The effect of amino acid composition on the thermogenic response to amino acid infusion was studied in 20 spontaneously breathing postoperative coronary bypass patients and 6 healthy volunteers. On the 1st postoperative day, patients received either a balanced amino acid solution (2510 kJ/24 h) or an amino acid solution consisting primarily (88.8%) of branched-chain amino acids (BCAAs; 2510 kJ/24 h) for 6 h. Another group of patients receiving only hypocaloric glucose served as control subjects. The volunteers randomly received the same balanced amino acid solution (2510 kJ/24 h) or the same BCAA-enriched solution (2510 kJ/24 h) for 6 h. In the coronary bypass patients, both infusion regimens resulted in an increase in resting energy expenditure (REE) (p < 0.01). The thermogenic response to nutrition was 28.5 +/- 5.8 and 46.9 +/- 4.3% (mean +/- SE) in the patients receiving the balanced and BCAA-enriched solutions, respectively (p < 0.05). REE did not change in the control group. Alveolar ventilation increased in both groups (p < 0.05), and the change was more prominent in patients receiving the BCAA-enriched solution. PaCO2 decreased significantly in both nutrition groups. In the healthy subjects, REE increased only during the balanced amino acid infusion (p < 0.05). The thermogenic response to the balanced amino acid solution was 20.7 +/- 4.2% (p < 0.05), whereas no thermogenic response to the BCAA-enriched solution was observed (-5.6 +/- 3.3%, NS). This difference was probably due to the smaller energy cost of BCAA metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)