Quercetin as a Multifunctional Flavonol: Molecular Insights and Therapeutic Applications.

Quercetin is one of the most abundant plant flavonoids and an intensively studied bioactive compound belonging to the class of functional heterocyclic compounds with outstanding biological activities, due to its numerous biological, pharmacological and therapeutic effects. This paper presents the main information on the chemical structure, dietary sources, absorption, metabolism and bioavailability of quercetin, highlighting the role of the intestinal microbiota in the formation of metabolites responsible for an important part of its effects. The limitations related to stability and bioavailability are analyzed as well as modern formulation strategies and advanced delivery systems aimed at improving the efficacy of the compound. The major biological activities of quercetin are also summarized, including antioxidant, anti-inflammatory, immunomodulatory, antiviral, cardioprotective, neuroprotective, metabolic and anticancer effects. Unlike previous reviews, which have generally addressed these topics separately, the present work integrates the gut-microbiota-mediated metabolism, cellular senescence, advanced delivery systems, combined therapies, and safety and regulatory considerations within a unified translational framework. Overall, this review emphasizes the need to relate the multifunctional biological potential of quercetin to its pharmacokinetic limitations, formulation performance and clinically achievable exposure while highlighting the need for standardized formulations and further clinical studies to validate its therapeutic benefits.