Tea and cancer chemoprevention: a comprehensive review.

Dietary components that are capable of inhibiting the growth of cancer cells without affecting the growth of normal cells are receiving considerable attention in developing novel cancer-preventive approaches. Tea, made from young leaves and leaf buds of the tea plant, 'Camellia sinensis', and the world's second most consumed beverage, has received a great deal of attention both from the general public and the scientific community because tea polyphenols are strong antioxidants, and tea preparations have inhibitory activity against tumorigenesis. Besides this, the wide spread consumption of tea throughout the world evoked the interest of the scientific community in the possibility of its use in cancer prevention. There are three main types of tea, all coming from the tea plant viz. black tea (fermented,) green tea (unfermented), or oolong tea (semi-fermented), classified based on the methods of brewing and processing. Inhibition of tumorigenesis by green or black tea preparations has been demonstrated in various animal models in different organs. Various epidemiological studies substantiate the correlation between tea consumption and cancer prevention; however, they have not yielded clear conclusions pertaining to the protective effects of tea consumption against cancer development in humans. Many mechanisms have been proposed for the inhibition of carcinogenesis by tea, including the modulation of signal transduction pathways (including growth factor-mediated, mitogen-activated protein kinase (MAPK)-dependent, and ubiquitin/proteasome degradation pathways ) that lead to the inhibition of cell proliferation and transformation; induction of apoptosis of preneoplastic and neoplastic cells, and inhibition of tumor invasion as well as angiogenesis. These mechanisms need to be evaluated, verified and corroborated in animal models and humans in order to gain more understanding on the effects of tea consumption on human cancer. Because the causative factors are different for different populations, tea consumption may affect carcinogenesis only in selected situations rather than having the general effect on all cancers. Although, on the basis of many epidemiological observations and numerous laboratory studies, it can be concluded that tea consumption is likely to have beneficial effects in reducing cancer risk in different populations, yet there is a need to define the population that could benefit from tea consumption. After careful evaluation of additional studies, it may be possible to recommend consumption of tea polyphenols by humans. Although considerable accumulating information provides a compelling body of evidence for the preventive potential of tea against cancer, naturally occurring tea polyphenols have yet to be evaluated in clinical intervention in human trials.