This thesis addresses the valorization of agri food co/by-products and former food products (FFPs) for their introduction into animal feed within a circular economy framework. Chapter 1 validated an HPLC-UV method for quantifying theobromine and caffeine, and assessed cocoa hulls (CHs) compliance with EU feed limits, while also characterising polyphenols and antioxidant potential; Chapter 2 profiled FFPs for nutrient composition and methylxanthine levels to estimate methylxanthines exposure in target animals and provide a safety assessment for FFP inclusion in animal diets; Chapter 3 evaluated hemp (Cannabis sativa L.) co-products for total phenolic content (TPC), antioxidant activity following green solvent extraction and ex-vivo digestion, and identified main phenolic compounds by HPTLC; and Chapter 4 produced a proposal for a harmonized feed-consumption database built on a standardized feed classification, including a model data structure and two applied case studies (genetically modified feed and contaminants). Results show that CHs exhibit positive polyphenol-linked antioxidant activity, but also contain theobromine at levels that constrain safe inclusion rates in complete feeds. FFPs were confirmed as nutritionally valuable and generally compliant with EU maximum limits when included at practical levels in livestock diets. However, species and stage-specific adjustments are required to avoid methylxanthine overexposure. Hemp co-products showed appreciable TPC and in vitro/ex vivo antioxidant activity, with extraction solvent and digestion affecting bioaccessibility. The database proposal demonstrates feasibility and practical value for harmonised exposure assessments. Overall, the findings support targeted and regulated reuse of agri food co/by-products and FFPs, but also highlight toxicological monitoring needs. A recommendation for the development of a harmonised animal consumption database is also provided for a potentially more harmonised risk assessment, which could also facilitate the safety assessment of circular feed.