The influence of positional distribution of fatty acids in native, interesterified and structure-specific lipids on lipoprotein metabolism and atherogenesis

The arrangement of fatty acids (FA) on the glycerol backbone of triacylglycerols (TAG) among natural fats has been indicated to be responsible for specific effects on lipoprotein metabolism and atherogenesis which may not be predicted from the unsaturated index alone. Typically, hypercholesterolemic animal fats such as bovine milk fat and lard contain mainly saturated FA on the sn-2 position. In contrast, in most vegetable oils such as cocoa butter unsaturated FA occupy the sn-2 position and saturated FA are usually located in the sn-1 and sn-3 positions. Human and animal feeding studies are discussed in which the TAG stereospecific composition of the fed fats has been altered via chemical or enzymatic interesterification. These processes either rearrange FA in each TAG position to one third of its total content within a fat, or generate novel synthetic fats from the interchange of FA between discrete TAG molecules from different fat sources. Such studies have shown effects of interesterification of milk fat, lard, and peanut oil on fat absorption, plasma cholesterol, and TAG concentrations and atherogenesis. Kinetic rat studies using radiolabeled FA and cholesterol have indicated a slowed chylomicron metabolism in structure-specific TAG containing a saturated FA at the sn-2 position. On the other hand, negligible effects of TAG structure on chylomicron and lipoprotein metabolism have been observed in recent humans and animal studies from the feeding of interesterified forms of palm oil in which palmitic acid is enriched at the sn-2 position. Metabolic parameters not measured in these investigations, however, such as alterations in platelet aggregation activity and in free cholesterol concentrations at the level of serum and liver have been noted from other interesterified palm oil feeding studies pointing to the need for further research. The role of TAG structure in natural, interesterified and structure-specific fats on FA and cholesterol absorption, chylomicron clearance, and lipoprotein metabolism is discussed as well as the confounding aspects that need consideration in terms of these studies.