The metabolic consequences of critical illness: acute effects on glutamine and protein metabolism.

Net protein loss and large decreases in plasma glutamine concentration are characteristics of critical illness. We have used [2-15N]glutamine and [1-13C]leucine to investigate whole body glutamine and leucine kinetics in a group of critically ill patients and matched healthy controls. Glutamine appearance rate (Ra,Gln) was similar in both groups. However, in the patients, the proportion of Ra,Gln arising from protein breakdown was higher than in the control group (43 +/- 3 vs. 32 +/- 2%, P < 0.05). Glutamine metabolic clearance rate (MCR) was 92 +/- 8% higher (P < 0.001), whereas plasma glutamine concentration was 38 +/- 5% lower (P < 0.001) than in the control group. Leucine appearance rate (whole body proteolysis) and nonoxidative leucine disposal (whole body protein synthesis) were 59 +/- 14 and 49 +/- 15% higher in the patients (P < 0.001). Leucine oxidation and MCR were increased in the patients by 104 +/- 37 and 129 +/- 39%, respectively (P < 0.05). These results demonstrate that critical illness is associated with a major increase in protein turnover. The acute decrease in plasma glutamine concentration and the unaltered plasma Ra,Gln suggest that the increase in proteolysis is insufficient to meet increased demand for glutamine in this severe catabolic state.