EVIDENCE THAT PALMITIC ACID IS ABSORBED AS SN-2 MONOACYLGLYCEROL FROM HUMAN-MILK BY BREAST-FED INFANTS

Milk fatty acids consist of about 20-25% palmitic acid (16:0), with about 70% of 16:0 esterified to the sn-2 position of the milk triacylglycerols. Hydrolysis of dietary triacylglycerols by endogenous lipases produces sn-2 monoacylglycerols and free fatty acids, which are absorbed, reesterified, and then secreted into plasma. Unesterified 16:0 is not well absorbed and readily forms soaps with calcium in the intestine. The positioning of 16:0 at the sn-2 position of milk triacylglycerols could explain the high coefficient of absorption of milk fat. However, the milk lipase, bile salt-stimulated lipase, has been suggested to complete the hydrolysis of milk fat to free fatty acids and glycerol. These studies determined whether 16:0 is absorbed from human milk as sn-2 monopalmitin by comparison of the plasma triacylglycerol total and sn-2 position fatty acid composition between breast-fed and formula-fed term gestation infants. The human milk and formula had 21.0 and 22.3% of 16:0, respectively, with 54.2 and 4.8% 16:0 in the fatty acids esterified to the 2 position. The plasma triacylglycerol total fatty acids had 26.0 +/- 0.6 and 26.2 +/- 0.6% of 16:0, and the sn-2 position fatty acids had 23.3 +/- 3.3 and 7.4 +/- 0.7% of 16:0 in the three-month old exclusively breast-fed (n = 17) and formula-fed (n = 18) infants, respectively. Marked differences were found in the plasma total and the 2 position phospholipid percentage of 20:4 omega 6, i.e., 11.6 +/- 0.3 and 6.9 +/- 0.6 (total), 17.7 +/- 1.4 and 9.7 +/- 0.6 (sn-2 position) and percentage of 22:6 omega 3, 4.6 +/- 0.3 and 2.1 +/- 0.3 (total), 5.6 +/- 0.6 and 2.0 +/- 0.2. (sn-2 position) for the breast-fed and formula-fed infants, respectively. These studies provide convincing evidence that 16:0 is absorbed from human milk as sn-2 monoacyl-glycerol. The metabolic significance of the differences in positional distribution of fatty acids in the plasma lipids of breast-fed and formula-fed infants is not known.