Tea Polyphenol Epigallocatechin Gallate and the Gut–Health Axis: Unraveling Structural Characteristics, Metabolic Pathways, and Systemic Benefits

Dietary components significantly impact human health, influencing diverse physiological processes from metabolic homeostasis to cognitive function and aging. Tea, a widely consumed functional beverage rich in antioxidants, has gained attention for its health benefits. Epigallocatechin gallate (EGCG), the most abundant and bioactive catechin in green tea, is renowned for its potent biological activities. However, the direct absorption of EGCG is limited due to its low oral bioavailability, with a substantial portion reaching the colon where it interacts extensively with gut microbiota. This microbial interplay is crucial for EGCG's biotransformation and the realization of its health-promoting potential, yet the underlying mechanisms remain to be fully elucidated. This review synthesizes EGCG's structural features, metabolism, and interactions with gut microbiota, summarizing its roles in gut health and systemic effects through gut-related axes, and outlines future research. First, it elaborates EGCG's structural features, as a flavan-3-ol with a polyphenolic structure containing multiple hydroxyl groups, whose antioxidant and bioactive properties are associated with the specific arrangement of benzene rings and the gallate moiety. Second, it outlines its metabolic process, limited absorption in the small intestine, enzymatic metabolism in the small intestine and liver (including methylation, glucuronidation, and sulfation), and extensive biotransformation in the colon by gut microbiota into metabolites such as epigallocatechin and gallic acid. Third, it explores its effects on the gut, modulating gut microbiota composition by promoting beneficial bacteria and inhibiting pathogenic strains, enhancing intestinal barrier function by upregulating tight junction proteins, and promoting the production of short-chain fatty acids. Finally, it elucidates how EGCG modulates key gut-related health pathways and its broader implications for systemic health through various interconnected gut axes, including the gut-liver, gut-brain, gut-renal, and gut-lung axes, and concludes by outlining prospective research directions aimed at further elucidating the potential of EGCG in promoting health.