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.

Muscle protein synthesis is slower in healthy older men and women than in young adults, but whether this results from relative disuse rather than aging is unclear. The present study was done to examine rates of myofibrillar protein synthesis before and after a 3-mo progressive resistance exercise program in young and old men and women. Protein synthesis was determined by incorporation of the tracer L-[1-13C]leucine into myofibrillar proteins obtained from the vastus lateralis muscle by needle biopsy. Before exercise, mean fractional myofibrillar synthesis was 33% slower (P < 0.01) in nine older subjects (62-72 yr old, 5 men and 4 women) than in 9 young subjects (22-31 yr old, 5 men and 4 women). Initial strength, as determined by three-repetition-maximum tests, was significantly less in the older group. Strength and training weights increased similarly in young and old groups, when expressed in relation to baseline values. Posttraining myofibrillar synthesis was determined on the day after the final training session. There was not a significant change in fractional myofibrillar synthesis in either the young or the old group after training, and the rate in the older group remained 27% slower (P < 0.05). Whole body protein turnover increased approximately 10% only in the younger group, and 24-h urinary 3-methylhistidine excretion (an index of myofibrillar proteolysis) was not significantly affected by training. These data suggest that the slower myofibrillar synthesis rate in older subjects cannot be explained by disuse.

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.

Obesity could be due to excess energy intake or decreased energy expenditure (EE). To evaluate this, we studied 18 obese females (148±8% of ideal body weight IIBW1, mean±SD) before and after achieving and stabilizing at IBW for at least 2 mo and a control group of 14 never obese females (< 110% of IBW or < 30% fat).In the obese, reduced obese, and never obese groups, the percent of body fat was 41±4%, 27±4%, and 25±3%; total energy expenditure (TEE) was 2704±449, 2473±495, and 2259±192 kcal/24 h; while resting metabolic rate was 1496±169, 1317±159, and 1341±103 kcal/24 h, respectively. 15 obese subjects who withdrew from the study had a mean initial body composition and EE similar to the subjects who were successful in achieving IBW. In 10 subjects followed for at least one year after stabilizing at IBW there was no significant relationship between the deviation from predicted TEE at IBW and weight regain.These studies indicate that, in a genetically heterogeneous female population, neither the propensity to become obese nor to maintain the obese state are due to an inherent metabolic abnormality characterized by a low EE. (J. Clin. Invest. 1993.

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