New developments in fat emulsions.

In Western countries, the transport of triacylglycerols by plasma triacylglycerol-rich lipoproteins (chylomicra and very-low-density lipoproteins (VLDL)) accounts for approximately 100 g lipids/d and represents a major pathway in the delivery of energy to tissues. Fat emulsions have been developed to resemble endogenous chylomicra (Wretlind, 1981). However, if the particle size of emulsion droplets is within the range of chylomicra, significant differences in composition can be found between both types of particles; indeed, no cholesteryl ester is dissolved in the triacylglycerol core of artificial particles and no apoprotein is present on their surface; their phospholipid content is higher than that of chylomicra. this difference being particularly marked in 10% emulsions. In fact, it is possible by ultracentrifugation in saline (9 g sodium chlorideA) for 30 min to demonstrate that fat emulsions are made up of not one but two different particle populations: one consisting of triacylglycerol-rich particles and the other of phospholipids. These phospholipid-rich particles, which resemble liposomes, are present in a much greater proportion in 10% than in 20% emulsions. They have a number of important metabolic features which have been recently reviewed (Carpentier, 1989). Their elimination from the plasma is much slower than that of infused triacylglycerols. High concentrations of liposomal particles substantially delay the clearance of triacylglycerols (Carpentier et al. 1987) and form phospholipid-free cholesterol complexes which accumulate in the low-density-lipoprotein (LDL) density range (Griffin et al. 1979: Untracht, 1982). Preterm neonates appear to be particularly sensitive to the presence of a high liposomal content in the emulsions. When infused at a dose of 2 g triacylglycerols/ kg per d in the parenteral regimen of low-birth-weight infants, 10% emulsion induces high plasma levels of phospholipids, free cholesterol and triacylglycerols (Haumont et al. 1989a). These effects can be substantially reduced by the use of 20% emulsions which even allow increases of parenteral lipid intake (Haumont er al. 1989h). The triacylglycerol fatty acid pattern of lipid emulsions also markedly differs from that of endogenous lipoproteins. For many years, soya-bean oil and to a lesser extent safflower oil have been used as sources of triacylglycerols. They contain exclusively long-chain triacylglycerols (LCT) and a majority of polyunsaturated fatty acids. More recently, a new emulsion has been developed, which provides an equal ponderal proportion of medium (mainly octanoate)and long-chain fatty acids, both being present in the triacylglycerol core. In vitro studies have demonstrated medium-chain triacylglycerol (MCT) emulsions to be a better substrate than LCT emulsions for hydrolysis by lipoprotein lipase and hepatic lipase. This is, at least partly, due to the much greater solubility of MCT v. LCT into phospholipid bilayers, which makes them more available for the endothelial bound