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In an attempt to develop strategies for enhancing the nutritional value of sheep milk fat, dairy ewe diet was supplemented with 3 incremental levels of marine algae (MA), in combination with sunflower oil, to evaluate the effects of these marine lipids on milk fatty acid (FA) profile and animal performance. Fifty Assaf ewes in mid lactation were distributed in 10 lots of 5 animals each and allocated to 5 treatments (2 lots per treatment): no lipid supplementation (control) or supplementation with 25 g of sunflower oil/kg of DM plus 0 (SO), 8 (SOMA(1)), 16 (SOMA(2)), or 24 (SOMA(3)) g of MA (56.7% ether extract)/kg of DM. Milk production and composition, including FA profile, were analyzed on d 0, 3, 7, 14, 21, and 28 of treatment. Neither intake nor milk yield were significantly affected by lipid addition, but all MA supplements decreased milk fat content from d 14 onward, reaching a 30% reduction after 28 d on SOMA(3). This milk fat depression might be related not only to the joint action of some putative fat synthesis inhibitors, such as trans-9,cis-11 C18:2 and probably trans-10 C18:1, but also to the limited ability of the mammary gland to maintain a desirable milk fat fluidity, that would have been caused by the noticeable increase in trans-C18:1 together with the lowered availability of stearic acid for oleic acid synthesis through Delta(9)-desaturase. Furthermore, all lipid supplements, and mainly MA, reduced the secretion of de novo FA (C6:0-C14:0) without increasing the yield of preformed FA (>C16). Supplementation with sunflower oil plus MA resulted in larger increases in cis-9,trans-11 C18:2 than those observed with sunflower oil alone, achieving a mean content as high as 3.22% of total FA and representing a more than 7-fold increase compared with the control. Vaccenic acid (trans-11 C18:1) was also significantly enhanced (on average +794% in SOMA treatments), as was C22:6 n-3 (DHA) content, although the transfer efficiency of the latter, from the diets to the milk, was very low (5%). However, the highest levels of MA inclusion (SOMA(2) and SOMA(3)) reduced the milk n-6:n-3 ratio, but MA supplements caused an important increase in trans-10 C18:1, which would rule out the possibility that this milk has a healthier fat profile before determining the specific role of each individual FA and ensuring that this trans-FA is at least innocuous in relation to cardiovascular disease risk.

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17Background: Milk fat intake is often associated with a high risk of suffering from 18 cardiovascular disease (CVD) due to its high saturated fat content. However, not all 19 saturated fatty acids (SFA) are equal and they present structural differences that 20 promote distinct effects on the biological processes. In addition, there is a growing 21 scientific consensus pointing to dairy fat as a natural source of bioactive components. 22 23 Scope and Approach: The present review provides the most recent knowledge on the 24 bioactive properties of fatty acids detected in dairy products and their potential effects 25 on consumer health. The metabolic processes that involve these fatty acids and serious 26 chronic diseases such as CVD, obesity, diabetes or cancer are explained and discussed 27 throughout the text based on in vitro, animal and human studies. Moreover, information 28 gaps are highlighted to inspire further research in the field. 29 30 Key Findings and Conclusions: Recent investigations support that milk SFA should no 31 longer be considered as a single group in terms of metabolism or negative effects in 32 case of excess. Even they suggest that individual SFA possess specific properties 33 associated with important physiological functions. Whole dairy products would also 34 promote human health due to the presence of certain bioactive fatty acids. Among them, 35 it is worth mentioning the maintenance of gut microbiota and weight control from short 36 and medium-chain SFA, the essential role of branched-chain SFA in gut health at birth 37 and the prevention of chronic inflammatory diseases by vaccenic and rumenic acids.38 39 Keywords: bioactive lipids, dairy fat, human health, trans fat, conjugated linoleic acid 40 41 42 48 total fat, while the remaining 2% comprises diacylglycerides, monoacylglycerides, free 49 fatty acids (FFA), phospholipids, sterols and hydrocarbons. TAG composition is 50 extremely complex as more than 400 different fatty acids (FA) can be esterified in the 51 three positions (sn-1, sn-2 and sn-3) of the glycerol backbone at different 52 concentrations, which mainly depend on ruminant diet and its lactation stage (Jensen, 53

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