Also indexed as: Bioflavonoids
Flavonoids are a class of water-soluble plant pigments. Flavonoids are broken down into
categories, though the issue of how to divide them is not universally agreed upon. One system
breaks flavonoids into isoflavones, anthocyanidins, flavans, flavonols, flavones, and
flavanones.1 Some of the best-known flavonoids, such as genistein in soy, and
quercetin in onions, can be considered
subcategories of categories. Although they are all structurally related, their functions are
different. Flavonoids also include hesperidin, rutin, citrus flavonoids, and a variety of
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?
Flavonoid deficiencies have not been reported.
How much is usually taken?
Flavonoid supplements are not required to prevent deficiencies in people eating a healthy
diet. Healthcare practitioners commonly recommend 1,000 mg of citrus flavonoids taken one to
three times per day. Alternatively, 240–600 mg of bilberry (standardized to 25% anthcyanosides) may be
taken per day.
Are there any side effects or interactions?
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.2 3 These side effects
subsided when treatment was discontinued.
In 1980, quercetin was reported to induce
cancer in animals.4 Most further research did not find this to be true,
however.5 6 While quercetin is mutagenic in test tube studies, it does
not appear to be mutagenic in animal studies.7 In fact, quercetin has been found to
inhibit both tumor promoters8 and human cancer cells.9 People who eat
high levels of flavonoids have been found to have an overall lower risk of getting a
wide variety of cancers,10 though
preliminary human research studying only foods high in quercetin has found no relation to
cancer risk one way or the other.11 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.12
The flavonoids work in conjunction with vitamin
C. Citrus flavonoids, in particular, improve the absorption of vitamin C.13
Are there any drug
Certain medicines may interact with flavonoids. Refer to drug interactions for a list of those medicines.
1. Peterson J, Dwyer J. Taxonomic classification helps identify
flavonoid-containing foods on a semiquantitative food frequency questionnaire. J Am Diet
2. Bar-Meir S, Halpern Z, Gutman M, et al. Effect of (+)-cyanidanol-3 on
chronic active hepatitis: a double-blind controlled trial. Gut
3. Conn HO. Cyanidanol: will a hepatotrophic drug from Europe go west?
Hepatology 1983;3:121–3 [review].
4. Pamukcu AM, Yalciner S, Hatcher JF, Bryan GT. Quercetin, a rat
intestinal and bladder carcinogen present in bracken fern (Pteridium
aquilinum). Cancer Res 1980;40:3468–72.
5. Hirono I, Ueno I, Hosaka S, et al. Carcinogenicity examination of
quercetin and rutin in ACI rats. Cancer Lett 1981;13:15–21.
6. Saito D, Shirai A, Matsushima T, et al. Test of carcinogenicity of
quercetin, a widely distributed mutagen in food. Teratog Carcinog Mutagen
7. Aeschbacher H-U, Meier H, Ruch E. Nonmutagenicity in vivo of
the food flavonol quercetin. Nutr Cancer 1982;2:90.
8. Nishino H, Nishino A, Iwashima A, et al. Quercetin inhibits the action
of 12-O-tetradecanoylphorbol-13-acetate, a tumor promoter. Oncology
9. Kuo SM. Antiproliferative potency of structurally distinct dietary
flavonoids on human colon cancer cells. Cancer Lett 1996;110:41–8.
10. Knekt P, Jävinen R, Seppänen R, et al. Dietary flavonoids
and the risk of lung cancer and other malignant neoplasms. Am J Epidemiol
11. Hertog M, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and
cancer risk in the Zutphen Elderly Study. Nutr Cancer 1994;22:175–84.
12. Stavric B. Quercetin in our diet: from potent mutagen to probable
anticarcinogen. Clin Biochem 1994;27:245–8.
13. Vinson JA, Bose P. Comparative bioavailability to humans of ascorbic
acid alone or in a citrus extract. Am J Clin Nutr 1988;48:601–4.
14. Vinson JA, Bose P. Comparative bioavailability of synthetic and
natural vitamin C in guinea pigs. Nutr Rep Int 1983;27:875–9.