Nutritional factors affecting the fatty acid composition of bovine milk.

The predominant fatty acids in milk are the long-chain fatty acids myristic, palmitic and stearic. These saturated fatty acids account for 75% of the total fatty acids, with a further 21% occurring as monounsaturated fatty acids of which the most prevalent is oleic acid. Only 4 g/100 g of the milk fatty acids are polyunsaturated, occurring mainly as linoleic and linolenic acids. All milk fatty acids are derived, almost equally, from either de novo synthesis or directly from preformed fatty acids in the diet. There are four main dietary sources of fatty acids: forages, oilseeds, fish oil and fat supplements. The digestive tract exerts a profound influence on the fate of dietary fatty acids. The short-chain saturated free fatty acids are absorbed through the walls of the rumen or abomasum into the bloodstream. The medium- and longer-chain saturated fatty acids pass into the small intestine, diffuse across the membrane wall where they are incorporated into lipoproteins and enter the bloodstream via the lymphatic system. The majority of unsaturated fatty acids are extensively hydrogenated in the rumen. However, recent work has shown that the levels of certain saturated fatty acids can be reduced and the levels of oleic, linoleic and linolenic fatty acids increased by feeding oilseeds rich in mono- or polyunsaturated fatty acids. In addition, work reported here has confirmed that eicosapentaenoic and docosahexaenoic acids can be transferred to milk when a diet containing fish oil is fed, but the transfer efficiencies are low.