Bioflavonoids – or simply flavonoids – are a group of water-soluble plant pigments that are responsible for the colors of many flowers and fruits. Flavonoids are found in a wide range of foods and supplementation is usually not required by individuals eating a healthy diet.
Bioflavonoids are often called "semi-essential" nutrients because they are an important part of our regular nutritional needs and support health as anti-inflammatory, antihistaminic and antiviral agents. They possess antioxidant properties that help fight poor health and aging. Flavonoids are referred to as "biological response modifiers" because they help to modify the body's reaction to allergens, viruses and carcinogens. The flavonoids help protect vitamin C, and citrus flavonoids in particular improve the absorption of vitamin C.
Flavonoids can be divided into categories, but how to divide them is not universally agreed upon. One system breaks them into isoflavones, anthocyanidins, flavans, flavonols, flavones and flavanones. Some of the best-known flavonoids, such as genistein in soy and quercetin in onions, could be considered subcategories. Other flavonoids include catechin, hesperidin, rutin and naringin. Although they are all structurally related, their functions are different.
When doctors recommend supplementation, the most common amounts suggested are 1,000mg of citrus flavonoids or 400mg of quercetin, each taken three times per day. No side-effects have been linked to the flavonoids except for catechin, which can occasionally cause reversible fever, hives and anemia from the breakdown of red blood cells.