Body composition studies suggest that the decrease in lean tissues is primarily related to loss of muscle mass with age ( Cohn et al. This fall in RMR is attributable to a loss of lean tissue with aging since there is little or no change when RMR is expressed relative to lean body mass ( Visser et al. 1995), resulting in decreased total energy requirement to maintain equilibrium. Total energy expenditure declines due to a reduction in resting metabolic rate (RMR) 3 and an even greater reduction in physical activity ( McGandy et al. Protein turnover, aging, N τ-methylhistidine, magnetic resonance imaging, elderly humansĪging is associated with many changes that may affect body protein metabolism. These findings have potential implications for the nutrition of both normal and sick elderly persons. Thus, the decrease in the contribution by muscle to whole-body protein metabolism with age is associated with an increase from 62 to 74% ( P < 0.001) in the contribution by nonmuscle lean tissues. d −1) and volume (7.5 ± 0.3 vs 8.0 ± 0.5 L) did not differ between age groups.This contrasted with intraabdominal lean tissue, where the rate of protein catabolism (13.8 ± 0.6 vs. d −1) and a trend ( P = 0.08) for lower muscle volume (19.7 ± 1.5 vs.There was a lower ( P < 0.004) rate of muscle protein catabolism in the elderly (1.8 ± 0.2 vs.
There were no significant age or gender effects on rates of protein kinetics per L total lean tissue. Whole-body protein kinetics, using the 60-h oral glycine method, and muscle and nonmuscle protein catabolism, based on protein kinetic data, urinary N τ-methylhistine excretion and lean tissue volumes defined by whole-body magnetic resonance imaging, from eight healthy elderly subjects (5 females and 3 males, mean age 71.5 y) were compared with those of seven young persons (3 females and 4 males, mean age 28 y). We tested the hypothesis that nonmuscle lean tissue mass and its rate of protein catabolism remain constant with aging despite changes in the proportional contribution of these tissues to whole-body protein metabolism.
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