Flavonoids are phytonutrients with powerful antioxidant properties. They are abundant in many fresh fruits and vegetables. They were once lumped together as "Vitamin P," but researchers have discovered there are more than 1,500 different substances that can be called flavonoids. They fall into three general categories:

• Flavones  -- including the flavonoid apigenin, which is found in chamomile
• Flavonols -- such as quercetin, in grapefruit; rutin, in buckwheat; and ginkgoflavonglycosides, in gingko biloba.
• Flavanones -- hesperidin in citrus fruits; silybin in milk thistle

The biologic activities of flavonoids include action against allergies, inflammation, free radicals, hepatotoxins, platelet aggregation, microbes, ulcers, viruses, and tumors. Flavonoids also inhibit specific enzymes. For example, flavonoids block the angiotensin-converting enzyme (ACE) that raises blood pressure. By blocking the "suicide" enzyme cyclooxygenase that breaks down prostaglandins, they prevent platelet stickiness and hence platelet aggregation. Flavonoids also protect the vascular system and strengthen the tiny capillaries that carry oxygen and essential nutrients to all cells.

Additionally, flavonoids can block the enzymes that produce estrogen, thus reducing the risk of estrogen-induced cancers. One way they do this is by blocking estrogen synthsase, an enzyme that works overtime in binding estrogen to receptors in several organs. Although their way of doing so is not yet fully understood, flavonoids also appear to retard development of cataracts in individuals with inborn errors in sugar metabolism such as diabetes. Cataracts can be a complication of diabetes because diabetics, unable to metabolize sugar normally, build up damaging levels of "alcohol sugars." These in turn cause clouding of the lens of the eye (cataract). It is suspected that flavonoids prevent cataracts by blocking aldose-reductase, a digestive enzyme that can convert the sugar galactose into the potentially harmful form of galacticol.