Fifteen patients undergoing gastric bypass surgery for morbid obesity received preoperatively a standard crystalline amino acid solution containing 15.6% branched-chain amino acids. During the first five postoperative days, the patients were randomized to receive one of three amino acid solutions (9.0 to 10.3 g of nitrogen per day) of different branched-chain amino acid content. The branched-chain amino acid concentrations of the mixtures were 15.6%, 50%, and 100% by weight with five patients in each group. Whole-body amino acid appearance and oxidation were estimated using a continuous intravenous infusion of L-(U-14C)-tyrosine preoperatively and on the third postoperative day. Daily postoperative nitrogen balance, fractional albumin renewal rate, and whole-body tyrosine appearance, oxidation, and incorporation into protein were not significantly different among the three groups. This study suggests that an adequate nitrogen intake of a balanced amino acid mixture, as well as a solution enriched with branched-chain amino acids, maintains protein homeostasis and supports protein synthesis similarly in well-nourished patients following major abdominal surgery. A diet containing only branched-chain amino acids in isomolar ratios was as effective at maintaining protein retention and whole-body protein synthesis and albumin renewal postoperatively when compared with a standard amino acid formula.