Origin of Fatty Acids and Influence of Nutritional Factors on Milk Fat

Ruminant milk fat is a complex lipid due to the large impact of the rumen on absorbed nutrients and unique aspects of the physiology of milk fat synthesis in the mammary gland. Approximately 60% of milk fatty acids originate from the blood and include a large amount of saturated and trans fatty acids that originate from microbial metabolism of dietary unsaturated fatty acids and odd-and branched-chain fatty acids from microbial synthesis. The remaining fatty acids are made in the mammary gland and include a short-and medium-chain fattyacids (<16 carbons) due to unique characteristics of the de novo synthesis pathway. Milk fat is the most variable component of milk and is largely influenced by the diet. Milk fat can be reduced by ruminal synthesis of bioactive trans fatty acids and can be increased by feeding certain rumen-inert fats. Milk fat is also highly heritable and follows a seasonal pattern. Past research provides a strong understanding of the biochemistry and physiology of milk fat synthesis that are reviewed. However, the complexity of the rumen microbial ecosystemmakes it difficult to predict the impact of diet and other factors on rumen metabolism of unsaturated fatty acids and resulting effects on milk fat synthesis.