Beyond Antioxidants: The Microbial Metabolic Landscape of Anthocyanins and Their Downstream Health Implications

Background/Objectives: Anthocyanins are dietary pigments associated with reduced risk of chronic diseases, yet their low systemic bioavailability challenges the traditional direct antioxidant hypothesis. This review aims to reconceptualize anthocyanin bioactivity by proposing the gut microbiome as a key mediator that biotransforms these compounds into bioactive metabolites responsible for systemic health effects. Methods: This review synthesizes evidence on the microbial metabolism of anthocyanins and includes a structured appraisal of the literature using an evidence evaluation framework analogous to GRADE, focusing on their transit to the colon, enzymatic biotransformation by gut microbiota, and resulting production of phenolic metabolites such as protocatechuic acid (PCA). It also examines the role of specific bacterial taxa (e.g., Bifidobacterium and Lactobacillus) in enhancing bioavailability and explores the downstream cellular pathways modulated by these metabolites. Results: Gut microbiota convert anthocyanins into smaller phenolic metabolites such as PCA, syringic acid, gallic acid, and other respective metabolites, which achieve plasma concentrations up to 100-fold higher than parent compounds and can cross the blood–brain barrier. These metabolites exert systemic effects by modulating key signaling pathways (NF-κB and Nrf2) and restoring redox homeostasis. Additionally, beneficial gut bacteria enhance anthocyanin bioavailability and support the production of short-chain fatty acids (SCFAs). Conclusions: Systemic health benefits of anthocyanins are largely mediated by gut microbiota through the generation of bioactive metabolites. This microbiota-driven process redefines the mechanistic understanding of anthocyanin action and highlights the microbiome as a critical determinant of their efficacy in preventing cardiometabolic and neurodegenerative diseases.