Excipient Nanoemulsion-Based Enhancement of Spinach ( <i>Spinacia oleracea</i> ) Quercetin Bioavailability and Antioxidant Efficacy: A Comprehensive In Vitro–In Vivo Evaluation of Gastrointestinal Fate

Dietary quercetin, derived from fruits and vegetables, exhibits multiple health benefits. However, its efficacy is strictly limited by its poor bioavailability. In this study, food-grade excipient nanoemulsions were coingested with spinach to modulate quercetin's gastrointestinal fate, enhancing its bioavailability and antioxidant activity. Digestion, biodistribution, and absorption were assessed using the INFOGEST in vitro model and an in vivo rat study in conjunction with FRAP analysis. Compared to the control, excipient nanoemulsions significantly increased in vitro quercetin bioaccessibility by 1.73-fold and in vivo bioavailability by 46.6% (<i>P</i> < 0.05). Moreover, there was a 1.47-fold increase in the in vivo antioxidant activity. These enhancements were attributed to the excipient nanoemulsions' ability to increase solubilization via mixed micelles, protect quercetin from degradation, and prolong intestinal residence, thereby improving epithelial absorption. Overall, this study shows how the bioavailability and bioactivity of health-promoting molecules in fruits and vegetables could be improved by using excipient-based strategies.