Flavonoids (or bioflavonoids), also collectively known as Vitamin P and citrin, are 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 (examples: quercetin, rutin).
- ''isoflavonoids'', derived from 3-phenylchromen-4-one (3-phenyl-1,4-benzopyrone) structure
- ''neoflavonoids'', derived from 4-phenylcoumarine (4-phenyl-1,2-benzopyrone) structure.
The three flavonoid classes above are all ketone-containing compounds, and as such, are flavonoids and flavonols. This class was the first to be termed "bioflavonoids." The terms flavonoid and bioflavonoid have also been more loosely used to describe non-ketone polyhydroxy polyphenol compounds which are more specifically termed flavanoids, flavan-3-ols, or catechins (although catechins are actually a subgroup of flavanoids).
Flavonoids are widely distributed in plants fulfilling many functions.
Flavonoids are the most important plant pigments for flower coloration producing yellow or red/blue pigmentation in petals designed to attract pollinator animals.
Flavonoids secreted by the root of their host plant help ''Rhizobia'' in the infection stage of their symbiotic relationship with legumes like peas, beans, clover, and soy. Rhizobia living in soil are able to sense the flavonoids and this triggers the secretion of Nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses such as ion fluxes and the formation of a root nodule.
They also protect plants from attacks by microbes, fungi and insects.
Flavonoids (specifically flavanoids such as the catechins) are "the most common group of polyphenolic compounds in the human diet and are found ubiquitously in plants". Flavonols, the original bioflavonoids such as quercetin, are also found ubiquitously, but in lesser quantities. Both sets of compounds have evidence of health-modulating effects in animals which eat them.
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. Resulting from experimental evidence that they may modify allergens, viruses, and carcinogens, flavonoids have potential to be biological "response modifiers", such as anti-allergic, anti-inflammatory, anti-microbial and anti-cancer activities shown from in vitro studies.
Antioxidant activity in vitro
Flavonoids (both flavonols and flavanols) are most commonly known for their antioxidant activity in vitro.
Consumers and food manufacturers have become interested in flavonoids for their possible medicinal properties, especially their putative role in prevention of cancers and cardiovascular diseases. Although physiological evidence is not yet established, the beneficial effects of fruits, vegetables, and tea or even red wine have sometimes been attributed to flavonoid compounds rather than to known micronutrients, such as vitamins and dietary minerals.
Alternatively, research conducted at the Linus Pauling Institute and evaluated by the European Food Safety Authority indicates that, following dietary intake, flavonoids themselves are of little or no direct antioxidant value. As body conditions are unlike controlled test tube conditions, flavonoids and other polyphenols are poorly absorbed (less than 5%), with most of what is absorbed being quickly metabolized and excreted.
The increase in antioxidant capacity of blood seen after the consumption of flavonoid-rich foods is not caused directly by flavonoids themselves, but most likely is due to increased uric acid levels that result from metabolism of flavonoids. 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."
Other potential health benefits
Physiological processing of unwanted flavonoid compounds induces so-called Phase II enzymes that also help to eliminate mutagens and carcinogens, and therefore may be of value in cancer prevention. Flavonoids could also induce mechanisms that may kill cancer cells and inhibit tumor invasion.
Research also indicated that only small amounts of flavonoids may be needed for possible benefits. Taking large dietary supplements likely provides no extra benefit and may pose risks. However, certainty of neither a benefit nor a risk has been proven yet in large-scale human intervention trials.
Capillary stabilizing agents
Bioflavonoids like rutin, monoxerutin, diosmin, troxerutin and hidrosmin have potential vasoprotective proprieties still under experimental evaluation.
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