Targets and procedures for altering ruminant meat and milk lipids

Beef and dairy products suffer from a negative health image, related to the nature of their lipid fraction. Rumen lipid metabolism involves the presen ce of saturated lipids in ruminant tissues. Lipolysis, fatty acid biohydrog enation and formation of microbial fatty acids in the rumen and their effec ts on rumen outflow of fatty acids are discussed. Special emphasis is given to the formation of trans-fatty acids and the possibilities of decreasing biohydrogenation. Small differences in intestinal digestibilities of fatty acids are mentioned, followed by a discussion on transfer of absorbed fatty acids into milk and adipose tissue lipids. The preferential retention of p olyunsaturated fatty acids as well as the balance between synthesis and inc orporation of fatty acids in tissues is described. Dietary means for the mo dification of milk fat are listed, with special emphasis on the possibiliti es for enrichment in polyunsaturated fatty acids and the presence of conjug ated Linoleic acids. A description of the nature and development of fat dep ots in beef cattle is followed by a discussion of breed, conformation and f eed effects on adipose tissue distribution and fatty acid composition. Spec ial emphasis is given to the very lean Belgian Blue double-muscled breed. T he review ends with a consideration of the limits to the modification of ru minant fats, involving considerations of consumer acceptance as well as ani mal welfare and environmental effects.