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1 0 - 0 4 - 2 0 0 6 Effects of leucine supplementation on the body composition and protein status of rats submitted to food restriction
Donato J Jr, Pedrosa RG, Cruzat VF, Pires IS, Tirapegui J. OBJECTIVE: Several mechanisms have been proposed to explain the effect of high protein intake in weight loss diets on body composition. Some investigators have suggested that part of the beneficial effects of these diets might be attributedto the higher ingestion of branched-chain amino acids (BCAAs). Thus, the objective of a higher protein intake would not be to increase substrate availability for protein synthesis but to stimulate the anabolic properties observed after increased ingestion of BCAAs (leucine, valine, and isoleucine), in particular leucine. Several lines of in vitro and in vivo evidence have indicated that leucine stimulates protein synthesis by activating the initiation of the translation process. Initiation is one of the main steps that regulate protein synthesis because it determines the efficiency with which the ribosomal apparatus is used during the translation of mRNA. Leucine acts on this process mainly by mediating the hyperphosphorylation of eukaryotic initiation factor binding protein-1, which ultimately increases the formation of the eukaryotic initiation factor-4F ribosomal complex that is responsible for the binding of mRNA to the 43S preinitiation complex. In addition, leucine may favor protein synthesis by increasing the cellular apparatus responsible for the translation process, resulting in a greater capacity of cellular protein synthesis. In this case, leucine stimulates the synthesis of ribosomal protein and RNA and elongation factors by increasing the phosphorylation state of the 70-kDa ribosomal protein S6 kinase-1. Several studies have suggested that leucine phosphorylates eukaryotic initiation factor binding protein-1 and protein S6 kinase-1 by activating the enzyme mammalian target of rapamycin. However, most of these studies only evaluated the acute effects of leucine administration. The effect of long-term supplementation has been investigated in a study conducted on rats supplemented with leucine for 12 d, which showed an increase of the protein synthesis rate in adipose tissue, gastrocnemius muscle, and liver. In addition, nitrogen retention was observed in carcasses of rats that were supplemented long term with leucine during the phase of nutritional recovery after a period of protein malnutrition. The effect of long-term leucine supplementation (with other BCAAs) in the condition of increased protein catabolism has been evaluated in a study that subjected wrestlers to 19 d of caloric restriction. The combination of caloric restriction and BCAA supplementation was more effective in the reduction of body fat than was caloric restriction alone. However, no significant changes were observed in the LM or body weight of these athletes, although body weight tended to be lower in the group supplemented with BCAA. In contrast, in another study BCAA supplementation did not alter weight loss or body composition in individuals with type 2 diabetes who underwent a physical training program for 2 mo. In view of the evidence indicating that leucine possesses an anabolic action in terms of protein metabolism and favorably affects body composition in catabolic situations, the present study tested the hypothesis that long-term low-dose leucine supplementation favorably alters the body composition and protein nutritional status of adult rats that are subjected to 50% food restriction. METHODS: Adult male Wistar rats were submitted to 50% food restriction for 6 weeks. The control group received the standard AIN-93M diet and the leucine group received the same diet supplemented with 5.91 g L-leucine/kg ration. We then determined carcass chemical composition, serum leptin, albumin and total protein concentrations, and protein, DNA and RNA concentrations in gastrocnemius muscle and liver. RESULTS: No difference in final body weight was observed between groups. However, the leucine group presented a lower amount of body fat (P < 0.05). [Tabel]
Leptin concentration showed a directly proportional correlation with the amount of body fat (r = 0.88, P < 0.05), but no significant difference in serum leptin concentration was observed between groups (P = 0.08). Regarding protein nutritional status, liver protein concentration was higher in the leucine group (P < 0.05). In the gastrocnemius muscle, a higher RNA concentration (P < 0.05) and a tendency towards higher DNA concentration (P = 0.06) were observed in the leucine group. CONCLUSION: The results indicate that low-dose leucine supplementation increases body fat loss and improves liver protein status and the capacity of muscle protein synthesis in rats submitted to FR.
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[Definitie:] "An ergogenic aid is any substance or phenomenon that enhances performance."
(Wilmore and Costill) 
