Synthesis and secretion of fat in human milk and its role in infant development

抄録

Approximately 99% of the fat in huma milk is secreted into the alveoli by the mammary secretory epithelial cell (lactocyte) in membrane bound milk fat globules (MFG). The MFG, unlike small molecules such as lactose, have little effect on the osmotic balance between milk and blood and therefore they can be stored in large amounts in the alveolar lumen.Approximately 98% of the fat in the MFG are tri-acylglycerols (TAG). Importantly, different species, e. g. women, rabbits, cows and elephants, have distinctive combinations of fatty acids esterified as TAG in their milk.We have measured 24-h milk production, fat content and fatty acid composition at 1, 2, 4, 6, 9, and 12 months of lactation in 5 women.Mean (±SD) milk production (375.5-1: 153.5mL/24-h/breast) differed between breasts, between women and with stage of lactation (p<0.05). Whereas the fat content (35.5±7.86g/L) and the percentage composition of 18: 1n9 (32.24±3.3), 18: 2n6 (9.18±2.66), 18: 3n3 (0.76±0.21), 20: 4n6 (0.37±0.07), 22: 5n3 (0.17±0.04), and 22: 6n3 (0.2±0.07) differed only between women and with stage of lactation (p<0.05).In contrast, the amount delivered to the infant differed (p<0.05) between women only for 18: 3n3, 22: 5n3 and 22: 6n3 and no differences in amounts delivered were observed for any of these fatty acids from 1 to 12 months of lactation.Each child received a mean (±SD) of 8.27-2.84 g 18: 1n9; 2.38±0.98g 18: 2n6;194±73mg 18: 3n3;92±31mg 20: 4n6;43±14 mg 22: 5n3 and 49±21 mg 22: 6n3 every 24-h from breastmilk over the first year of life.These results indicate that variation in percentage composition of individual fatty acids (e.g.18: 2n6) does not always translate to variation in the amount delivered to the infant.
Milk fat not only accounts for approximately 50% of the infant's energy intake, but also is responsible for the supply of the essential and long-chain polyunsaturated fatty acids that are required for the optimal development of the infant.For example, arachidonic acid (20: 4n6) and eicosapentaenoic acid (20: 5n3) are essential precursors for the synthesis of prostaglandins and immunomodulatory eicosanoids.On the other hand, docosahexaenoic acid (22: 6n3, DHA) is a major polyunsaturated fatty acid in the membranes of the cerebral cortex and retina and higher intakes of DHA have been associated with higher ratings in intelligence tests particularly in children born prematurely. Fatty acids and mono-acylglycerols released by hydrolysis of TAG in the infant's digestive tract have a detergent like lytic action and inactivate enveloped viruses, gram positive and gram negative bacteria, fungi and protozoa.The membrane surrounding the human MFG contains mucin filaments that may act as a decoy to pathogenic micro-organisms (e.g.E coli).Micro-organisms recognise mucin filaments as membrane docking sites from which to launch an invasive infection and are thereby lured away from the membrane docking sites on the epithelial cells lining the infant's digestive tract.
Despite the importance of milk fat to the infant, it is the most variable component of human milk.It varies, over the course of a feed, over the course of the day, with stage of lactation, from one lactation to the next, between breasts, and between women. The major predictors of the fat content of milk over the course of a day have been shown to be the length of the interval between breastfeeds, the fat content at the end of the previous breastfeed, the amount of milk removed at the previous breastfeed and the amount of milk removed at the current feed.However, we have found that the fat content of milk is determined primarily by the amount of milk (degree of fullness) in the breast.For example, if the baby sleeps overnight, the mother's breasts will be full of milk in the morning and the fat content of fore-milk (milk obtained before a breastfeed) will be very low.