Thermoregulation and aging.

ACCURATE REGULATION OF BODY temperature is essential for optimal function and survival. The elderly are at increased risk of both hypothermia and hyperthermia when exposed to extreme temperatures. Two new investigations show that it does not take extreme temperatures to see differences between the aged and the adult. In this issue of the American Journal of Physiology—Regulatory, Integrative and Comparative Physiology, DeGroot and Kenney (9) showed in humans that core body temperature dropped with mild ambient cooling in the aged, while it remained high in young subjects. This finding provides support for a deficit in autonomic thermoregulation, even with mild cooling. The drop in core body temperature, however, was not compatible with heat loss as calculated with a model on the measured variables, suggesting that the presently adopted models may require adaptation for aging studies. We suggest that it may be of value to evaluate whether the ratio between core and shell body compartments, as proposed by Aschoff and Wever (2), could help to resolve the discrepancy between observed and modeled heat loss in elderly. Moreover, we propose that the cutaneous blood flow and temperature of the extremities of the body require a closer examination. Although these parts are rich in arteriovenous anastomoses (AVAs) and are therefore extremely well suited for heat loss regulation; their temperature is usually hardly, if at all, present in calculations of mean skin temperature and heat loss to the environment. Not only core and shell modeling but also distal skin measurements may improve our understanding of agerelated deficits in autonomic thermoregulation. In a nonhuman primate model, the mouse lemur, Aujard et al. (3) showed also that behavioral thermoregulation changes with age. Selection of ambient temperature was studied during the inactive (daytime) and active (nocturnal) circadian phases, both during a long photoperiod (simulated summer season) and a short photoperiod (simulated winter season). Old animals selected significantly higher ambient temperatures than adult animals in all conditions, except when it was needed most: during the “winter days,” that is, the period during which both the circadian and the seasonal phase dictate a low activity level, and, moreover, ambient temperature is normally low. This suggests that aged animals, although they do seek higher ambient temperatures in general, may not optimally utilize photoperiodic information to seasonally adapt behavioral thermoregulation. This idea has to be confirmed in future studies. Humans need to regulate their body temperature within restricted limits to function well, and even more essential, to survive. Epidemiological studies suggest that most of the deaths due to hypothermia or hyperthermia occur in elderly subjects (4, 6, 8). The reason why the elderly especially are at such an increased risk of thermoregulatory deficits is not a new question, yet still important and unsolved. Is it because their capability to sense temperature is compromised, because their thermoregulatory capacities are limited or because their physiology is less tolerant to endure extreme temperatures? Previous studies suggested that probably all these possibilities contribute in the case of extreme temperatures. Age-related changes, however, may also occur under less extreme temperatures (reviewed in Ref. 22). Investigation of thermoregulation in the range of mildly elevated or decreased ambient temperature is of importance, because even small changes in body temperature may strongly affect basal functions, including vigilance (16), and because in most of the fatal or near-fatal cases, the exposure to extreme ambient temperatures will be preceded by exposure to mildly elevated or decreased temperatures. If thermoregulation does not start in time during the drift from thermoneutrality, it may be more difficult to make up for body cooling or heating at a subsequent phase of extreme ambient temperature. Two new reports now specifically address the issue of age-related changes in thermoregulation in a nonextreme range of ambient temperatures. The first study, reported by DeGroot and Kenney (9) in this issue of the American Journal of Physiology—Regulatory, Integrative and Comparative Physiology, demonstrates age-related deficits in “autonomic” thermoregulation in humans. The second study, of Aujard and colleagues (3), shows that in a nonhuman primate model, age-related changes in “behavioral” thermoregulation differ depending on the season and on the circadian phase or sleep/wake state. DeGroot and Kenney (9) confined a quite respectable number of thirty-six young and forty-six aged subjects to a semirecumbent position and exposed them to an ambient temperature that slowly drifted downward from thermoneutrality. Meanwhile, continuous assessments were made of skin temperature, skin and forearm blood flow, arterial blood pressure, oxygen consumption, and subjective thermal sensation. Subjects were exposed just until sustained involuntary shivering occurred, which was after a median duration of about 80 min for both young and elderly subjects. Although this finding at first sight might suggest integrity of at least the autonomic shivering response, group differences in core body temperature actually indicated that the shivering response occurred at a much lower core body temperature in elderly subjects. At baseline, core (esophageal) temperature was slightly, near-