Glossary: Flavonoids

flavonoids

Also known as: bioflavonoids

What is it?

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 other supplements.

While they are not considered essential, some flavonoids do support health 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-cholesterol from oxidative damage. Others, such as the anthocyanidins from bilberry, may help protect the lens of the eye from cataracts. Animal research suggests that naringenin may have anticancer activity.2 Soy isoflavones are also currently being studied to see if they help fight cancer.

Years ago, quercetin was reported to induce cancer in animals.5 Most further research did not find this to be true, however.6 7 While quercetin is mutagenic in test tube studies, it does not appear to be mutagenic in real animals.8 In fact, quercetin has been found to inhibit both tumor promoters9 and human cancer cells.10 People who eat high levels of flavonoids have been found to have an overall lower risk of getting a wide variety of cancers,11 though preliminary human research studying only foods high in quercetin has found no relation to cancer risk one way or the other.12 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.13 The flavonoids help protect vitamin C; citrus flavonoids, in particular, improve the absorption of vitamin C.14 15 Certain medications may interact with flavonoids.

Ranking	Health Concerns
Primary	Capillary fragility (hesperidin, quercitin, rutin) Chronic venous insufficiency (rutin) Hepatitis (catechin) Injury (minor) (for prevention only) Night blindness (bilberry)
Secondary	Bruising Chronic venous insufficiency (hesperidin) Cold sores Diabetes (bilberry) Hemorrhoids Macular degeneration (bilberry) Retinopathy (bilberry)
Other	Atherosclerosis (quercetin, (bilberry) Cataracts (quercetin, (bilberry) Diabetes (quercetin) Edema (water retention) (quercetin, rutin) Gingivitis (periodontal disease) Glaucoma (rutin) Hay fever (quercetin, hesperidin, rutin) High cholesterol (quercetin) Menopause (hesperidin) Menorrhagia (heavy menstruation) Peptic ulcer (quercetin) Retinopathy (quercetin) Varicose veins (bilberry)

References

1. Peterson J, Dwyer J. Taxonomic classification helps identify flavonoid-containing foods on a semiquantitative food frequency questionnaire. J Am Dietet Assoc 1998;98:682–5.

2. So FV, Guthrie N, Chambers AF, et al. Inhibition of human breast cancer cell proliferation and delay of mammary tumorigenesis by flavonoids and citrus juices. Nutr Cancer 1996;26:167–81.

3. Bar-Meir S, Halpern Z, Gutman M, et al. Effect of (+)-cyanidanol-3 on chronic active hepatitis: a double-blind controlled trial. Gut 1985;26:975–9.

4. Conn HO. Cyanidanol: will a hepatotrophic drug from Europe go west? Hepatology 1983;3:121–3 [review].

5. 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.

6. Hirono I, Ueno I, Hosaka S, Takanashi H, et al. Carcinogenicity examination of quercetin and rutin in ACI rats. Cancer Lett 1981;13:15–21.

7. Saito D, Shirai A, Matsushima T, et al. Test of carcinogenicity of quercetin, a widely distributed mutagen in food. Teratog Carcinog Mutagen 1980;1:213–21.

8. Aeschbacher H-U, Meier H, Ruch E. Nonmutagenicity in vivo of the food flavonol quercetin. Nutr Cancer 1982;2:90.

9. Nishino H, Nishino A, Iwashima A, et al. Quercetin inhibits the action of 12-O-tetradecanoylphorbol-13-acetate, a tumor promoter. Oncology 1984;41:120–3.

10. Kuo SM. Antiproliferative potency of structurally distinct dietary flavonoids on human colon cancer cells. Cancer Lett 1996;110:41–8.

11. Knekt P, Jävinen R, Seppänen R, et al. Dietary flavonoids and the risk of lung cancer and other malignant neoplasms. Am J Epidemiol 1997;146:223–30.

12. Hertog M, Feskens EJM, Hollman PCH, et al. Dietary flavonoids and cancer risk in the Zutphen Elderly Study. Nutr Cancer 1994;22:175–84.

13. Stavric B. Quercetin in our diet: from potent mutagen to probable anticarcinogen. Clin Biochem 1994;27:245–8.

14. 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.

15. Vinson JA, Bose P. Comparative bioavailability of synthetic and natural vitamin C in Guinea pigs. Nutr Rep Intl 1983;27 (4):875.