Myofibrillar protein synthesis in young and old men.

We tested the hypothesis that healthy older men (> 60 yr old) have a slower rate of myofibrillar protein synthesis than young men (< 35 yr old). Myofibrillar protein synthesis was determined by the in vivo incorporation of L-[1-13C]leucine into myofibrillar proteins obtained by muscle biopsy. Subjects were eight young (21-31 yr) and eight older (62-81 yr) men, all healthy and moderately active. There was no significant difference in the mean height and weight of the two age groups, but the older group had 12% less lean body mass (determined by 40K counting) and 21% less muscle mass (estimated by urinary creatinine excretion). Upper leg strength was approximately one-third lower in the older subjects according to isokinetic dynamometry. The fractional rate of myofibrillar 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 myofibrillar protein synthesis, estimated as total myofibrillar mass (from creatinine excretion) times the fractional synthesis rate, was 44% slower in the older group (1.4 vs. 2.5 g/h, P < 0.001). Whole body protein synthesis, assessed as the difference between leucine disappearance rate and leucine oxidation, was marginally slower (8%, P = 0.10) in the older group, but not when the data were adjusted for lean body mass. Myofibrillar protein synthesis was a smaller fraction of whole body protein synthesis in the older group (12 vs. 19%). Reduced myofibrillar protein synthesis may be an important mechanism of the muscle atrophy associated with aging.