Glycerol kinetics with parenteral lipid emulsions (long-chain triglycerides, medium-chain triglycerides, and structured lipids) in rats

Several studies have reported that parenteral lipid emulsions containing medium-chain triglycerides (MCT) and structured lipids (SL) are better utilized than those containing long-chain triglycerides (LCT). The objective of this study was to test the hypothesis that parenteral LCT require more extensive modification via hydrolysis and reesterification (triglyceride-free fatty acid [TG-FFA] recycling) for effective utilization, whereas MCT and SL do not. As an index of TG-FFA cycling activity, we measured glycerol and palmitate kinetics in rats (204 to 243 g) fed parenterally one of three isocaloric (250 kcal/kg/d) isonitrogenous (1.5 g N/kg/d) diets with half of the nonprotein energy from glucose and the rest from either LCT, LCT plus MCT, or SL for 5 days. Two experiments were performed. On day 5, rats were given a 7- to 8-hour infusion of either ⁵H₂ Glycerol and 1-¹⁴C Palmitate bound to albumin to measure palmitate and glycerol kinetics (experiment 1), or U-¹³C glucose to determine the proportion of endogenous glycerol production derived from glucose (experiment 2). Data are presented as means ± SEM. Endogenous glycerol production was significantly higher with LCT (11.33 ± 2.89 mmol/kg/h) than with SL (2.91 ± 0.62 mmol/kg/h). The value for the physical mixture of LCT plus MCT (5.46 ± 1.29 mmol/kg/h) fell midway between that for LCT and SL (P = NS). There were no significant differences in palmitate kinetics or oxidation. The increased glycerol production is due to the mobilization of endogenous triglyceride and is consistent with a higher rate of TG-FFA cycling being involved in the metabolism of LCT than of SL. The energy expended in TG-FFA cycling with LCT can account for the apparent poorer caloric effectiveness of LCT when compared with SL.