Biochemical precursors of ruminant milk fat biosynthesis:
In ruminant mammary epithelial cells, approximately $50\%$ of fatty acids (all $C_4$ to $C_{14}$ short- and medium-chain fatty acids, and ~50% of palmitic acid $C_{16:0}$) are synthesized de novo in the cytosol:

• Unlike non-ruminants (which utilize glucose via ATP-citrate lyase), ruminants lack functional ATP-citrate lyase and NADP-malate dehydrogenase in the mammary gland.
• Instead, ruminants utilize blood-borne ruminal fermentation products: Acetate ($C_2$) provides roughly $75\%$ of the carbon for de novo synthesis via acetyl-CoA carboxylase and fatty acid synthase, while Beta-hydroxybutyrate (BHBA, $C_4$, produced by rumen epithelium from butyrate) provides the initial 4-carbon priming unit for approximately $25\%$ of fatty acid chains.