Whole-body protein turnover in the healthy elderly.

We tested the hypothesis that aging affects whole-body protein turnover via altered fat-free mass (FFM). Whole-body protein kinetics were estimated by the 60-h oral [15N]glycine method. Results from 16 healthy, elderly subjects (8 men and 8 women with a mean age of 72.6 y) were compared for age and sex effects with those of 15 lean young subjects (8 men and 7 women with a mean age of 28.4 y) during isoenergetic formula diets. Per kilogram body weight, nitrogen flux was lower only as an effect of age (P = 0.006) whereas age and female sex significantly lowered synthesis and breakdown (P < or = 0.04). However, per kilogram FFM, no significant age or sex effects on rates of protein kinetics remained. Age and female sex contributed significantly to decreased muscle protein catabolism (based on 3-methylhistidine excretion), both in absolute terms and as a percentage of whole-body protein breakdown in the elderly compared with the young: 20.2% compared with 30.9% in women and 27.9% compared with 39.8% in men. No significant age or sex effects on rates of nonmuscle lean tissue protein breakdown were observed with or without correction for body composition. We conclude that the lower rates of flux, synthesis, and breakdown per kilogram body weight in elderly compared with young persons are due to changes in body composition with aging because rates are not different per kilogram FFM. However, there is a reduced contribution by muscle to whole-body protein catabolism in older persons. This has potential implications for the nutrition of both normal and sick elderly persons.