Quercetin: A flavonol widely distributed in plants. It is an antioxidant, like many other phenolic heterocyclic compounds. Glycosylated forms include RUTIN and quercetrin.Flavonols: A group of 3-hydroxy-4-keto-FLAVONOIDS.Flavonoids: A group of phenyl benzopyrans named for having structures like FLAVONES.Onions: Herbaceous biennial plants and their edible bulbs, belonging to the Liliaceae.Antioxidants: Naturally occurring or synthetic substances that inhibit or retard the oxidation of a substance to which it is added. They counteract the harmful and damaging effects of oxidation in animal tissues.Glycosides: Any compound that contains a constituent sugar, in which the hydroxyl group attached to the first carbon is substituted by an alcoholic, phenolic, or other group. They are named specifically for the sugar contained, such as glucoside (glucose), pentoside (pentose), fructoside (fructose), etc. Upon hydrolysis, a sugar and nonsugar component (aglycone) are formed. (From Dorland, 28th ed; From Miall's Dictionary of Chemistry, 5th ed)Plant Extracts: Concentrated pharmaceutical preparations of plants obtained by removing active constituents with a suitable solvent, which is evaporated away, and adjusting the residue to a prescribed standard.Flavanones: A group of FLAVONOIDS characterized with a 4-ketone.Phenols: Benzene derivatives that include one or more hydroxyl groups attached to the ring structure.Polyphenols: A large class of organic compounds having more than one PHENOL group.Catechin: An antioxidant flavonoid, occurring especially in woody plants as both (+)-catechin and (-)-epicatechin (cis) forms.