Available lipid emulsions made from soybean or safflower oil are classified as long-chain triglycerides (LCT). In contrast, medium-chain triglyceride (MCT) emulsions have different physical properties and are metabolized by other biochemical pathways. To compare the differences between these two fat emulsions, the authors studied 12 surgical patients and 6 volunteers. These subjects were randomly assigned to receive parenteral nutrition with MCT or LCT emulsion. Measurement of arterial and venous concentration differences across the forearm demonstrated that muscle utilization was significantly improved with MCT administration. There was also a trend toward improved nitrogen balance in the MCT group, and less weight loss in the postoperative period also was observed in this group. During the fat clearance test, the serum ketone concentrations were significantly higher in the MCT than the LCT group. The improvement in nitrogen retention may be associated with increasing ketone and insulin levels. Fat emulsions containing 50% MCT are safe for use in parenteral nutrition and may provide an alternate fuel that improves protein metabolism.