In experiments, numerous natural products have been found to inhibit xanthine oxidase in vitro or in model animals (mice, rats). These include three flavonoids that occur in many different fruits and vegetables: kaempferol, myricetin, and quercetin. More generally, planar flavones and flavonols with a 7-hydroxyl group inhibit xanthine oxidase. An essential oil extracted from Cinnamomum osmophloeum inhibits xanthine oxidase in mice. The natural product propolis from selected sources inhibits xanthine oxidase in rats; the specific substance responsible for this inhibition has not been identified, and the generality of these findings is unknown. An extract of leaves of Pistacia integerrima also inhibits xanthine oxidase at a level that appears to merit further research.
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