Background:This study reports the effects of ingesting branched‐chain amino acids (leucine, valine, and isoleucine) on protein metabolism in four men.Methods:To calculate leg protein synthesis and breakdown, we used a new model that utilized the infusion of L‐[ring‐13C6]phenylalanine and the sampling of the leg arterial‐venous difference and muscle biopsies. In addition, protein‐bound enrichments provided for the direct calculation of muscle fractional synthetic rate. Four control subjects ingested an equivalent amount of essential amino acids (threonine, methionine, and histidine) to discern the effects of branched‐chain amino acid nitrogen vs the effects of essential amino acid nitrogen. Each drink also included 50 g of carbohydrate.Results:Consumption of the branched‐chain and the essential amino acid solutions produced significant threefold and fourfold elevations in their respective arterial 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 amino acids. However, net leg balance of phenylalanine was unchanged by either drink. Direct measurement of protein synthesis by tracer incorporation into muscle protein (fractional synthetic rate) revealed no changes within or between drinks. Whole‐body phenylalanine flux was significantly suppressed by each solution but to a greater extent by the branched‐chain amino acids (15% and 20%, respectively) (p<.001).Conclusions:These results suggest that branched‐chain amino acid ingestion suppresses whole‐body proteolysis in tissues other than skeletal muscle in normal men.(Journal of Parenteral and Enteral Nutrition19:47–54, 1995)