Glycerol turnover and kinetics of exogenous fat in surgical patients

In the United States, glucose has been until recently the sole non-protein energy substrate utilized in total parenteral nutrition (TPN). However, recent reports show that the administration of massive glucose loads to surgical patients cause a high incidence of pulmonary and hepatic complications. Indirect calorimetry data indicate that these hypermetabolic patients seem to utilize endogenous fat preferentially to carbohydrate. Therefore, we have undertaken some studies on the metabolism of endogenous and exogenous fat. Peripheral lipolysis - as estimated by measurements of glycerol turnover - is increased after injury and during sepsis. There is not correlation between the turnover of glycerol and the plasma concentration in surgical patients. High glucose intake induces a marked decrease in the glycerol turnover of nutritionally depleted subjects but has only a slight effect on the lipolytic rate of injured and septic patients. The rates of free fatty acid turnover and oxidation are elevated in surgical patients. High carbohydrate loads reduce FFA oxidation rate less in hypermetabolic patients than in depleted subjects. The utilization of exogenous fat was studied by associating an estimation of the oxidation rate to the measure of the clearance after the injection of 14C -Intralipid in surgical patients. Both oxidation and clearance rates are increased in these patients in relation with the severity of the injury. A dissociation between the variations of the clearance and the oxidation rates was observed when TPN was given to surgical patients. In such conditions, the clearance rate does not indicate the further utilization of a fat emulsion. Administration of TPN containing exogenous fat seem to induce less pulmonary and hepatic complications than TPN with glucose alone. In conclusion, kinetic measurements are needed for studying the metabolism of endogenous fat in surgical patients. An increased utilization of endogenous and exogenous fat takes place in these patients. Including exogenous fat as part of their parenteral diet is indicated but the optimal proportion of glucose and fat has not been precisely determined yet.