Responses to mild cold stress are predicted by different individual characteristics in young and older subjects

Older individuals' ability to maintain core temperature during cold stress is impaired; however, the relative importance of individual characteristics that influence this response are unknown. The purpose of this study was to determine the relative influence of individual characteristics on core temperature and tissue insulation (I_t) during mild cold stress. Forty-two young (23 ± 1 yr, range 18-30 yr) and 46 older (71 ± 1 yr, range 65-89 yr) subjects, varying widely in muscularity, adiposity, and body size, underwent a transient cooling protocol during which esophageal temperature (T_es) was measured continuously and I_t was calculated using standard equations. Multiple-regression analyses were performed to determine predictors of T_es and I_t, and standardized regression coefficients were analyzed to determine the relative influence of each predictor. Candidate predictors included age, sex, weight, body surface area, body surface area-to-mass ratio, sum of skinfolds, percent fat, appendicular skeletal muscle mass, and thyroid hormone concentrations (triiodothyronine, thyronine). The sum of skinfolds explained 67% (P < 0.01) of the T_es variance in young subjects vs. 2% (P = 0.30) in older subjects. Conversely, appendicular skeletal muscle mass explained a greater portion of the variance in older subjects for both T_es (older: 28%, P < 0.01; young: 8%, not significant) and I_t (older: 46%, P < 0.01; young: 17%, P < 0.01). The T _es residual variance was considerably larger in older subjects (59-72% vs. 14-42% in young subjects), possibly due to varying rates of physiological aging. These results suggest that the relative influence of individual characteristics changes with aging.