Polyphenols are potent antioxidants found in both green and black tea. Although most of the research on tea has been done in the laboratory and on animals, there is strong evidence that tea offers positive heath benefits to humans. Herbal teas do not contain polyphenols in any significant amount.

Polyphenols, particularly the flavonoids, are among the most potent plant antioxidants, and they protect and recycle other antioxidants, such as vitamin E. They are also known to protect against heart disease. One of the ways in which polyphenols protect the heart is by lowering the production of the protein endothelin-1, which causes blood vessels to constrict and reduces the flow of oxygen to the heart. They also chelate metal ions such as iron, lead, and copper. This prevents them from being absorbed by the body where they would promote free-radical generation. The polyphenols present in green tea may promote weight loss.

Fruits and vegetables have significant variations in polyphenol content, depending on the part of the plant used (leaves are the highest), cultivation and harvesting methods, degree of ripeness, storage conditions and so on. A flavonoid intake of about 150-300 mg per day would be obtained from eating the recommended five fruit servings each day.