Milk Fat: Origin of Fatty Acids and Influence of Nutritional Factors Thereon

Ruminant milk fat is of unique composition among terrestrial mammals, due to its great diversity of component fatty acids. The diversity arises from the eVects of ruminal biohydrogenation on dietary unsaturated fatty acids and the range of fatty acids synthesized de novo in the mammary gland. Forty to sixty per cent of milk fatty acids are long-chain (predominantly C18) fatty acids derived from the diet, dependent on the amount of fat in the diet. Fatty acids from C4 to C14 are synthesized de novo in the mammary gland whereas C16 arises from both diet and de novo synthesis. Milk fat is the most variable component of milk, both in concentration and composition. In dairy cattle, both the concentration and composition of milk fat are inXuenced by the diet. Concentration is reduced by feeding diets that contain large proportions of readily-fermentable carbohydrates (starch) and unsaturated fat. Conversely, the percentage of fat in milk can be increased by feeding rumen-inert fats. In ruminants, in contrast with nonruminants, dietary fats have little eVect on milk fat composition. Nevertheless, subtle changes in composition and manufacturing functionality can be eVected by feeding diVerent fats. Those fatty acids synthesized de novo, especially C12 to C16, and oleic acid (C18:1) show greatest variation when supplemental fats are fed. Modern developments in the manufacture of rumen-protected and rumen-inert fats, together with increased understanding of ruminal and D.L. Palmquist . Professor Emeritus, Department of Animal Sciences, Ohio Agricultural Research and Development Center/The Ohio State University, Wooster, OH 44691, USA. Advanced Dairy Chemistry, Volume 2: Lipids, 3rd edition. Edited by P.F. Fox and P.L.H. McSweeney, Springer, New York, 2006.