Distribution of trans-octadecenoic acid in the major glycerolipids of human milk.

The intramolecular distributions of fatty acids in triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine of human milk were determined. Milk donated from 10 mothers during their 2nd week of lactation was analyzed. Based on pancreatic lipase digestion of milk triacylglycerols, the 14:0 and 16:0 fatty acids were observed predominantly in the sn-2 position, while 18:0, 18:1, and 18:2 were observed predominantly in the sn-1,3 positions. The average trans-18:1 content of the triacylglycerols was 3.4% and was greater in the sn-1,3 positions. Phosphatidylcholine and phosphatidylethanolamine were digested by phospholipase A2. As in other tissues, saturated fatty acids were esterified at the sn-1 position and polyunsaturated fatty acids at the sn-2 position of the phospholipids. The proportion of trans-18:1 averaged 2.5% in phosphatidylcholine and 3.7% in phosphatidylethanolamine. The proportion of trans-18:1 was slightly greater in the sn-2 position of the phospholipids. In conclusion, there appeared to be a reverse positional distribution of saturated and polyunsaturated fatty acids in triacylglycerol compared to phosphatidylcholine and phosphatidylethanolamine. When compared to other mammalian tissues, there were only small differences observed in trans-18:1 content between positions within a glycerolipid class and between glycerolipids of human milk.