Lipid Metabolism in the rumen: new insights on lipolysis and biohydrogenation with an emphasis on the role of endogenous plant factors.

Diet composition is the major factor influencing the fatty acid composition of meat and milk from ruminants because the fatty acids (FA) which reach the duodenum are, at least in part, of dietary origin as well as the result of rumen microbial biohydrogenation (BH) of dietary lipids. In this review, effects of synthesis of conjugated linoleic (CLA) and linolenic (CLNA) acid isomers in the rumen, effects of the lipids in herbage, and plant endogenous factors on synthesis of nutraceutical fatty acids are discussed. Discovery of beneficial FA in ruminant products, such as CLA and other omega-3 FA, stimulated many studies in the last 20 years, including those on the roles of minor FA intermediates on rumen BH and mammary gland Metabolism. Much of this research was targeted at identifying the intermediates formed during BH as well as the rumen microbial ecology involved in these processes. However, shifting the research to feedstuff endogenous factors which influence lipolysis (LP) and losses of polyunsaturated FA in the rumen may be of interest in identifying nutritional strategies to manipulate FA profiles in ruminant products. The presence of FA with healthful properties in milk or meat from ruminants can be enhanced by inclusion of fresh forages in their diet. Hence, there is increasing interest in the crucial role of endogenous LP, plant secondary metabolites (PSM) and polyphenol oxidase (PPO) on ruminal BH. To better understand the pathways through which PSM or PPO impact FA metabolism, characterization of lipids in fresh forages suggests the important role of the diet matrix on the ruminal fate of lipids. A critical discussion of the role of odd chain branched FA (OBCFA) is also reported, including potential impacts on rumen microbial metabolism. Finally, new insights into lipid metabolism from in vitro techniques are discussed.