Thermoregulation at rest and during exercise in healthy older adults.

Epidemiological evidence of increased mortality among older men and women resulting from hyper- and hypothermia should not be interpreted as implying that aging per se confers an intolerance to environmental extremes. Relatively few studies have attempted to delineate the effects of chronological age from concomitant factors (such as decreases in VO2max, lowered habitual activity levels, alterations in body composition, etc.) in determining thermoregulatory responses to rest and exercise in hot environments. When the effects of chronic diseases and sedentary life-style are kept to a minimum, heat tolerance appears to be minimally compromised by age. VO2max is more important than age in predicting body temperature during exertion, even though some of the physiological mechanisms associated with heat dissipation (especially control of skin blood flow and distribution of cardiac output) are closely associated with chronological age. Dehydration effects may be magnified in older individuals, and rehydration may be compromised by age-related differences in thirst sensitivity and renal function. The efficacy of various interventions in improving thermoregulatory responses of older individuals (e.g., aerobic training and heat acclimation) has not been studied adequately. Older men and women are capable of acclimating to hot conditions, but the time course of physiological changes underlying acclimation with age may be different. Another intervention that holds promise is hormone replacement therapy in postmenopausal women, which acutely affects temperature regulation and control of body fluids in a positive direction. The chronic effects of hormone replacement therapy on thermoregulation during exercise and environmental stresses are not known. Tolerance to cold exposure under resting conditions may be less dependent on age and aerobic fitness than on body composition. Studies of older and younger subjects exercising in cold environments are lacking altogether, including important studies into the possible preventive effects of regular physical activity on physiological responses to cold stress.