Please note that green tea contains mainly catechins, which are relatively simple phenolic compounds. Black tea and many fruits and vegetables contain mainly complex polyphenols, also called polymeric polyphenols, or condensed tannins. Both simple and complex polyphenols, often present side by side, have been found to have a wide range of health benefits. Ellagic acid, for instance, a powerful anti-carcinogen, is also present in many kinds of berries, including blueberries and raspberries, as well as in cherries and pomegranates. Hence, for instance, the well-established effectiveness of coffee in decreasing iron levels, or helping fight certain bacterial and viral infections.
Tannins are very common in the plant world.
The wide distribution of tannins in the plant kingdom is probably related both to their antioxidant and antimicrobial properties. The presence of tannins in wood, for instance, is likely to be a key reason for the durability of wood. The fact that chocolate doesn't spoil in spite of its high fat content is also due to these fascinating polyphenols. Also, in spite of containing sugar, chocolate, like tea and other flavonoid-rich foods, appears to help prevent cavities. There is emerging evidence that thanks to their antimicrobial action, flavonoids can help prevent dental decay and oral diseases.