Distribution of protein turnover changes with age in humans as assessed bywhole-body magnetic resonance image analysis to quantify tissue volumes

We tested the hypothesis that nonmuscle lean tissue mass and its rate of pr otein catabolism remain constant with aging despite changes in the proporti onal contribution of these tissues to whole-body protein metabolism. Whole- body protein kinetics, using the 60-h oral [(15) N]glycine method, and musc le and nonmuscle protein catabolism, based on protein kinetic data, urinary N-tau-methylhistine excretion and lean tissue volumes defined by whole-bod y magnetic resonance imaging, from eight healthy elderly subjects (5 female s and 3 males, mean age 71.5 y) were compared with those of seven young per sons (3 females and 4 males, mean age 28 y). There were no significant age or gender effects on rates of protein kinetics per L total lean tissue. The re was a lower (P < 0.004) rate of muscle protein catabolism in the elderly (1.8 +/- 0.2 vs, 2.6 +/- 0.1 g L-1.d(-1)) and a trend (P = 0.08) for lower muscle volume (19.7 +/- 1.5 vs, 25.0 +/- 2.4 L), This contrasted with intr aabdominal lean tissue, where the rate of protein catabolism (13.8 +/- 0.6 vs. 13.2 +/- 0.9 g.L-1.d(-1)) and volume (7.5 +/- 0.3 vs 8.0 +/- 0.5 L) did not differ between age groups. Thus, the decrease in the contribution by m uscle to whole-body protein metabolism with age is associated with an incre ase from 62 to 74% (P < 0.001) in the contribution by nonmuscle lean tissue s. These findings have potential implications for the nutrition of both nor mal and sick elderly persons.