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 (V̇O₂max). Thus, as V̇O₂max declines, any absolute task represents a higher relative V̇O₂max (% V̇O₂max) 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 (Q̇), a lower BV may also be associated with an inability to increase Q̇ appropriately. These effects are particularly important under conditions of heat stress, where a larger increase in Q̇ 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. Furthermore, basal metabolic rate (and thus basal heat production) decreases 20% from age 30 to age 70, due, in large part, to the loss of active muscle mass. This decrement may be associated with a decreased capacity for shivering thermogenesis. The combination of these changes may lend to relatively lower body temperatures and an increased risk of hypothermia during cold stress.