Camellia sinensis is one of about 200 species in the genus Camellia, which is native to Asia and includes several species valued as oil plants and ornamentals. It has glossy dark green leaves and attractive white flowers; if grown in the wild it is a small tree, but in cultivation it is a shrub kept pruned to less than two meters tall. The leaf buds and leaves are individually harvested by hand and are used to produce tea, which is, after water, the most popular beverage in the world, consumed by billions of people. The choice of leaves used, from the tiniest leaf buds alone to the larger leaves on the lower part of the plant, affects chemical content and flavor, and a number of varieties are recognized. Preparation methods define the difference between green tea and black tea: green tea is quickly sun-dried and roasted, whereas black tea is allowed to sit for extended periods while damp, resulting in fermentation of the leaves before roasting. Oolong tea is midway between these two extremes. Other flavorings may also be added to certain types of tea; for example, Earl Gray tea is flavored with bergamot. Tea contains caffeine and other stimulant alkaloids; it also contains tannins and has a somewhat astringent effect, so strong tea is used as a folk remedy for diarrhea.

Tea also contains polyphenolic compounds called catechins, which have been shown in numerous in vitro and animal studies to have several different biological mechanisms that act to inhibit cancers. For example, many studies have shown that green tea is active against breast cancer cell lines or implanted tumors in animals, and tea extracts have been given to human patients to shrink breast tumors prior to surgery. The most potent anticancer compound is epigallocatechin gallate (EGCG); however, the presence of other catechins has been shown to increase the activity of EGCG. Catechins are present in greater quantity in green tea, as they are destroyed during the fermentation process for black tea, yet some studies suggest that black tea also has an anticancer effect attributable to other compounds. It has also been noted that Asians, who consume much more green tea than Westerners, have lower rates of certain cancers. Some epidemiological studies have suggested that consumers of green tea had lower rates of some cancers, but results are inconsistent. A very large study involving residents of Hiroshima and Nagasaki found almost identical cancer rates in those who consumed over five cups a day and those who consumed one cup or less. Two studies showed significant correlation between green tea consumption and reduced breast cancer rates only in women with certain genotypes (high-activity ACE or low-activity COMT, the latter being a gene whose functions include elimination of tea catechins from the body). There is also some evidence that green tea consumption may reduce the risk of recurrence of stage I and II breast cancers. However, despite impressive results from lab studies, the evidence for a cancer-preventive effect in actual human beings is still rather limited.

Additionally, tea has an antioxidant effect, which has been demonstrated in human studies at levels normally consumed. One human study has shown that green tea reduces oxidative damage in smokers, which might have an effect on the incidence of cancer as well as other chronic diseases. For example, there is some epidemiological evidence that tea consumption may be protective against heart disease and stroke, and possible protective effects against neurodegenerative diseases have been remarked upon in multiple reviews. Also, one human trial has shown that high doses of green tea extract significantly reduced provirus load in healthy carriers of HTLV-1, a virus associated with leukemia in some people. Evidence from human studies remains quite limited. As to safety, a wealth of human experience has shown that consumption of tea beverage at normal levels is safe (except that pregnant women are advised to limit the amount of caffeine they consume), and human trials have demonstrated the safety of decaffeinated green tea extract at consumption levels that, if taken as tea, would cause unpleasant side effects related to the high caffeine content.

Selected References

Chang H. T. and B. Bartholomew. 1984. Camellias. Timber Press: Portland, OR.

Crespy, V., and G. Williamson. 2004. A review of the health effects of green tea catechins in in vivo animal models. J. Nutr. 134:3431S-3440S.

Erba, D., P. Riso, A. Bordoni, P. Foti, P. L. Biagi, and G. Testolin. 2005. Effectiveness of moderate green tea consumption on antioxidative status and plasma lipid profile in humans. J. Nutr. Biochem. 16:144-149.

Hakim, I. A., R. B. Harris, S. Brown, H. H. Chow, S. Wiseman, S. Agarwal, and W. Talbot. 2003. Effect of increased tea consumption on oxidative DNA damage among smokers: a randomized controlled study. J. Nutr. 133:3303S-3309S.

Inoue, M., K. Tajima, M. Mizutani, H. Iwata, T. Iwase, S. Miura, K. Hirose, N. Hamajima, and S. Tominaga. 2001. Regular consumption of green tea and the risk of breast cancer recurrence: follow-up study from the Hospital-based Epidemiologic Research Program at Aichi Cancer Center (HERPACC), Japan. Cancer Lett. 167:175-182.

Nagano, J., S. Kono, D. L. Preston, and K. Mabuchi. 2001. A prospective study of green tea consumption and cancer incidence, Hiroshima and Nagasaki (Japan). Cancer Causes Control 12:501-508.

Pettigrew, J. 2004. The tea companion: a connoisseur?s guide. Running Press: Philadelphia, PA.

Saleem, M., V. M. Adhami, I. A. Siddiqui, and H. Mukhtar. 2003. Tea beverage in chemoprevention of prostate cancer: a mini-review. Nutr. Cancer 47:13-23.

Sealy, J. R. 1958. A revision of the genus Camellia. The Royal Horticultural Society, London.

Seely, D., E. J. Mills, P. Wu, S. Verma, and G. H. Guyatt. 2005. The effects of green tea consumption on incidence of breast cancer and recurrence of breast cancer: a systematic review and meta-analysis. Integr. Cancer Ther. 4:144-155.

Vinson, J. A. 2000. Black and green tea and heart disease: a review. Biofactors 13:127-132.

Wu, A. H., C. C. Tseng, D. Van Den Berg, and M. C. Yu. 2003. Tea intake, COMT genotype, and breast cancer in Asian-American women.

Yang, C. S., P. Maliakal, and X. Meng. 2002. Inhibition of carcinogenesis by tea. Annu. Rev. Pharmacol. Toxicol. 42:25-54.

Yuan, J. M., W. P. Koh, C. L. Sun, H. P. Lee, and M. C. Yu. 2005. Green tea intake, ACE gene polymorphism and breast cancer risk among Chinese women in Singapore. Carcinogenesis 26:1389-1394.

Zhen, Y.-S., ed. 2002. Tea: bioactivity and therapeutic potential. Medicinal and aromatic plants ? industrial profiles vol. 17. Taylor & Francis: London and New York.