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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-[sup.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. J. Nutr. 133: 1198-1205, 2003. KEY WORDS: * aging * muscle protein synthesis * leucine * chronic * rats

Aging induces a dysregulation of immune and inflammation functions that may affect protein synthesis rates in lymphoid tissue and plasma proteins. We quantified in vivo synthesis rates of thymus, spleen and plasma proteins, including albumin and acute phase proteins, in adult (8 mo old) and old (22 mo old) rats using the flooding dose method [L-(1-[sup.13]C) phenylalanine]. Immunosenescence was reflected by thymus atrophy and spleen hypertrophy in old rats but not in adult rats. A low albumin plasma level associated with high concentrations of fibrinogen, [[alpha].sub.2]-macroglobulin, [[alpha].sub.1]-acid glycoprotein and proteins other than albumin revealed a low grade inflammation in old rats. Protein fractional synthesis rates (FSR) and protein synthesis efficiencies of thymus were 29 and 26% lower in old than in adult rats, respectively; these variables did not differ in spleen. Protein absolute synthesis rates (ASR) of the thymus and spleen were 76% lower and 67% greater in old than adult rats, respectively. The FSR and ASR of albumin and other plasma proteins were greater in old than in adult rats. Protein synthesis measurement is a valuable nonimmunological tool to assess, in vivo, immune and inflammatory variables. Alterations in secondary lymphoid organs and plasma protein synthesis may contribute to the significant repartitioning of amino acids in old compared with adult rats and may be involved in sarcopenia. KEY WORDS: * aging * inflammation * lymphoid tissues * protein synthesis * rats

Aging is characterized by a progressive loss of muscle mass. A decrease of muscle protein synthesis stimulation has been detected in the postprandial state and correlated to a decrease of muscle protein synthesis sensitivity to leucine in vitro. This study was undertaken to examine the effect of a leucine-supplemented meal on postprandial (PP) muscle protein synthesis during aging. Adult (8 mo old) and old (22 mo old) rats were fed a semiliquid 18.2% protein control diet for 1 mo. The day of the experiment, rats received no food (postabsorptive group) or either an alanine or leucine-supplemented meal for 1 h (postprandial groups: PP and PP + Leu groups, respectively). Muscle protein synthesis was assessed in vivo 90-120 min after the meal distribution using the flooding dose method (1-[sup.13]C phenylalanine). Plasma leucine concentrations were significantly greater in the PP + Leu group compared with the PP group at both ages. Muscle protein synthesis was significantly greater in the adult PP group, whereas it was not stimulated in the old PP group. When supplemented with leucine, muscle protein synthesis in old rats was stimulated and similar to that observed in adults. We conclude that acute meal supplementation with leucine is sufficient to restore postprandial stimulation of muscle protein synthesis in old rats. Whether chronic leucine meal supplementation may limit muscle protein wasting during aging remains to be verified. KEY WORDS: * aging * protein synthesis * leucine * muscle * rats