The Effect of Rumen-Protected Conjugated Linoleic Acid on Milk Composition and Milk Energy Output of Holstein Cows and Its Potential Mechanism

Conjugated linoleic acid (CLA) can regulate fatty acid metabolism and increase the content of unsaturated fatty acids in milk during the nutritional regulation process in dairy cows. However, its effects on milk composition and milk energy output in dairy cows remain unclear. We aimed to investigate whether RP-CLA reduces milk energy output during early lactation while lowering milk fat, as well as whether it affects milk protein synthesis and the potential mechanisms. We examined the milk composition of Holstein fresh cows at different stages by administering RP-CLA for 7 days and analyzing related gene expression in the MAC-T cell line. RP-CLA did not significantly alter milk yield, lactose, or blood ketone levels in early lactation. Starting RP-CLA from day 21 postpartum reduced milk fat by about 26% (3.74% vs 2.78%) on day 28 postpartum, slightly increased milk protein, and improved blood glucose on day 24. No such effects were seen with RP-CLA beginning on day 7 postpartum. RP-CLA supplementation saved 6.45 and 11.43% milk energy output during days 7-14 and 21-28, respectively. We hypothesize that the limited RP-CLA regulation effect during days 7-21 may relate to intense lipid mobilization under negative energy balance. In MAC-T cells under low glucose, t-10, c-12 CLA decreased the expression of <i>SREBP1</i> and <i>AMPK</i>, while increased <i>CDK1</i>, indicating its role in reducing fat synthesis, easing energy deficiency, promoting cell proliferation, and enhancing protein synthesis. Similarly, under normal glucose, t-10, c-12 CLA suppressed SREBP1 and elevated <i>CDK1</i> expression, with <i>AMPK</i> reduction only at low doses. Inhibiting milk fat synthesis with an SREBP1 inhibitor slightly upregulated <i>CDK 1</i> without significantly affecting <i>AMPK</i>. In conclusion, RP-CLA has the potential to alleviate negative energy balance in dairy cows during days 21-35 postpartum. t10, c12 CLA can inhibit milk fat synthesis by reducing the expression of SREBP1 and AMPK in mammary epithelial cells, while increasing the expression of CDK1, thereby improving the cellular energy status and promoting milk protein synthesis. The energy required for the increase in milk protein is not derived from the energy saved by the inhibition of milk fat synthesis by CLA.