Dietary triacylglycerol structure and its role in infant nutrition.

Human milk TG are a remarkable example of stereo-specific positioning of fatty acids with structures that are highly conserved and unusual. Not only does human milk contain high amounts of fat and 16:0, but ~70% of the 16:0 is esterified at the TG stereo-specifically numbered (sn)-2 position, with preferential positioning of 18:1(n-9) and 18:2(n-6) at the TG sn-1,3 positions. The milk TG structures and digestive lipases combine to enable efficient digestion and absorption of 16:0 by conserving 16:0 in sn-2 monoacylglycerols, which are absorbed, reassembled, and secreted in plasma conserving the original milk TG configuration; these studies are reviewed in this article. The reason why the mammary gland invests in enzymes to provide the infant with 20-25% milk fatty acids as 16:0 rather than selecting against 16:0 is unknown, yet likely has a purpose given the mammary gland capacity for 10:0, 12:0, and 14:0 synthesis. Recent advances in the development-, tissue-, and species-specific activity of enzymes of TG synthesis and knowledge that dietary TG structures are maintained postabsorption suggest that the purpose of the milk TG structures is more sophisticated than simply avoiding 16:0 malabsorption. The overall aim is to expand consideration of fatty acids in the infant diet from a simple view of average fatty acid compositions to the complex lipids and molecular structures in which fatty acids are provided to tissues during early life and the biology through which the unique features of human milk enable the infant to grow and thrive on a high-fat, high-saturated-fat milk diet.