Research on flavonoids and other polyphenols truly began after 1995. 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 the most abundant antioxidants in the diet.

Polyphenols occur in all plant foods and contribute to the beneficial health effects of vegetables and fruit. Their contribution to the antioxidant capacity of the human diet is much larger than that of vitamins. The total intake of polyphenols in a person's diet could amount to 1 gram a day, whereas combined intakes of beta-carotene, vitamin C, and vitamin E from food most often is about 100 mg a day.

Recent studies have shown that dietary fiber and polyphenols of vegetables and fruits improve lipid metabolism and prevent the oxidation of low density lipoprotein cholesterol

Several other studies have shown that a group of polyphenol antioxidant compounds found in grapes, green tea, soybeans and wine may lower the risk of a range of cancers, but exactly how these powerful compounds work has remained unclear.

The green tea polyphenols contributed to minimizing tumor development by governing the amount of vascular endothelial growth factor (VEGF) in the serum of the prostate cancer mouse model.