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A few thousand different flavonoids have so far been identified. Many of these compounds serve as antioxidants or play other important roles in maintaining the health of body. One characteristic of nearly all flavonoids is that they enhance the functionality of vitamin C, itself a powerful antioxidant. They are sometimes (redundantly) called bioflavonoids. Some of the well-known supplements, such as genistein in soy, and quercetin in onions, are also flavonoids. Although they are all structurally related, their functions are different. Flavonoids also include hesperidin, rutin, citrus flavonoids, and a variety of other supplements. Quercetin is a flavonoid that serves as the backbone for many other flavonoids, including the citrus flavonoids rutin, quercitrin, and hesperidin. Quercetin is consistently the most active of the flavonoids in experimental studies, and many medicinal plants owe much of their activity to their high quercitin content.

Flavonoids health benefits

Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation. For example, it inhibits both the manufacture and release of histamine and other allergic/inflammatory mediators. In addition, it exerts potent antioxidant activity and vitamin C-sparing action. Quercetin's proven anti-inflammatory properties help the body counter allergic reactions to pollen. Quercetin also seems to reduce inflammation in the lungs and other air passages, making breathing easier.

In addition to possesing antioxidant activity and an ability to increase intracellular levels of vitamin C, rutin, and hesperidin exert many beneficial effects on capillary permeability and blood flow. They exhibit also some of the anti-allergy and anti-inflammatory benefits of quercetin. Green tea polyphenols are potent antioxidant compounds that have demonstrated greater antioxidant protection than vitamins C and E. Green tea may also increase the activity of antioxidant enzymes. Green tea polyphenols inhibit cancer by blocking the formation of cancer-causing compounds and suppressing the activation of carcinogens. Proanthocyanidins extracts demonstrate a wide range of pharmacological activity. They increase intracellular vitamin C levels, decrease capillary permeability and fragility , scavenge oxidants and free radicals, inhibit destruction of collagen the most abundant protein in the body. The flavonoid rutin has been found to help prevent stomach ulcers by protecting the stomach lining. Flavonoids have also been shown to reduce the 'stickiness' of blood platelets and thus reduce the formation of blood clots, which reduces the risks of certain cardiovascular diseases.

Sources of flavonoids

Dietary sources of flavonoids include tea, red wine, fruits, vegetables and legumes. Flavanones are in citrus, isoflavones in soy products, anthocyanidins in wine and bilberry, and flavans in apples and tea. Citrus flavonoids are found in citrus fruits, rutin in buckwheat, epigallocatechin gallate (EGCG) ingreen tea, anthocyanidins in bilberry, and naringenin in grapefruit. Oligomeric proanthocyanidins, are found in grape seeds and skins. Quercetin is found in onions, tea, and apples. Polyphenols are the most abundant group of compounds in fresh tea leaves and are found in green and black tea beverages at 30-42% and 3-10% of the total dry matter, respectively. Many medicinal plants contain bioflavonoids such as ginkgo biloba, hawthorn and Chinese scullcap.