Evaluation of amino acid infusions as protein-sparing agents in normal adult subjects.

Blackburn et al. (1, 2) reported nitrogen equilibrium and/or positive balance in surgical patients maintained on a 3% amino acid solution at a level of 1 g/kg body weight per day. We have measured nitrogen balance and fat mobilization in six normal adult volunteers durig the infusion of two compositionally different amino acid solutions at 1 g/kg body weight per day, and compared these to values noted during isocaloric infusion of glucose solutions. Nitrogen balance was negative during all infusion periods, but was significantly less negative (P smaller than 0.01) during amino acid infusion periods (--5.8 plus or minus 1.1 g/day) than during glucose infusion (--9.8 plus or minus 1.8 g/day). No significant differences in nitrogen balance were noted between amino acid infusion periods, despite compositional differences in solutions. Elevated plasma levels of free fatty acids (0.71 plus or minus 0.37 mM) and ketone bodies (1.3 plus or minus 0.3 mM) observed during amino acid infusion periods reflected the availability of body lipids as energy substrate. The disparity between our results and those reported by Blackburn et al. (1, 2) may in part reflect the previous nutritional debilitation of the surgical subjects studied by these investigators.