Lack of Correlation Between Nitrogen Balance and Serum Insulin Levels During Protein Sparing with and Without Dextrose

This study tested the hypothesis that the decreased insulin levels observed during protein sparing therapy are the major factor accounting for the favorable nitrogen balance seen in surgical patients receiving amino acid infusions. Eight morbidly obese patients received amino acid infusions, with and without added dextrose, for sequential 4-day study periods after major abdominal surgery, in a crossover design. Daily infusion of 3 liters of 5% amino acids increased lipolysis compared to a dextrose control period. This was evidenced by a small rise in free fatty acid levels from 0.76 ± 0.10 to 0.85 ± 0.05 mM and a rise in ketone body levels from 0.15 ± 0.02 to 0.44 ± 0.04 him (P < 0.01). Serum insulin levels fell from 33.3 ± 5.9 to 22.3 ± 2.6 μU per ml, and nitrogen balance improved from −12.3 ± 4.7 to −1.3 ± 1.8 g per day (P < 0.001). The addition of 50 g of dextrose to each liter of amino acid solution resulted in the return of insulin, free fatty acid, and ketone body levels to values noted during the dextrose infusion period, but did not affect nitrogen balance. The order of the amino acid and the amino acid plus dextrose infusion periods did not affect the results. Glucagon and growth hormone levels did not differ significantly between study periods. The improved nitrogen balance noted during peripheral amino acid infusion is not attributable to insulin changes alone, but is related both to the availability of an adequate exogenous supply of essential amino acids, and the ability of the body to mobilize endogenous lipid supplies.