Ability of tannins to modulate ruminal lipid metabolism and milk and meat fatty acid profiles

Tannins are a large, diverse and complex group of phenolic compounds that may be detrimental, innocuous or beneficial to animal nutrition and health depending on a number of factors (e.g., type, amount ingested, consumer animal species or the basal diet). The ability of tannins to modulate ruminal biohydrogenation and, consequently, the fatty acid (FA) composition of milk and meat is a relatively recent finding that has attracted interest among ruminant nutritionists. In this review, we take a close look at the effects of tannins on the broadest possible range of FA, including less abundant compounds that have previously received little attention. Tannins are able to improve the concentrations of potentially beneficial FA, such as 18:3n-3, 18:2n-6, trans-11 18:1 and conjugated linoleic and linolenic acids, both in milk and meat, which may contribute to meeting consumer demand for health-promoting foods. These positive responses to tannins can be explained by their modulatory action on specific biohydrogenation steps and pathways. Thus, higher contents of dietary polyunsaturated FA in ruminant-derived products, as well as increased or decreased accumulation of other biohydrogenation intermediates (e.g., cis and trans 18:3, 18:2 and 18:1 isomers) and changes in lipids of bacterial origin (odd- and branched-chain FA and dimethylacetals), are somewhat common findings when the effects of tannins are examined. In contrast, de novo synthesized FA are less frequently affected by these plant secondary metabolites, which also applies to other FA (e.g., trans-10 18:1) that might be detrimental to animal performance (i.e., milk fat synthesis) and perhaps to human health. However, further studies are required to unravel the reasons for certain erratic responses to tannins. This paper reviews the roles of tannin chemical and structural diversity, dosage, interaction with other dietary ingredients, differences among ruminant species and variations over time in the reaction of rumen microbiota in the influences of these phenolic compounds on milk and meat FA profiles.