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Polyphenols are a group of chemical substances found in plants, characterized by the presence of more than one phenol group per molecule. Polyphenols are generally further subdivided into hydrolyzable tannins, which are gallic acid esters of glucose and other sugars; and phenylpropanoids, such as lignins, flavonoids, and condensed tannins.
Classification and Nomenclature
The subdivision 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 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 the eponymously named 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).
|Class/Polymer:||hydrolyzable tannins||Flavonoid, condensed tannins||Lignins|
Polyphenols are also grouped and classified by the type and number of phenolic subcomponents present. More than one subcomponent can be present on a given polyphenol.
|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.
High levels of polyphenols can generally be found in the fruit skins.
Polyphenols were once briefly known as Vitamin P. However they were quickly found out to be non-essential and reclassified. The health benefits of specific polyphenols such as quercetin are well-established, 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.  Polyphenols have also been investigated as a source of additional health benefit in organic produce, but no conclusion was made.  Polyphenols bind with nonheme iron (e.g. from plant sources) in vitro in model systems.  This may decrease its absorption by the body.
- P. M. Dewick, The Biosynthesis of Shikimate Metabolites, Natural Product Reports 12:579-607 (1995)
- 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.