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Milk contains high levels of nutrients such as proteins, fatty acids, phospholipids, vitamins and minerals [1]. Among these nutrients, milk fat plays an important role in determining the quality and energy composition of dairy products [2]. Milk fat contains large amount of saturated fatty acids (SFAs) and unsaturated fatty acids (UFAs). Recently, evidence has shown that high levels of SFAs pose a potential risk to human health such as cardiovascular disease (CVD) [3]. In contrast, there is epidemiological evidence suggesting that dietary monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) have beneficial effects for preventing CVD by favorably affecting a number of risk factors for CVD, including plasma lipids and lipoproteins. For example, oleic acid has a protective effect against retinopathy [4]. Similarly, eico-sapentaenoic acid (EPA) and docosahexaenoic acid (DHA) play beneficial roles in preventing diabetes, atherosclerosis and arthritis [5]. Many strategies have been investigated to enhance the unsaturated fatty acid content of milk [6-8].
Milk triglycerides are derived from two sources: the biosynthesis of fatty acids and their subsequent esterification within the mammary gland and the uptake of lipids from plasma into the mammary gland [9]. Fatty acids in the mammary gland can also be generated by two pathways: short- and medium-chain fatty acids (C4-C14) can be synthesized de novo under the control of several key factors and enzymes including SREBP-1 (sterol regulatory element binding protein 1), ACACA (acetyl-coenzyme A carboxylase alpha), FAS (fatty acid synthase), and ACSS2 (acyl-CoA synthetase short-chain family member 2). Long-chain fatty acids (C > 16) are taken up from plasma via transporters such as fatty acid binding protein (FABP), fatty acid transport protein (FATP) and cluster of differentiation 36 (CD36) [10]. Acetate and 3-hydroxybutyrate are the main sources of a de novo synthesis for short-and medium-chain FAs [11]. These FAs in the mammary gland can be desaturated by stearoyl-CoA desaturase (SCD) and a synthesis of cis-9 unsaturated FA and can then be esterified to glycerol sequentially via glycerol-3phosphate acyl transferase (GPAT), acyl glycerol phosphate acyl transferase (AGPAT), and diacylglycerol acyltransferase (DGAT) [11]; subsequently, the triglycerides are secreted into the milk as fat globules. The composition of milk fatty acids is highly variable and depends on several factors such as the animal breed, the lactating stage, the dietary...