MYOFIBRILLAR PROTEIN-SYNTHESIS IN YOUNG AND OLD MEN

We tested the hypothesis that healthy older men (>60 yr old) have a sl ower rate of myofibrillar protein synthesis than young men (<35 yr old ). Myofibrillar protein synthesis was determined by the in vivo incorp oration o L-[1-C-13]leucine into myofibrillar proteins obtained by mus cle biopsy. Subjects were eight young (21-31 yr) and eight older (62-8 1 yr) men, all healthy and moderately active. There was no significant difference in the mean height and weight of the two age groups, but t he older group had 12% less lean body mass (determined by K-40 countin g) and 21% less muscle mass (estimated by urinary creatinine excretion ). Upper leg strength was approximately one-third lower in the older s ubjects according to isokinetic dynamometry. The fractional rate of my ofibrillar protein synthesis was 28% slower in the older group (0.039 +/- 0.009 vs. 0.054 +/- 0.010 %/h, mean +/- SD, P < 0.01). Total myofi brillar protein synthesis, estimated as total myofibrillar mass (from creatinine excretion) times the fractional synthesis rate, was 44% slo wer in the older group (1.4 vs. 2.5 g/h, P < 0.001). Whole body protei n synthesis, assessed as the difference between leucine disappearance rate and leucine oxidation, was marginally slower (8%, P = 0.10) in th e older group, but not when the data were adjusted for lean body mass. Myofibrillar protein synthesis was a smaller fraction of whole body p rotein synthesis in the older group (12 vs. 19%). Reduced myofibrillar protein synthesis may be an important mechanism of the muscle atrophy associated with aging.