Cold-induced thermoregulation and biological aging.

Aging is associated with diminished cold-induced thermoregulation (CIT). The mechanisms accounting for this phenomenon have yet to be clearly elucidated but most likely reflect a combination of increased heat loss and decreased metabolic heat production. The inability of the aged subject to reduce heat loss during cold exposure is associated with diminished reactive tone of the cutaneous vasculature and, to a lesser degree, alterations in the insulative properties of body fat. Cold-induced metabolic heat production via skeletal muscle shivering thermogenesis and brown adipose tissue nonshivering thermogenesis appears to decline with age. Few investigations have directly linked diminished skeletal muscle shivering thermogenesis with the age-related reduction in cold-induced thermoregulatory capacity. Rather, age-related declines in skeletal muscle mass and metabolic activity are cited as evidence for decreased heat production via shivering. Reduced mass, GDP binding to brown fat mitochondria, and uncoupling protein (UCP) levels are cited as evidence for attenuated brown adipose tissue cold-induced nonshivering thermogenic capacity during aging. The age-related reduction in brown fat nonshivering thermogenic capacity most likely reflects altered cellular signal transduction rather than changes in neural and hormonal signaling. The discussion in this review focuses on how alterations in CIT during the life span may offer insight into possible mechanisms of biological aging. Although the preponderance of evidence presented here demonstrates that CIT declines with chronological time, the mechanism reflecting this attenuated function remains to be elucidated. The inability to draw definitive conclusions regarding biological aging and CIT reflects the lack of a clear definition of aging. It is unlikely that the mechanisms accounting for the decline in cold-induced thermoregulation during aging will be determined until biological aging is more precisely defined.