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flavonoid

[fley-vuh-noid]

or flavone

Organic compound, any member of a class of biological pigments containing no nitrogen that are found in many plants. They include anthoxanthins, which give yellow colours, often to flower petals, and anthocyanins, largely responsible for the red colouring of buds and young shoots and the purple and purple-red colours of autumn leaves. Their biological function is unknown; they may attract pollinators and seed dispersers.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

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Flavonoid

The term flavonoid (or bioflavonoid) refers to a class of plant secondary metabolites. According to the IUPAC nomenclature, 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-phenylcoumarine (4-phenyl-1,2-benzopyrone) structure.

Flavonoids are most commonly known for their antioxidant activity. However, it is now known that the health benefits they provide against cancer and heart disease are the result of other mechanisms. 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.

Biosynthesis

Flavonoids are synthesized by the phenylpropanoid metabolic pathway in which the amino acid phenylalanine is used to produce 4-coumaroyl-CoA. This can be combined with malonyl-CoA to yield the true backbone of flavonoids, a group of compounds called chalcones, which contain two phenyl rings (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.

Biological effects

Flavonoids are widely distributed in plants fulfilling many functions including producing yellow or red/blue pigmentation in flowers and protection from attack by microbes and insects. The widespread distribution of flavonoids, their variety and their relatively low toxicity compared 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-inflammatory , anti-microbial and anti-cancer activity.

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 vitamins.

Health benefits aside from antioxidant values

In 2007, research conducted at the Linus Pauling Institute and published in Free Radical Biology and Medicine indicates that inside the human body, flavonoids themselves are of little or no direct antioxidant value. 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 acid levels 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."

Their research also indicated that only small amounts of flavonoids are necessary to see these medical benefits. Taking large dietary supplements provides no extra benefit and may pose some risks.

Diarrhea

A study done at Children's Hospital & Research Center Oakland, in collaboration with scientists at Heinrich Heine University in Germany, has shown that epicatechin, quercetin and luteolin can inhibit the development of fluids that result in diarrhea by targeting the intestinal cystic fibrosis transmembrane conductance regulator Cl– transport inhibiting cAMP-stimulated Cl– secretion in the intestine.

Important flavonoids

Quercetin

Quercetin is a flavonoid and, to be more specific, a flavonol (see below), that constitutes the aglycone of the glycosides rutin and quercitrin. In studies, quercetin is found to be the most active of the flavonoids, 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 histamine and other allergic/inflammatory mediators. 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. 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.

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.

Epicatechin

Epicatechin improves blood flow and thus seems good for cardiac health. 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 wine and up to three times that of green tea in in-vitro tests. But in the test outlined above it now appears the beneficial antioxidant effects are minimal as the antioxidants are rapidly excreted from the body.

Oligomeric proanthocyanidins

Proanthocyanidins extracts demonstrate a wide range of pharmacological activity. Their effects include increasing intracellular vitamin C levels, 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 citrus fruits, berries, ginkgo biloba, onions, parsley, pulses, tea (especially white and green tea), red wine, seabuckthorn, and dark chocolate (with a cocoa content of seventy percent or greater).

Citrus

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 hesperidin exert beneficial effects on capillary permeability 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. 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.

Ginkgo

Leaf extract from the Ginkgo tree is widely marketed as an herbal supplement. The active ingredients are flavoglycosides.

Tea

Green tea flavonoids are potent antioxidant compounds, thought to reduce incidence of cancer and heart disease. The major flavonoids in green tea are the kaempferol and catechins (catechin, epicatechin, epicatechin gallate, and epigallocatechin gallate (EGCG)).

In producing teas such as oolong tea and 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 tea is the least processed of teas and is shown to present the highest amount of catechins known to occur in camellia sinensis.

Wine

Grape skins contain significant amounts of flavonoids as well as other polyphenols. 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 resveratrol.

Dark chocolate

Flavanoids exist naturally in cacao, but because they can be bitter, they are often removed from chocolate, even the dark variety.

Subgroups

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

Flavones are divided into four groups:

Group	Skeleton				Examples
	Description	Functional groups		Structural formula
	Description	3-hydroxyl	2,3-dihydro	Structural formula
	-				Luteolin, Apigenin, Tangeritin
or	--				Quercetin, Kaempferol, Myricetin, Fisetin, Isorhamnetin, Pachypodol, Rhamnazin
	--				Hesperetin, Naringenin, Eriodictyol, Homoeriodictyol
or or	---				Taxifolin (or Dihydroquercetin), Dihydrokaempferol

Isoflavones

Isoflavones
:Isoflavones use the 3-phenylchromen-4-one skeleton (with no hydroxyl group substitution on carbon at position 2).
:Examples: Genistein, Daidzein, Glycitein