Abstract: Milk fat is the most variable component subjected to dietary changes. Diets high in concentrates or containing unsaturated fatty acids always lead to Milk Fat Depression Syndrome (MFD), which is highly detrimental to the dairy industry. The theories trying to explain the MFD are mainly classified into two aspects. In one kind of theories, it is thought that milk fat precursor deficiency is the direct cause for MFD, which is proved incomplete recently. The other kind of theory considers that during rumen biohydrogenation, some unique fatty acid is formed, which inhibit mammary fatty acid synthesis including trans fatty acid theory and biohydrogenation theory. Molecular mechanism underlying mammary fat synthesis involved two key regulation proteins SREBP and S14. This paper gives a review on biohydrogenation theory.

Key words: milk fat depression; biohydrogenation; trans fatty acid; conjugated linoleic acid; spot14 