Free Fatty Acid Mobilization and Oxidation During Total Parenteral Nutrition in Trauma and Infection

Abstract

Free fatty acid (FFA) metabolism was studied in 18 traumatized and/or septic patients. Each patient was studied while receiving 5% dextrose (D5W) and after 4-7 days of total parenteral nutrition (TPN). Nonprotein energy during TPN was given entirely as glucose (Glucose System) or as equal portions of i.v. fat and glucose (Lipid System). Plasma FFA concentrations were in the normal range on D5W and decreased markedly with TPN. FFA turnover was higher than normal on D5W and did not decrease significantly with TPN. The poor correlation between these 2 variables emphasizes the need to perform kinetic studies to characterize FFA metabolism in trauma and sepsis. Plasma FFA oxidation and net whole body fat oxidation measured by indirect calorimetry were in the normal range on D5W, 35 and 82%, respectively, of resting energy expenditure (REE). With a glucose intake averaging 108% of REE, plasma FFA oxidation and net fat oxidation decreased to 17 and 13%, respectively, of REE. Nonprotein respiratory quotients increased only to 0.94 despite administration of glucose in excess of REE, indicating an abnormal persistence of fat oxidation. During D5W administration, plasma FFA accounted for < 1/2 of the total fat oxidation, indicating that unlabeled fat, such as tissue or plasma triglycerides not in rapid equilibrium with plasma FFA, accounted for the bulk of fat oxidation. Glucagon concentrations which were high on D5W did not decrease significantly with TPN. Insulin concentrations were normal on D5W and increased in response to TPN. The abnormal hormonal milieu may account for much of the abnormal fat metabolism. Administration of large amounts of glucose decreased FFA oxidation much more than FFA mobilization. Thus, the infused glucose acts to increase the rate of futile cycling of FFA in these acutely ill patients.