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Polyphenols are a group of chemical substances found in plants, characterized by the presence of more than one phenol unit or building block per molecule. Polyphenols are generally divided into hydrolyzable tannins (gallic acid esters of glucose and other sugars) and phenylpropanoids, such as lignins, flavonoids, and condensed tannins.
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Chemistry
Classification and Nomenclature
The division of polyphenols into tannins, lignins, and flavonoids is derived from the variety of simple polyphenolic units derived from secondary plant metabolism of the shikimate pathway[1] as well as classical divisions based upon the relative importance of each base component to different fields of study. Tannin chemistry originated in the importance of tannic acid to the tanning industry; lignins to the chemistry of soil and plant structure; and flavonoids to the chemistry of plant secondary metabolites for plant defense, and flower color (e.g. from anthocyanins).
The largest and best studied polyphenols are the flavonoids, which include several thousand compounds, among them the flavonols, flavones, catechins, flavanones, anthocyanidins, and isoflavonoids.[2]
The most abundant polyphenols are the condensed tannins, found in virtually all families of plants, and comprising up to 50% of the dry weight of leaves. The convergent evolution of tannin-rich plant communities has occurred on nutrient-poor acidic soils throughout the world. Tannins were once believed to function as anti-herbivore defenses, but more and more ecologists now recognize them as important controllers of decomposition and nitrogen cycling processes. As concern grows about global warming, there is great interest to better understand the role of polyphenols as regulators of carbon cycling, particularly in northern boreal forests.
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| Class/Polymer: | hydrolyzable tannins | Flavonoid, condensed tannins |
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Polyphenols are also classified by the type and number of phenolic subcomponents present. More than one subcomponent can be present on a given polyphenol.
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| Examples: coumaric acid derived lignins, kaempferol | Examples: catechin, quercetin, caffeic and ferulic acid derived lignins, hydroxytyrosol esters | Examples: gallocatechins (EGCG), tannins, myricetin, sinapyl alcohol derived lignins | Examples: resveratrol | Examples: almost all flavonoids | Examples: arbutin |
The phenolic unit can often be esterified or methylated. It can also be found dimerized or further polymerized, creating a new class of polyphenol. For example, ellagic acid is a dimer of gallic acid and forms the class of ellagitannins, or a catechin and a gallocatechin can combine to form the red compound theaflavin, a process which also results in the large class of brown thearubigins in tea.
In Foods
Notable sources of polyphenols include berries, tea, beer, grapes/wine, olive oil, chocolate/cocoa, coffee, walnuts, peanuts, pomegranates, yerba mate, and other fruits and vegetables.
High levels of polyphenols can generally be found in the fruit skins.
Health benefits
Polyphenols were once briefly known as Vitamin P. However they were quickly found out to be non-essential and reclassified.citation needed The possible health benefits of specific polyphenols such as quercetin are well-established,citation needed and there are less well-established claims of health benefits from all types of polyphenols.
Research indicates that polyphenols may have antioxidant characteristics with potential health benefits. They may reduce the risk of cardiovascular disease and cancer.[3] Polyphenols have also been investigated as a source of additional health benefit in organic produce, but no conclusion was made.[4] Polyphenols bind with nonheme iron (e.g. from plant sources) in vitro in model systems.[5] This may decrease its absorption by the body.
See also
- List of phytochemicals and foods in which they are prominent
- List of antioxidants in food
- Tannin
- Catechin
- Benzenediol
- Polyphenon
- Phenolic compounds in wine
References
- ^ P. M. Dewick, The Biosynthesis of Shikimate Metabolites, Natural Product Reports 12:579-607 (1995)
- ^ Clinical Guide to Nutrition and Dietary Supplements in Disease Management, by Jennifer R. Jamison, ISBN 0-443-07193-4, page 525.
- ^ Arts, I.C. and P.C. Hollman, "Polyphenols and disease risk in epidemiologic studies." American Journal Clinical Nutrition, 2005. 81(1 Suppl): p. 317S-325S.
- ^ Nutrition Perspectives Vol 30, No. 3 May/June 2005
- ^ E. Matuschek, U. Svanberg (2002) "Oxidation of Polyphenols and the Effect on In vitro Iron Accessibility in a Model Food System", Journal of Food Science 67 (1), pp. 420–424.
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