Bioflavonoid is a generic term used to describe biologically active members of the group of plant-derived compounds known as flavonoids. Bioflavonoids were first discovered in 1936 by Nobel-prize winning scientist and vitamin C research pioneer Albert Szent-Gyorgi, who originally named the group of compounds "vitamin P." Since then, over 4,000 flavonoids have been identified and classified according to chemical structure. Bioflavonoid supplements are available as individual flavonoids, such as quercetin (derived from onions), and as multiple bioflavonoid complexes (derived from citrus fruits), which are often combined with vitamin C. Compounds commonly featured in citrus bioflavonoid supplements include hesperidin, rutin, naringin, and quercitin.
Most of the health benefits attributed to citrus bioflavonoids relate to their antioxidant activity, which has been demonstrated in numerous in vitro and animal studies. Citrus bioflavonoids are derived from fruits high in vitamin C, and they appear to act synergistically with the vitamin to neutralize free radicals. Bioflavonoids' antioxidant properties are thought to be particularly beneficial for capillary strength.
Although the optimum daily bioflavonoid intake has not been determined, some supplement advocates recommend 2,000 to 6,000 mg of citrus bioflavonoids per day. Citrus bioflavonoids appear to have very low toxicity, and appear to be safe at this dosage range. Research indicates that some flavonoids found in grapefruit juice may interfere with an enzyme that breaks down certain drugs, increasing the drugs' activity. For this reason, people taking prescription medications should consult a doctor or pharmacist before taking bioflavonoid supplements.