Antioxidant and Health-Related Effects of Tannins: From Agri-Food By-Products to Human and Animal Health

<b>Background:</b> Agri-food by-products are increasingly recognized as valuable sources of tannins, whose antioxidant properties represent the primary driver of their biological activity across human and animal health. The strong redox-modulating capacity of condensed and hydrolysable tannins provides a unifying mechanistic explanation for their effects on inflammation, metabolism, gut integrity and neuroprotection. <b>Methods:</b> This narrative review synthesizes evidence obtained through a structured literature search across major databases, selecting studies that investigated antioxidant mechanisms of tannin-rich matrices from plant- and processing-derived residues. <b>Results:</b> Condensed tannins, particularly proanthocyanidins, consistently display potent antioxidant activity through radical scavenging, metal chelation and activation of endogenous defenses, thereby underpinning their anti-inflammatory, anti-ischemic, neuroprotective and metabolic actions. Hydrolysable tannins similarly exert strong antioxidative effects that support antimicrobial activity, enzyme modulation and protection against neuroinflammation. In animals, the antioxidant capacity of tannins translates into improved oxidative balance, enhanced immune status, reduced tissue damage, better feed efficiency and mitigation of oxidative stress-linked methane emission pathways. <b>Conclusions:</b> Antioxidant activity emerges as the central, cross-species mechanism through which tannins mediate diverse health benefits. Tannin-rich agri-food by-products therefore represent promising sustainable antioxidant resources, although their efficacy remains influenced by tannin class, degree of polymerization and dosage, warranting further mechanistic and translational research.