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The History, Mechanism, and Modification of Milk Fat Synthesis in Ruminants
Milk fat is economically valuable and is important to many dairy products. Diet-induced milk fat depression (MFD) was first described over a century ago and remains a common problem observed under both intensive and extensive management. For decades theories proposing a shortage of substrate for milk fat synthesis were popular, but failed to withstand rigorous mechanistic investigation. The biohydrogenation theory fifteen years ago established that MFD is caused by an inhibition of mammary synthesis of milk fat by specific fatty acids produced as intermediates in ruminal biohydrogenation. During MFD, lipogenic capacity and transcription of key lipid synthesis genes in the mammary gland are down-regulated in a coordinated manner. Mechanistic work has established that expression of sterol response element-binding protein 1 (SREBP1) and Spot 14 during MFD. Molecular investigation in multiple research groups provides strong evidence for SREBP1 as a central signaling pathway in the regulation of mammary FA synthesis. In addition, SREBP1c and Spot 14 knock-out mice exhibit reduced milk fat similar to the magnitude and pattern of MFD in the cow. Application of molecular biology approaches has provided the latest chapter in the regulation of milk fat synthesis. Although our understanding of the causative factors and mechanism has made great strides diet-induced milk fat depression remains an issue because of the complexity of rumen fermentation and the need to feed high energy diets.
Keywords: Milk fat depression, conjugated linoleic acid