Diet-induced milk fat depression in dairy cows results in increased trans-10, cis-12 CLA in milk fat and coordinate suppression of mRNA abundance for mammary enzymes involved in milk fat synthesis.

Milk composition can be altered by diet, and one example is milk fat depression (MFD) in dairy cows. The biohydrogenation theory of MFD has implicated unique fatty acids formed by altered rumen biohydrogenation of PUFA; one example is trans-10, cis-12 conjugated linoleic acid (CLA). In the present study, we induced MFD with a high concentrate/low forage (HC/LF) diet and examined milk composition, milk fatty acid changes and mammary lipogenic mRNA abundance to determine the mechanism involved. The HC/LF diet reduced milk fat percentage by 25% and yield by 27% with no effect on dietary intake, milk production, protein or lactose. Milk fatty acids synthesized de novo in the mammary gland and fatty acids taken up from circulation were reduced to a similar extent (molar basis). MFD was also characterized by the appearance of trans-10, cis-12 CLA in the milk fat. We analyzed mammary mRNA abundance for lipogenic genes and detected reductions for acetyl CoA carboxylase (ACC), fatty acid synthase (FAS), fatty acyl CoA ligase, glycerol phosphate acyltransferase (GPAT) and acylglycerol phosphate acyltransferase (AGPAT). There was no effect on the milk protein gene, kappa-casein. The reductions in mRNA were also correlated with the appearance of trans-10, cis-12 CLA in the milk fat for ACC, FAS, lipoprotein lipase and GPAT. This study demonstrates that diet-induced MFD involves coordinated effects on mRNA for mammary lipid synthesis pathways, and provides support for a mechanism involving alterations in transcriptional activation of these genes.