Cows' milk fat and human nutrition.

Composition of milk fa t Cows’ milk fat consists mainly of triglycerides (98-99 g/100 g) together with smaller amounts of phospholipids (0 .9 ~ / I W g), cholesterol (0.3 g/100 g), carotenoids (6-7 pg/g), vitamin A (6-8 pg/g), vitamin D (0-5 pg cholecalciferoV1 oo g) and trace concentrations of other fat-soluble substances such as lanosterol, P-sitosterol, saturated hydrocarbons, squalene, glyceryl ethers and vitamin K (Kon, 1972; Kuzdzal-Savoie, Kuzdzal, Langlois & Trehin, 1973). The composition of human milk fat is similar to that of cows’ milk fat. Many analyses have been reported for the fatty acid compositions of cows’ and human milk fat, but the representative values given (as weight percentages) by Patton, McCarthy, Evans & Lynn (1960) and Breckenridge, Marai 8z Kuksis (1969) will be used in the following comparison. Saturated acids from 4:o to IO:O account for about 9% of the fatty acids in cows’ milk fat, but are not normally detected as components of human milk fat. In cows’ milk fat the contents of 12:o (3%) and 16:1 ( I % ) are usually somewhat smaller, and the content of 14:o (12%) is usually somewhat greater than in human milk fat ( I Z : ~ , 5.6%, 16:1, 3.3%; 14:0, 7.0%). The concentrations of 16:o (25%) and 18:1 (310/0) in cows’ milk fat produced in the summer months are similar to the corresponding concentrations observed in human milk fat, but cows’ milk fat produced in the winter months contains greater concentrations of 16:o (3470) and smaller concentrations of 18:1 (22%). Invariably, cows’ milk fat contains greater concentrations of 18:o (14%) but much smaller concentrations of 1 8 2 (1 .6%) than does human milk fat (18:0, 6%; 18:2, 10%). Human milk fat may contain up to I% 20:4 whereas cows’ milk fat contains only trace concentrations of this tetraenoic acid. Because of the importance of certain unsaturated fatty acids in nutrition and metabolism it is necessary to examine in some detail the nature of the unsaturated fatty acids in cows’ milk fat. The diet of the cow normally contains appreciable amounts of oleic acid (18: I cis-9 or 18: I (n-9)), linoleic acid (18:2 cis-g,cis-12 or 1 8 2 (72-6)) and linolenic acid (18:z cis-g,cis-15 or 18:3 (n-3)) but these dietary unsaturated acids are extensively hydrogenated by the microorganisms in the rumen. The mechanisms involved in these ruminal biohydrogenation processes are complex and result in the formation of a variety of trans isomers such as 18:2 c i s -g , t rans-~~, 18:z trans-II,cis-IS, 18:1 trans-11 and 18:1 tram-9 which are absorbed from the intestine and are incorporated into ruminant tissue and milk lipids (see reviews by Dawson & Kemp, 1970; Viviani, 1970; Moore, 1974). The concentration of trans isomers in cows’ milk fat varies considerably with the diet and the efficiency of the hydrogenation process operating in the rumen under a