Functional consequences of sarcopenia: effects on thermoregulation.

The loss of skeletal muscle mass with aging (sarcopenia), and related changes in body size and composition, may impact body temperature and thermoregulation in both hot and cold environmental conditions. Sarcopenia alters the thermal properties of the body as a passive system because of differences in water content, and thus specific heat, of muscle and adipose tissue. With respect to active thermoregulation in warm environments, differences in fat-free weight (FFW) can explain more than 80% of the variance in absolute blood volume (BV) among individuals (Allen et al., 1956) and BV, in turn, profoundly influences the cardiovascular responses to exercise and heat stress. For example, a lower BV for a given body weight may explain more than half of the variability in maximal oxygen uptake (VO2max). Thus, as VO2max declines, any absolute task represents a higher relative VO2max (% VO2max) and proportionately greater cardiovascular strain. Because BV is an important determinant of left-ventricular filling pressure, and because older individuals rely more on the Frank-Starling mechanism to increase cardiac output (Q), a lower BV may also be associated with an inability to increase Q appropriately. These effects are particularly important under conditions of heat stress, where a larger increase in Q is necessary to perfuse both skin and active muscle vascular beds. With exposure to cold, age-related changes in body composition affect the insulation provided by the peripheral tissues, especially in the limbs. This results in an increased reliance on peripheral vasoconstriction to minimize heat loss, yet this vasoconstriction is attenuated in older individuals.(ABSTRACT TRUNCATED AT 250 WORDS)