Green tea contains high levels of polyphenols called catechins. Generally, the catechins in green tea have been reported to have the highest antioxidant activity. In other words, the lining of the mouth is a good place for absorption of tea polyphenols into the body. EGCG, an excellent antioxidant, is the most actively studied of the tea polyphenols, but other tea constituents might be important against cancer, either singly or in combination. Caffeine, when consumed at the levels that occur in tea and coffee, can alter the amounts or activities of some enzymes in the body which metabolize carcinogens and other foreign compounds.
Caffeine also increases the activity of detoxification enzymes under some circumstances.
However, caffeine also has widely-known pharmacological properties that render it less suitable than more benign dietary compounds. Conclusions In many experimental animal and human studies, a test compound is usually administered at doses far above the levels typically encountered in the human diet. Perhaps the most encouraging finding from animal experiments is that cancer inhibition by tea occurs at concentrations that are dietarily relevant. This concentration of tea is very effective in many animal studies, and since people find even higher concentrations of tea quite palatable, future human trials should examine tea concentration as a variable. Other important issues to consider are brewing time, type of tea, frequency of tea consumption and timing of tea consumption relative to food intake.