Age-related loss of muscle protein may involve a decreased response to anabolic factors of muscle protein synthesis through dysregulation of translation factors. To verify this hypothesis, we simultaneously investigated muscle protein synthesis and expression of some factors implicated in insulin signal transduction during hyperinsulinemia and hyperaminoacidemia in 6 young (25+/-1 year; mean+/-sem) and 8 elderly subjects (72+/-2 year). Incorporation of L-[1-13C] leucine in muscle proteins (fractional synthesis rate, FSR) was measured in vastus lateralis, before and during a euglycemic hyperinsulinemic hyperaminoacidemic clamp, together with Western blot analysis of protein kinase B (PKB), mTOR, 4E-BP1, and S6K1 phosphorylation. In basal state, muscle protein FSR was reduced in elderly in comparison with young subjects (0.061+/-0.004% per hour) vs 0.082+/-0.010% per hour, elderly vs. young, P<0.05). During clamp, muscle protein FSR was stimulated in young (0.119+/-0.006% per hour; P<0.05), but this response was significantly lower in elderly subjects (0.084+/-0.005% per hour, P<0.05 vs young subjects). Phosphorylation of PKB, mTOR, and 4E-BP1 were similarly increased by insulin and amino acid in both groups, except for S6K1 phosphorylation, which was not stimulated in elderly subjects. In conclusion, 1) response of muscle protein synthesis to insulin and amino acid is impaired in elderly humans; 2) a defect in S6K1 pathway activation may be responsible for this alteration. This modification is a mechanistic basis of sarcopenia development during aging.

The present study was designed to assess the effects of dietary leucine supplementation on muscle protein synthesis and whole body protein kinetics in elderly individuals. Twenty healthy male subjects (70 ± 1 years) were studied before and after continuous ingestion of a complete balanced diet supplemented or not with leucine. A primed (3.6 μmol kg −1 ) constant infusion (0.06 μmol kgphenylalanine was used to determine whole body phenylalanine kinetics as well as fractional synthesis rate (FSR) in the myofibrillar fraction of muscle proteins from vastus lateralis biopsies. Whole body protein kinetics were not affected by leucine supplementation. In contrast, muscle FSR, measured over the 5-h period of feeding, was significantly greater in the volunteers given the leucine-supplemented meals compared with the control group (0.083 ± 0.008 versus 0.053 ± 0.009% h −1 , respectively, P < 0.05). This effect was due only to increased leucine availability because only plasma free leucine concentration significantly differed between the control and leucine-supplemented groups. We conclude that leucine supplementation during feeding improves muscle protein synthesis in the elderly independently of an overall increase of other amino acids. Whether increasing leucine intake in old people may limit muscle protein loss during ageing remains to be determined.

Aging is characterized by a decrease of muscle mass associated with a decrease in postprandial anabolism. This study was performed to gain a better understanding of the intracellular mechanisms involved in the stimulation of muscle protein synthesis by amino acids and their role in the decrease of muscle sensitivity to food intake during aging. The effects of amino acids or leucine alone were assessed in vitro on epitrochlearis muscle from young, adult and old rats. Protein synthesis was assessed by incorporation of radiolabeled phenylalanine into protein and p70 S6 kinase activity by incorporation of (32)P into a synthetic substrate. Amino acids, at physiologic concentrations, stimulated muscle protein synthesis (P < 0.05) and leucine reproduced this effect. The intracellular targets of amino acids were phosphatidylinositol 3' kinase and the rapamycin-sensitive pathways mammalian target of rapamycin (mTOR)/p70 S6 kinase. In old rats, the sensitivity of muscle protein synthesis to leucine was lower than in adults (P < 0.05) and this paralleled the lesser ability of leucine to stimulate the rapamycin-sensitive pathways (P < 0.05). We demonstrated that amino acids and leucine stimulate muscle protein synthesis and that aging is associated with a decrease in this effect. However, because aged rats are still able to respond normally to high leucine concentrations, we hypothesize that a nutritional manipulation increasing the availability of this amino acid to muscle could be beneficial in maintaining the postprandial stimulation of protein synthesis.

Acute leucine supplementation of the diet has been shown to blunt defects in postprandial muscle protein metabolism in old rats. This study was undertaken to determine whether the effect of leucine persists in a 10-d experiment. For this purpose, adult (9 mo) and old (21 mo) rats were fed a semiliquid 18.2 g/100 g protein standard diet during the 8-h dark period for 1 mo. Then, each group was given either a leucine-supplemented meal or an alanine-supplemented meal (as the control meal) for 1 h and the standard diet the rest of the feeding period. On d 10, rats were fed either no food (postabsorptive group) or the supplemented meal for 1 h. Muscle protein synthesis was assessed in vivo 90-120 min after meal distribution using the flooding dose method (1-(13)C phenylalanine). Leucinemia was similar in rats of both ages in the postabsorptive state. Postprandial plasma leucine concentrations were one- to twofold greater after the leucine meal than after the control meal. In the postabsorptive state, leucine supplementation did not modify the muscle protein synthesis rate in old rats but enhanced it to the postprandial rate in adult rats. As expected, muscle protein synthesis was stimulated by the control meal in adult rats but not in old rats. The leucine meal restored this stimulation in old rats but did not further stimulate muscle protein synthesis in adult rats. In conclusion, the beneficial effect of leucine supplementation on postprandial muscle protein anabolism persists for at least 10 d. The long-term utilization of leucine-rich diets may therefore limit muscle protein wasting during aging.