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Article

Distribution of Protein Turnover Changes with Age in Humans as Assessed by Whole-Body Magnetic Resonance Image Analysis to Quantify Tissue Volumes¹

José A. Morais^2*,, Robert Ross, Réjeanne Gougeon, Paul B. Pencharz^§, Peter J. H. Jones and Errol B. Marliss

McGill Nutrition and Food Science Centre and ^* Division of Geriatric Medicine, Royal Victoria Hospital, Montreal, Canada; School of Health and Physical Education, Queen’s University, Kingston, Ontario, Canada; Research Institute, Hospital for Sick Children, Departments of Paediatrics and Nutritional Sciences, University of Toronto, Toronto, Ontario, Canada; and ^§ School of Dietetics and Human Nutrition, Mac Donald Campus, McGill University, Montreal, Canada

²To whom correspondence should be addressed.

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. Whole-body protein kinetics, using the 60-h oral [¹⁵ N]glycine method, and muscle and nonmuscle protein catabolism, based on protein kinetic data, urinary N-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). There were no significant age or gender effects on rates of protein kinetics per L total lean tissue. There 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 intraabdominal 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 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. These findings have potential implications for the nutrition of both normal and sick elderly persons.

KEY WORDS: • protein turnover • aging • N-methylhistidine • magnetic resonance imaging • elderly humans

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