Quercetin in Functional Foods: Properties, Applications, Challenges, and Strategies to Improve Bioavailability

The growing interest in functional foods has increased attention to plant-derived bioactive compounds with potential health benefits. Quercetin, one of the most abundant dietary flavonols in fruits and vegetables, has attracted particular interest because of its antioxidant and anti-inflammatory activities. However, its incorporation into functional foods remains limited by poor aqueous solubility, chemical instability, limited gastrointestinal bioaccessibility, and low oral bioavailability. This review examines the physicochemical properties, natural sources, and major application barriers of quercetin, with particular emphasis on recent multi-strategy approaches, ranging from food-grade biopolymer nanoparticles and excipient emulsions to glycosylation and cyclodextrin inclusion complexation. Rather than treating improved bioaccessibility as equivalent to improved bioavailability, the review compares how these strategies affect gastrointestinal solubilisation, epithelial absorption, and systemic exposure. Current evidence indicates that these approaches can improve quercetin dispersibility, formulation stability, intestinal solubilisation, or systemic bioavailability through different mechanisms. However, evidence from human studies remains limited. Further research should therefore validate these strategies through human pharmacokinetic studies and evaluations of their application in real food products.