Theabrownins from dark tea: formation, gut microbiota-mediated biotransformation, health benefits, and potential in functional food applications

Theabrownins (TBs) are the major polyphenol-derived pigments and key bioactive components in dark tea formed through microbial fermentation or enzymatic oxidation. This review provides a comprehensive overview of the formation pathways, structural complexity, and diverse biological functions of TBs, highlighting recent advances in their production and application. Emerging evidence indicates that TBs exert significant health-promoting effects, including anti-obesity, anti-diabetic, anti-inflammatory, antioxidant, anti-cancer, anti-photodamage, and gut microbiota-modulating activities. These effects are mediated through multiple molecular mechanisms involving the regulation of bile acid metabolism, intestinal microbial composition, circadian rhythm genes, and signaling pathways such as AMPK/PGC1α, PI3K/Akt, FoxO/PPAR, FXR/FGF15, NRF2, and NF-κB. Additionally, TBs influence apoptosis, autophagy, and epigenetic modifications, thereby further contributing to their therapeutic potential. Despite promising bioactivities, challenges remain in elucidating TBs' precise molecular structures, optimizing large-scale production, and translating findings from cell and animal studies into human clinical trials. Addressing these limitations is critical for advancing TBs from functional components to evidence-based nutraceutical ingredients. Overall, this review summarizes current knowledge on TBs' bioactivities and underlying mechanisms. It also provides insights for future research and industrial development, supporting the potential of TBs as natural agents for disease prevention and human health promotion.