Postprandial myofibrillar and whole body protein synthesis in young and old human subjects.

Rates of incorporation of leucine (using L-[1-13C]leucine as a tracer) into myofibrillar and whole body proteins were determined in healthy old (> 60 yr old, n = 7) and young (< 30 yr old, n = 9) men and women who were fed small meals (4% of daily energy) every 30 min. There was no difference in whole body incorporation of leucine into proteins in the young (148 +/- 5 mumol.h-1.kg lean body mass-1, means +/- SE) and old groups (150 +/- 3 mumol.h-1.kg lean body mass-1). However, the fractional myofibrillar protein synthesis in the vastus lateralis muscle was 28% slower in the older group (0.063 +/- 0.004 vs. 0.088 +/- 0.003 %/h, P < 0.001). Extrapolation of these results to whole body myofibrillar synthesis (fractional rate x myofibrillar mass estimated by creatinine excretion) indicated that, in the older group, total myofibrillar synthesis was 43% slower (1.8 +/- 0.2 vs. 3.1 +/- 0.2 g/h, P < 0.01) and that their myofibrillar synthesis was a smaller portion of whole body protein synthesis (15 +/- 1 vs. 23 +/- 1%, P < 0.001). Compared with age-matched postabsorptive subjects, whole body protein synthesis was approximately 25% faster, and fractional myofibrillar synthesis was approximately 50% faster in these fed subjects, both young and old. We conclude that myofibrillar synthesis is slower in older subjects during both postabsorptive and postprandial conditions but that aging does not impair the stimulatory effect of feeding on protein synthesis.