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Background
The transition period is considered the most challenging time in a dairy cow’s production cycle [1]. During this period, cows’ lactation energy requirements increase dramatically, while the feed intake is low and increases slowly, resulting in a state of negative energy balance (NEB). When the cow is in a severe NEB situation, a large amount of nonesterified fatty acids (NEFAs) released from the adipose tissue will go into the liver to produce energy to meet the postpartum lactation demand as well as be converted into ketone bodies or be re-esterified to triglyceride (TG) [2, 3, 4–5]. Thus, dairy cows in the early lactation stage are prone to ketosis (hyperketonemia) and fatty liver, which reduce production performance and impair health status [6, 7]. To some extent, these metabolic diseases are primarily caused by the dysregulation of lipid and glucose metabolism during the transition period [5, 8, 9].
Milk fat is one of the main nutritional components for offspring and is also one of the most complex lipids known in terms of composition and structure, which is directly related to the economic benefits of dairy farms and the healthy growth of calves. As the main component of milk fat, TG is synthesized in mammary epithelial cells from α-phosphoglycerides and fatty acids (FAs) and accounting for approximately 95% in milk fat [10]. In dairy cows, there are two pathways for the synthesis of FAs in milk, namely the de novo synthesized pathway in mammary gland epithelial cells and the direct uptake of FAs from the blood [11]. Thus, FAs exhibit a wide range of physiological activities and biological functions [12, 13]. However, disordered glucose and lipid metabolism can also negatively affect milk FA synthesis. Research has shown that the ability of cows to manage energy intake and demand during the transition period is one of the important factors determining the success or failure of later lactation performance [14]. Only cows who can successfully adapt to the onset of lactation and pass through the NEB period will have high production performance and health status for the entire lactation period [6].
Bile acids (BAs) are specific products of cholesterol catabolism in the liver and are a major component of human and animal bile [15]. There is growing evidence demonstrating...