What do they do?
While they are not considered essential nutrients, some flavonoids support health by strengthening capillaries and other connective tissue, and some function as anti-inflammatory, antihistaminic, and antiviral agents. Quercetin has been reported to block the "sorbitol pathway" that is linked to many problems associated with diabetes. Rutin and several other flavonoids may also protect blood vessels.
As antioxidants, some flavonoids, such as quercetin, protect LDL ("bad") cholesterol from oxidative damage. Others, such as the anthocyanidins from bilberry, purple cabbage, and grapes, may help protect the lens of the eye from cataracts. Animal research suggests that naringenin, found in grapefruit, may have anticancer activity.2 Soy isoflavones are also currently being studied to see if they help fight cancer.
In a small, preliminary trial, rutoside (500 mg twice daily), a derivative of the flavonoid, rutin, combined with vitamin C (500 mg twice daily) produced marked improvement in three women with progressive pigmented purpura (PPP), a mild skin condition.3 Although not a serious medical condition, cosmetic concerns lead persons with PPP to seek treatment with a variety of drugs. The vitamin C/rutoside combination represents a promising, non-toxic alternative to these drug treatments, but larger, controlled trials are needed to confirm these preliminary results.
Where are they found?
Flavonoids have been used in connection with the following conditions (refer to the individual health concern for complete information):
Who is likely to be deficient of flavonoids?
Flavonoid deficiencies have not been reported.
How much flavonoids is usually taken?
Are there any side effects or interactions with flavonoids?
No consistent side effects have been linked to the flavonoids except for catechin, which can occasionally cause fever, anemia from breakdown of red blood cells, and hives.4 5 These side effects subsided when treatment was discontinued.
In 1980, quercetin was reported to induce cancer in animals.6 Most further research did not find this to be true, however.7 8 While quercetin is mutagenic in test tube studies, it does not appear to be mutagenic in animal studies.9 In fact, quercetin has been found to inhibit both tumor promoters10 and human cancer cells.11 People who eat high levels of flavonoids have been found to have an overall lower risk of getting a wide variety of cancers,12 though preliminary human research studying only foods high in quercetin has found no relation to cancer risk one way or the other.13 Despite the confusion, in recent years experts have shifted their view of quercetin from concerns that it might cause cancer in test tube studies to guarded hope that quercetin has anticancer effects in humans.14
The flavonoids work in conjunction with vitamin C. Citrus flavonoids, in particular, improve the absorption of vitamin C.15 16
Are there any drug interactions?
Certain medicines may interact with flavonoids. Refer to drug interactions for a list of those medicines.