Quercetin is one of the most common flavonoids, isolable from many plant species (horse-chestnut, marigold, hawthorn, camomile, hypericum and ginko biloba). Foods rich in quercetin are: caper, leist, red grapes (and red wine), red onion, green tea, blueberry, apple, propolis and celery. Considered a natural inhibitor of various enzymes and therefore studied in the field of oncology. It is a natural antioxidant.