Effect of muscle mass decrease on age-related BMR changes

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Abstract
The long known and often studied decrease in basal metabolic rate (BMR) with age was recently shown to be related to changes in the proportion of lean-to-fat tissue mass in the body. Earlier studies showed the decline in basal O2 consumption (basal .ovrhdot.VO2) to be accounted for by a decline in lean body mass (LBM). The purpose of the present study was to identify and account for that component of the lean body responsible for the age-related decrease in basal .ovrhdot.VO2 in 959 participants in the Baltimore [Maryland, USA] Longitudinal Study aged from 20-97 yr. An age-independent linear relation was found betWeen paired values of basal .ovrhdot.VO2, a measure of total metabolic activity and 24-h creatinine excretion, a measure proportional to skeletal muscle mass. The relation is quantified by basal .ovrhdot.VO2 = 99.1 + 0.079 (creatinine excretion), where basal .ovrhdot.VO2 is expressed in ml/min (STPD) and creatinine excretion in mg/24 h. When muscle .ovrhdot.VO2 was subtracted from the whole-body basal .ovrhdot.VO2 for each subject, the remainder showed no age-related decreases. A linear relation was also found between anthropometrically derived lean body mass and 24-h creatinine excretion for men up to age 65. This relation attributed 24.8 kg of muscle mass for the excretion of 1 g creatinine in 24 h. Muscle mass may be wholly responsible for age-related decreases in BMR. These decreases may have both reversible and irreversible components.

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