The term flavonoid (or bioflavonoid) refers to a class of
plant secondary metabolites. According to the IUPACnomenclature, [http://www.iupac.org/goldbook/F02424.pdf Flavonoids (isoflavonoids and neoflavonoids).] , IUPAC Compendium of Chemical Terminology] they can be classified into:
*"flavonoids", derived from 2-phenylchromen-4-one (2-phenyl-1,4-benzopyrone) structure
*"isoflavonoids", derived from 3-phenylchromen-4-one (3-phenyl-1,4-benzopyrone) structure
*"neoflavonoids", derived from 4-phenyl
Flavonoids are most commonly known for their
antioxidantactivity. However, it is now known that the health benefits they provide against cancer and heart disease are the result of other mechanisms." [http://www.eurekalert.org/pub_releases/2007-03/osu-sfn030507.php Studies force new view on biology of flavonoids] ", by David Stauth, "EurekAlert!". Adapted from a news release issued by Oregon State University. URL accessed .] cite journal | last = Ververidis Filippos | coauthors = Trantas Emmanouil, Douglas Carl, Vollmer Guenter, Kretzschmar Georg, Panopoulos Nickolas | year = 2007| month = October | title = Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health | journal = Biotechnology Journal | volume = 2 | issue = 10 | pages = | accessdate = ] Flavonoids are also commonly referred to as bioflavonoids in the media – the terms are largely equivalent and interchangeable, for most flavonoids are biological in origin.
Flavonoids are synthesized by the
phenylpropanoid metabolic pathwayin which the amino acid phenylalanineis used to produce 4-coumaroyl-CoA. This can be combined with malonyl-CoAto yield the true backbone of flavonoids, a group of compounds called chalcones, which contain two phenylrings "(see polyphenols)". Conjugate ring-closure of chalcones results in the familiar form of flavonoids, the three-ringed structure of a flavone. The metabolic pathway continues through a series of enzymatic modifications to yield flavanones→ dihydroflavonols→ anthocyanins. Along this pathway, many products can be formed, including the flavonols, flavan-3-ols, proanthocyanidins (tannins) and a host of other polyphenolics.
Flavonoids are widely distributed in plants fulfilling many functions including producing yellow or red/blue
pigmentationin flowers and protection from attack by microbesand insects. The widespread distribution of flavonoids, their variety and their relatively low toxicitycompared to other active plant compounds (for instance alkaloids) mean that many animals, including humans, ingest significant quantities in their diet. Flavonoids have been referred to as "nature's biological response modifiers" because of strong experimental evidence of their inherent ability to modify the body's reaction to allergens, viruses, and carcinogens. They show anti-allergic, anti-inflammatorycite web|url=http://www.jci.org/cgi/content/full/107/2/135?ijkey=a1e09ce2dbca283cec170598f2410b15d5f4304f&keytype2=tf_ipsecsha|title=Therapeutic potential of inhibition of the NF-κB pathway in the treatment of inflammation and cancer|work=Yamamoto and Gaynor 107 (2): 135 -- Journal of Clinical Investigation|accessdate=] , anti-microbial [cite journal |author=Cushnie TPT, Lamb AJ |title=Antimicrobial activity of flavonoids |journal=International Journal of Antimicrobial Agents |volume=26 |issue=5 |pages=343-356 |year=2005 |pmid= 16323269 |doi=10.1016/j.ijantimicag.2005.09.002] and anti- canceractivity.
Consumers and food manufacturers have become interested in flavonoids for their medicinal properties, especially their potential role in the prevention of cancers and
cardiovascular disease. The beneficial effects of fruit, vegetables, and tea or even red wine have been attributed to flavonoid compounds rather than to known nutrients and vitaminscite web|url=http://www.frenchscout.com/polyphenols#procyanidins|title=Health benefits of oligomeric proanthocyanidins|author=Félicien Breton|year=2008] .
Health benefits aside from antioxidant values
In 2007, research conducted at the
Linus Pauling Instituteand published in " Free Radical Biology and Medicine" indicates that inside the human body, flavonoids themselves are of little or no direct antioxidant value. [cite journal |author=Lotito SB, Frei B |title=Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: cause, consequence, or epiphenomenon? |journal=Free Radic. Biol. Med. |volume=41 |issue=12 |pages=1727–46 |year=2006 |pmid=17157175 |doi=10.1016/j.freeradbiomed.2006.04.033] Unlike in the controlled conditions of a test tube, flavonoids are poorly absorbed by the human body (less than 5%), and most of what is absorbed is quickly metabolized and excreted from the body.
The huge increase in antioxidant capacity of blood seen after the consumption of flavonoid-rich foods is not caused directly by the flavonoids themselves, but most likely is due to increased
uric acidlevels that result from expelling flavonoids from the body. According to Frei, "we can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them. But this process of gearing up to get rid of unwanted compounds is inducing so-called Phase II enzymes that also help eliminate mutagens and carcinogens, and therefore may be of value in cancer prevention... Flavonoids could also induce mechanisms that help kill cancer cells and inhibit tumor invasion."
A study done at Children's Hospital & Research Center Oakland, in collaboration with scientists at
Heinrich Heine Universityin Germany, has shown that epicatechin, quercetinand luteolincan inhibit the development of fluids that result in diarrhea by targeting the intestinal cystic fibrosis transmembrane conductance regulatorCl– transport inhibiting cAMP-stimulated Cl– secretion in the intestine. [cite journal | last = Schuier | first = Maximilian | coauthors = Helmut Sies, Beate Illek, and Horst Fischer | year = 2005| month = October | title = Cocoa-Related Flavonoids Inhibit CFTR-Mediated Chloride Transport across T84 Human Colon Epithelia | journal = Journal of Nutrition | volume = 135 | issue = 10 | pages = | url = http://jn.nutrition.org/cgi/reprint/135/10/2320 | format = PDF | accessdate = ]
Quercetinis a flavonoid and, to be more specific, a flavonol (see below), that constitutes the aglyconeof the glycosides rutinand quercitrin. In studies, quercetin is found to be the most active of the flavonoids,Fact|date=May 2008 and many medicinal plants owe much of their activity to their high quercetin content. Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several initial processes of inflammation. For example, quercetin inhibits both the production and release of histamineand other allergic/inflammatory mediators.Fact|date=May 2008 In addition, it exerts potent antioxidant activity and vitamin C-sparing action. It may also help to prevent some types of cancer. Quercetin can be found in the herbal products based on Hawthorn, which are used for acute symptoms of congestive heart failure.Fact|date=May 2008 One study that people who ate quercetin-rich foods at least four times a week, on average, were 51% less likely to have lung cancer than those who ate none.Fact|date=May 2008
Please note the above comments on the article appear inaccurate. I have tried to locate the study that shows "people who ate quercetin-rich foods at least four times a week, on average, were 51% less likely to have lung cancer than those who ate none" but as far as I can tell, no such study exists. There are other articles on the internet claiming the same fact, but none link to ANY study or scientific journal I can find. As far as the american cancer society is concerned, there are no conclusive studies done on the topic. I quote "While some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans." - you can read the full article here : http://www.cancer.org/docroot/ETO/content/ETO_5_3x_Quercetin.asp . As far as I can tell, the only sites that appear to brashly claim the magical healing properties of quercetin are sites such as "alternativehealing.org" hardly a proven reliable source of medical claims. Please be aware that this article may have been modified by people within the nutrician industry ie. people without doctorates and an interest in promoting alternative medicine. Remember that anyone can call themselves a nutritionalist without any form of qualification. I would suggest that either the sources for the studies above are found and placed in the "citation needed" section or this section is removed altogether.
Epicatechinimproves blood flow and thus seems good for cardiachealth. Cocoa, the major ingredient of dark chocolate, contains relatively high amounts of epicatechin and has been found to have nearly twice the antioxidant content of red wineand up to three times that of green teain in-vitro tests. [J. Agric.Food Chem. (2003) 51: Lee et al.] cite web|url=http://news.bbc.co.uk/2/hi/health/.stm|title=Cocoa nutrient for 'lethal ills'|work=BBC News|accessdate=] But in the test outlined above it now appears the beneficial antioxidant effects are minimal as the antioxidants are rapidly excreted from the body.
Proanthocyanidins extracts demonstrate a wide range of pharmacological activity. Their effects include increasing
intracellular vitamin Clevels, decreasing capillary permeability and fragility, scavenging oxidants and free radicals, and inhibiting destruction of collagen, the most abundant protein in the body.
Important dietary sources
Good sources of flavonoids include all
citrusfruits, berries, ginkgo biloba, onions [Tsushida T., Suzuki, M. (1996) Content of flavonol glucosides and some properties of enzymes metabolizing the glucosides in onion. J. Jap. Soc. Food Sci. Technol., 43, 642-649. ] [Slimestad R, Fossen T, Vågen IM. (2007) Onions: a source of unique dietary flavonoids. J Agric Food Chem. 2007 Dec 12;55(25):10067-80. Epub 2007 Nov 13. PMID 17997520] , parsley[Justesen, U., and Knuthsen, P. Composition of flavonoids in fresh herbs and calculation of flavonoid intake by use of herbs in traditional Danish dishes. Food Chem., 2001, 73, 245-250. PMID 11192169] , pulses[Ewald, C., Fjelkner-Modig, S., Johansson, K., Sjöholm, I., and Åkesson. B. Effect of processing on major flavonoids in processed onions, green beans, and peas. Food Chem., 1999, 64, 231-235.] , tea(especially white and green tea), red wine, seabuckthorn, and dark chocolate(with a cocoacontent of seventy percent or greater).
The citrus bioflavonoids include
hesperidin(a glycoside of the flavanone hesperetin), quercitrin, rutin(two glycosides of the flavonol quercetin), and the flavone tangeritin. In addition to possessing antioxidant activity and an ability to increase intracellular levels of vitamin C, rutin and hesperidinexert beneficial effects on capillarypermeability and blood flow. They also exhibit some of the anti-allergy and anti-inflammatory benefits of quercetin. Quercetin can also inhibit reverse transcriptase, part of the replication process of retroviruses. [ Spedding, G., Ratty, A., Middleton, E. Jr. (1989) Inhibition of reverse transcriptases by flavonoids. "Antiviral Res" 12 (2), 99-110. PMID ] The therapeutical relevance of this inhibition has not been established. Hydroxyethylrutosides (HER) have been used in the treatment of capillary permeability, easy bruising, hemorrhoids, and varicose veins.
Leaf extract from the Ginkgo tree is widely marketed as an herbal supplement. The active ingredients are flavoglycosides.
Green teaflavonoids are potent antioxidantcompounds, thought to reduce incidence of cancer and heart disease. The major flavonoids in green tea are the kaempferoland catechins (catechin, epicatechin, epicatechin gallate, and epigallocatechin gallate(EGCG)).
In producing teas such as
oolong teaand black tea, the leaves are allowed to oxidize, during which enzymes present in the tea convert some or all of the catechins to larger molecules. However, green tea is produced by steaming the fresh-cut leaf, which inactivates these enzymes, and oxidation does not significantly occur. White teais the least processed of teas and is shown to present the highest amount of catechins known to occur in camellia sinensis.
Grape skins contain significant amounts of flavonoids as well as other polyphenols [James A. Kennedy, Mark A. Matthews, and Andrew L. Waterhouse, "Effect of Maturity and Vine Water Status on Grape Skin and Wine Flavonoids" Am. J. Enol. Vitic. 53:4:) [http://www.ajevonline.org/cgi/content/abstract/53/4/268 (abstract)] ] . Both red and white wine contain flavonoids; however, since red wine is produced by fermentation in the presence of the grape skins, red wine has been observed to contain higher levels of flavonoids, and other polyphenolics such as
Flavanoids exist naturally in cacao, but because they can be bitter, they are often removed from chocolate, even the dark variety [Editorial. 'The devil in the dark chocolate.' The Lancet. 2007; 370:2070 [http://www.thelancet.com/journals/lancet/article/PIIS014067360761873X/fulltext] ] .
Over 5000 naturally occurring flavonoids have been characterized from various plants. They have been classified according to their chemical structure, and are usually subdivided into the following subgroups (for further reading see ):
Flavones are divided into four groups: [ [http://www.ars.usda.gov/is/np/phenolics/illus/phenfig4.htm Phenolics:figure 4 ] ]
*:Isoflav"ones" use the 3-phenylchromen-"4-one" skeleton (with no hydroxyl group substitution on carbon at position 2).
Genistein, Daidzein, Glycitein
Flavan-3-ols and Anthocyanidins
*Flavan-3-ols (also known as Flavanols)
*:Flavan-"3-ols" use the 2-phenyl-3,4-dihydro-2"H"-chromen-"3-ol" skeleton.
Catechins( Catechin(C), Gallocatechin(GC), Catechin 3-gallate(Cg), Gallocatechin 3-gallate(GCg)), Epicatechins( Epicatechin(EC), Epigallocatechin(EGC), Epicatechin 3-gallate(ECg), Epigallocatechin 3-gallate(EGCg))
Anthocyanidins are the aglycones of anthocyanins. Anthocyanidins use the flavylium(2-phenylchromenylium) ion skeleton
Cyanidin, Delphinidin, Malvidin, Pelargonidin, Peonidin, Petunidin
Availability through microorganisms
A number of recent research articles have demonstrated the efficient production of flavonoid molecules from genetically-engineered microorganisms [Hwang EI, Kaneko M, Ohnishi Y, Horinouchi S. "Production of plant-specific flavanones by Escherichia coli containing an artificial gene cluster." Appl Environ Microbiol. 2003 May;69(5): PMID ] cite journal | last = Ververidis Filippos | coauthors = Trantas Emmanouil, Douglas Carl, Vollmer Guenter, Kretzschmar Georg, Panopoulos Nickolas | year =
2007| month = October | title = Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part II: Reconstruction of multienzyme pathways in plants and microbes | journal = Biotechnology Journal | volume = 2 | issue = 10 | pages = | accessdate = ] .
* [http://www.nal.usda.gov/fnic/foodcomp/Data/Flav/flav.pdf USDA Database of Flavonoid content of food] (pdf)
* [http://www.friedli.com/herbs/phytochem/flavonoids.html Flavonoids (chemistry)]
* [http://www.uky.edu/~dhild/biochem/17/lect17.htm Flavonoids (chemistry)]
* [http://www.news.cornell.edu/releases/Nov03/HotCocoa-Lee.bpf.html Cornell news on Cocoa]
* [http://my.webmd.com/content/article/88/99702.htm A Dark Chocolate a Day Keeps the Doctor Away]
* [http://www.webmd.com/alzheimers/news/20050920/antioxidant-in-green-tea-may-fight-alzheimers Antioxidant in Green Tea may fight Alzheimer's-EGCG]
* [http://www.jci.org/cgi/content/full/107/2/135?ijkey=a1e09ce2dbca283cec170598f2410b15d5f4304f&keytype2=tf_ipsecsha Therapeutic potential of the NF-kB pathway in the treatment of inflammatory disorders]
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