Condensed Tannins as Antioxidants in Ruminants—Effectiveness and Action Mechanisms to Improve Animal Antioxidant Status and Oxidative Stability of Products

SIMPLE SUMMARY: Condensed tannins (CTs) are secondary plant metabolites known for their antinutritional properties but also for their beneficial attributes for animal health and food quality, including antioxidant activity. Condensed tannins sources, such as plants and agro-industrial by-products or extracts prepared from these vegetal materials, have been used in ruminant diets to improve the animal antioxidant status and the oxidative stability of their products. However, this nutritional strategy has shown inconsistent results. Furthermore, unlike other phenolic compounds with low molecular weight, CTs are high molecular weight oligomers and polymers with poor bioavailability, which limit their absorption into circulation and direct antioxidant effect in living animals and post-mortem. Therefore, the action mechanism by which dietary CT exerts an antioxidant effect on ruminants is poorly understood. So, this review briefly presents the chemical structure of tannins, with particular emphasis on CT chemical structure, summarizes several studies focused on the effect of dietary CT sources on ruminants’ antioxidant status and oxidative stability of their products, and discusses the possible action mechanisms by which CT can exert such effects. ABSTRACT: Condensed tannins (CTs) are widely distributed in plants, and due to their recognized antioxidant activity are considered as possible natural antioxidants for application in ruminant diets. A wide range of CT-rich sources has been tested in ruminant diets, and their effects on animal antioxidant status and oxidative stability of their products are reviewed in the present work. Possible mechanisms underlying the CT antioxidant effects in ruminants are also discussed, and the CT chemical structure is briefly presented. Utilization of CT-rich sources in ruminant feeding can improve the animals’ antioxidant status and oxidative stability of their products. However, the results are still inconsistent. Although poorly understood, the evidence suggests that CTs can induce an antioxidant effect in living animals and in their products through direct and indirect mechanisms, which can occur by an integrated and synergic way involving: (i) absorption of CTs with low molecular weight or metabolites, despite CTs’ poor bioavailability; (ii) antioxidant action on the gastrointestinal tract; and (iii) interaction with other antioxidant agents. Condensed tannins are alternative dietary antioxidants for ruminants, but further studies should be carried out to elucidate the mechanism underlying the antioxidant activity of each CT source to design effective antioxidant strategies based on the use of CTs in ruminant diets.