The Effect of Rumen-Protected\nConjugated Linoleic\nAcid on Milk Composition and Milk Energy Output of Holstein Cows and\nIts Potential Mechanism

Conjugated linoleic\nacid (CLA) can regulate fatty acid metabolism\nand increase the content of unsaturated fatty acids in milk during\nthe nutritional regulation process in dairy cows. However, its effects\non milk composition and milk energy output in dairy cows remain unclear.\nWe aimed to investigate whether RP-CLA reduces milk energy output\nduring early lactation while lowering milk fat, as well as whether\nit affects milk protein synthesis and the potential mechanisms. We\nexamined the milk composition of Holstein fresh cows at different\nstages by administering RP-CLA for 7 days and analyzing related gene\nexpression in the MAC-T cell line. RP-CLA did not significantly alter\nmilk yield, lactose, or blood ketone levels in early lactation. Starting\nRP-CLA from day 21 postpartum reduced milk fat by about 26% (3.74%\nvs 2.78%) on day 28 postpartum, slightly increased milk protein, and\nimproved blood glucose on day 24. No such effects were seen with RP-CLA\nbeginning on day 7 postpartum. RP-CLA supplementation saved 6.45 and\n11.43% milk energy output during days 7–14 and 21–28,\nrespectively. We hypothesize that the limited RP-CLA regulation effect\nduring days 7–21 may relate to intense lipid mobilization under\nnegative energy balance. In MAC-T cells under low glucose, t-10, c-12\nCLA decreased the expression of <i>SREBP1</i> and <i>AMPK</i>, while increased <i>CDK1</i>, indicating\nits role in reducing fat synthesis, easing energy deficiency, promoting\ncell proliferation, and enhancing protein synthesis. Similarly, under\nnormal glucose, t-10, c-12 CLA suppressed SREBP1 and elevated <i>CDK1</i> expression, with <i>AMPK</i> reduction only\nat low doses. Inhibiting milk fat synthesis with an SREBP1 inhibitor\nslightly upregulated <i>CDK 1</i> without significantly\naffecting <i>AMPK</i>. In conclusion, RP-CLA has the potential\nto alleviate negative energy balance in dairy cows during days 21–35\npostpartum. t10, c12 CLA can inhibit milk fat synthesis by reducing\nthe expression of SREBP1 and AMPK in mammary epithelial cells, while\nincreasing the expression of CDK1, thereby improving the cellular\nenergy status and promoting milk protein synthesis. The energy required\nfor the increase in milk protein is not derived from the energy saved\nby the inhibition of milk fat synthesis by CLA.