Other studies showed that green tea and black tea inhibited lung, forestomach and liver tumor formation in mice, and esophageal tumor formation in rats. Among those animals bearing ACF, an average of 2.4 ACF per colon were found in the group given only the carcinogen, compared with 1.0 ACF and 1.6 ACF per colon in rats given the carcinogen plus green tea or black tea, respectively. Thus, both teas reduced the incidence and frequency of pre-cancerous lesions in the colon. Tea is a complex mixture of chemicals. To understand exactly how tea might prevent cancer, it has been separated into various fractions, and these fractions have subsequently been examined for inhibitory activity using different tests.
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.