ORAL BRANCHED-CHAIN AMINO-ACIDS DECREASE WHOLE-BODY PROTEOLYSIS

Background: This Study reports the effects of ingesting branched-chain amino acids (leucine, valine, and isoleucine) on protein metabolism i n four men. Methods: To calculate leg protein synthesis and breakdown we used a new model that utilized the infusion of L-[ring-C-13(6)]phen ylalanine and the sampling of the leg arterial-venous difference and m uscle biopsies. In addition, protein-bound enrichments provided for th e direct calculation of muscle fractional synthetic rate. Four control subjects ingested an equivalent amount of essential amino acids (thre onine, methionine, and histidine) to discern the effects of branched-c hain amino acid nitrogen vs the effects of essential amino acid nitrog en. Each drink also included 50 g of carbohydrate. Results: Consumptio n of the branched-chain and the essential amino acid solutions produce d significant threefold and fourfold elevations in their respective ar terial concentrations. Protein synthesis and breakdown were unaffected by branched-chain amino acids, but they increased by 43% (p <.05) and 36% (p < .03), respectively, in the group consuming the essential ami no acids. However, net leg balance of phenylalanine was unchanged by e ither drink. Direct measurement of protein synthesis by tracer incorpo ration into muscle protein (fractional synthetic rate) revealed no cha nges within or between drinks. Whole-body phenylalanine flux was signi ficantly suppressed by each solution but to a greater extent by the br anched-chain amino acids (15% and 20%, respectively) (p < .001). Concl usions: These results suggest that branched-chain amino acid ingestion suppresses whole-body proteolysis in tissues other than skeletal musc le in normal men.