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.
Polyphenolic compounds are natural components of a wide variety of plants; they are also known as secondary plant metabolites. Food sources rich in polyphenols include onion, apple, tea, red wine, red grapes, grape juice, strawberries, raspberries, blueberries, cranberries, and certain nuts. The average polyphenol / flavonoid intake in the U.S. has not been determined with precision, in large part, because there is presently no U.S. national food database for these compounds. (USDA scientists and their colleagues are in the process of developing a database for foods rich in polyphenols.) It has been estimated that in the Dutch diet a subset of flavonoids (flavonols and flavones) provide 23 mg per day. Earlier estimates of dietary intake that approximated 650 mg per day (Kuhnau, World Review of Nutrition and Dietetics, 1976;24:117) are generally thought to be too high as the estimate was based on data that were generated by "old" (less specific) methodology. Scientists at the Food Composition Laboratory, Beltsville Human Nutrition Research Center are currently developing new methodology for the accurate measurement of polyphenols in foods.
Polyphenols act as antioxidants. They protect cells and body chemicals against damage caused by free radicals, reactive atoms that contribute to tissue damage in the body. For example, when low-density lipoprotein (LDL) cholesterol is oxidized, it can become glued to arteries and cause coronary heart disease.
Polyphenols can also block the action of enzymes that cancers need for growth and they can deactivate substances that promote the growth of cancers. The polyphenol most strongly associated with cancer prevention is epigallocatechin-3-gallate, or EGCG.
By mechanisms still undefined, polyphenols 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.