Quercetin belongs to a class of water-soluble plant pigments called flavonoids. Quercetin acts as an antihistamine and has anti-inflammatory properties. As an antioxidant, it protects LDL cholesterol (the ¡°bad¡± cholesterol) from becoming damaged. A variety of evidence indicates that quercetin possesses potent antioxidant properties. Cardiologists believe that damage to LDL cholesterol is an underlying cause of heart disease. Quercetin blocks an enzyme that leads to accumulation of sorbitol, which has been linked to nerve, eye, and kidney damage in those with diabetes. However, no human research has demonstrated these actions of quercetin in people with diabetes patients.
Quercetin is considered a phytoestrogen (i.e., a plant substance with similar functions as that of estrogen). Some phytoestrogens are believed also to have antiestrogenic effects that might lead to reduced risks of certain cancers. Quercetin was found to have this antiestrogenic activity, by inhibiting breast cancer cells in a test tube.