study was to determine the responses of young and older subjects, matched for anthropometric characteristics, during mild cold stress. Thirty-six young (YS; 23 ± 1 years, range 18-30) and 46 older (OS; 71 ± 1 years, range 65-89) subjects underwent a slow transient cold air exposure from a thermoneutral baseline, during which esophageal (T_es) and mean skin temperatures (T _sk), O₂ consumption, and skin blood flow (SkBF; laser-Doppler flowmetry) were measured. Cold exposure was terminated at the onset of visible sustained shivering. Net metabolic heat production (M _net), heat debt, predicted change in midregion temperature (ΔT_mid), and tissue insulation (I_t) were calculated. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial pressure and expressed as percent change from baseline (ΔCVC _%base). There were no baseline group differences for T_es, but OS M_net was lower (OS: 38.0 ± 1.1; YS: 41.9 ± 1.1 W·m^-2, P < 0.05). T_es was well maintained in YS but fell progressively in OS (P < 0.01 for all timepoints after 35 min). The skin vasoconstrictor response to mild cold stress was attenuated in OS (42 ± 3 vs. 53 ± 4 ΔCVC_%base, P < 0.01). There were no group differences for T_sk or I_t, while M _net remained lower in OS (P < 0.05). The ΔT_mid did not account for the drop in T_es in OS. Healthy aged humans failed to maintain T_es; however, the mechanisms underlying this response are not clear.