Table 1.

A brief description of various plant polyphenols and their sources

Classes of polyphenols	Source and description
Phenolic acids	Phenolic acids are simple molecules such as caffeic acid, and coumaric acid. Phenolic acids form a diverse group that includes the widely distributed hydroxybenzoic and hydroxycinnamic acids. Hydroxycinnamic acid compounds (p-coumaric, caffeic acid, ferulic acid) occur most frequently as simple esters with hydroxy carboxylic acids or glucose, while the hydroxybenzoic acid compounds (p-hydroxybenzoic, gallic acid, ellagic acid) are present mainly in the form of glucosides. Ellagic acid is found in pomegranates. Coffee is particularly rich in bound phenolic acids, such as caffeic acid, ferulic acid, and p-coumaric acid. Phenolic acids found in blueberries include gallic acid, p-hydroxybenzoic acid, caffeic acid, p-coumaric acid and vanillic acid.
Flavonoids	Flavonoids are a subclass of polyphenols and are widely distributed in nature. The polyphenolic structure of flavonoids and tannins renders them quite sensitive to oxidative enzymes.
Anthocyanins	Anthocyanins and anthocyanidins are a large group of water-soluble pigments found in a large number of fruits, vegetables and flowers, particularly grapes, grape seed extract, and berries. Bilberry and other berries have a high concentration of anthocyanins.
Catechins or flavanols	These are primarily found in tea leaves. Tea leaves and grape seeds have the monomeric flavan-3-ols catechin, epicatechin, gallocatechin, epigallocatechin, epicatechingallate and epigallocatechin-3-gallate.
Flavones	Apigenin, luteolin. The herb chamomile has a good amount of apigenin.
Flavonols	Flavonols are found at high concentrations in onions, apples, red wine, broccoli, tea, and Ginkgo-Biloba. The most common flavonols are quercetin, kaempferol, and myricetin. Flavonols also include fisetin, isoquercitrin and hyperoside.
Flavanones	Flavanones are hesperidin and naringin.
Isoflavones	Genistein and daidzein are found in soy.
Lignans	Lignans are found in nuts and whole grain cereals. Flaxseed has a high content of lignan.
Proanthocyanidins	These are found in grapes, red wine, and pine bark. Pycnogenol is a pine bark extract. Grape seed extract provides a concentrated source of polyphenols, many of which are proanthocyanidins. Red wine is rich in the complex polyphenols, the proanthocyanidins. Proanthocyanidins share common properties with other polyphenols, in particular their reducing capacity and ability to chelate metal ions.
Procyanidins	Oligomeric catechins are found at high concentrations in red wine, grapes and grape seeds, cocoa, cranberry, apples, and some supplements such as pycnogenol. Apples contain many kinds of polyphenols, and the main components are oligomeric procyanidins.
Stilbenes	Resveratrol is found in the skin of dark colored grapes.
Tannins	Tannins are found in red wine, tea, and nuts. They are large molecules. Many flavonoids in foods also occur as large molecules (tannins). These include condensed tannins (proanthocyanidins), derived tannins and hydrolysable tannins.