Tea and Cancer Chemoprevention: A Comprehensive Review


Dietary components that are capable of inhibiting the growth of cancer cells without affecting the growth ofnormal cells are receiving considerable attention in developing novel cancer-preventive approaches. Tea, madefrom young leaves and leaf buds of the tea plant, ‘Camellia sinensis’, and the world’s second most consumedbeverage, has received a great deal of attention both from the general public and the scientific communitybecause tea polyphenols are strong antioxidants, and tea preparations have inhibitory activity againsttumorigenesis. Besides this, the wide spread consumption of tea throughout the world evoked the interest of thescientific community in the possibility of its use in cancer prevention. There are three main types of tea, allcoming 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 teapreparations has been demonstrated in various animal models in different organs. Various epidemiologicalstudies substantiate the correlation between tea consumption and cancer prevention; however, they have notyielded clear conclusions pertaining to the protective effects of tea consumption against cancer development inhumans. Many mechanisms have been proposed for the inhibition of carcinogenesis by tea, including themodulation 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 cellproliferation and transformation; induction of apoptosis of preneoplastic and neoplastic cells, and inhibition oftumor invasion as well as angiogenesis. These mechanisms need to be evaluated, verified and corroborated inanimal models and humans in order to gain more understanding on the effects of tea consumption on humancancer. Because the causative factors are different for different populations, tea consumption may affectcarcinogenesis only in selected situations rather than having the general effect on all cancers. Although, on thebasis of many epidemiological observations and numerous laboratory studies, it can be concluded that teaconsumption is likely to have beneficial effects in reducing cancer risk in different populations, yet there is aneed to define the population that could benefit from tea consumption. After careful evaluation of additionalstudies, it may be possible to recommend consumption of tea polyphenols by humans. Although considerableaccumulating information provides a compelling body of evidence for the preventive potential of tea againstcancer, naturally occurring tea polyphenols have yet to be evaluated in clinical intervention in human trials.