Effects of Nonglucose Substrates (Xylitol, Medium-Chain Triglycerides, Long-Chain Triglycerides) and Carnitine on Nitrogen Metabolism in Stressed Rats

To evaluate the efficacy of nonglucose energy substrates in promoting nitrogen retention and survival in stressed states, two series of studies were done. In study 1, 50 rats underwent cecal ligation/perforation and subsequent infusion for 24 hr with one of four isocaloric (220 kcal/kg/day), isonitrogenous (1.4 g/N/kg/day), isovolemic regimens which differed in caloric source: Glucose (GLU) + long-chain triglycerides (LCT) (50%:50%), GLU + LCT + medium-chain triglycerides (MCT) (50%:32%:18%), GLU + LCT/Carnitine (10 mg/dl) or GLU + LCT + Xylitol (XYL) (33%:33%:33%). The nitrogen-sparing effect of GLU + LCT was not enhanced by the addition of carnitine to facilitate LCT mitochondrial uptake or by MCT to bypass carnitine-dependent transport. In contrast, relative to GLU + LCT GLU + LCT + XYL decreased urinary 3-methylhistidine (3MH) excretion (p less than 0.01), and enhanced nitrogen retention (p less than 0.01 vs GLU + LCT). For study 2, 24 male rats were anesthetized, cannulated for TPN, and given a 25% burn. They were then randomized into three dietary groups. The diets were isocaloric (103 kcal/kg/day) and isonitrogenous (2.0 g N/kg/day) but differed in nonprotein calorie source: GLU + LCT (51%:49%), GLU + Glycerol (51%:49%) and XYL + LCT (51%:49%). As in the septic animals, N balance was best with the xylitol regimen (p less than 0.01). The polyol, xylitol, appears to have a significant nitrogen sparing effect in stressed animals.