Extracts of tea, especially green tea, and tea polyphenols have been shown to inhibit the formation and development of tumours at different organ sites in animal models. There is considerable evidence that tea polyphenols, in particular (−)-epigallocatechin-3-gallate, inhibit enzyme activities and signal transduction pathways, resulting in the suppression of cell proliferation and enhancement of apoptosis, as well as the inhibition of cell invasion, angiogenesis and metastasis. Here, we review these biological activities and existing data relating tea consumption to human cancer risk in an attempt to understand the potential use of tea for cancer prevention.Dietary factors can substantially influence human cancer risk. Many food items, beverages and dietary constituents have reported cancer-preventive or anticancer activities. Tea, a commonly consumed beverage derived from the dried leaves of the Camellia sinensis plant, has been studied extensively for its health benefits, including cancer prevention. These studies are important because tea is consumed by a large proportion of the world's population and is the most popular beverage after water. Therefore, tea is the major source of dietary flavonoids (or polyphenols) in many populations. Tea is a unique food item in that the major constituents, especially those in green tea, are well characterized, allowing their biological activities to be thoroughly studied. The relationship between tea consumption and human cancer can be investigated using retrospective and prospective studies in which tea consumption is assessed by dietary recalls or by specific urinary markers.Although different aspects of tea and cancer prevention have been covered by previous reviews [1][2][3][4][5][6][7][8][9][10] , this Review critically assesses existing data and discusses our current understanding of how tea constituents might prevent cancer. We use selected recent findings to illustrate the types of laboratory results that are available. We also briefly review the results of human epidemiological and intervention studies, interpret these results on the basis of our understanding of the biochemical properties of tea constituents (BOX 1) and discuss possible mechanisms by which tea polyphenols prevent cancer development. Finally, we discuss the opportunities and challenges presented by the study of tea for cancer prevention. We hope that this Review will have wide-ranging implications, as many of the issues discussed here might also be applicable to studies of other dietary materials.Correspondence to: Chung S. Yang, csyang@rci.rutgers.edu. NIH Public Access Author ManuscriptNat Rev Cancer. Author manuscript; available in PMC 2010 March 1. Tea constituents and their biochemical propertiesThe major catechins (a group of polyphenols) in green tea are (−)-epigallocatechin-3-gallate (EGCG), (−)-epigallocatechin (EGC), (−)-epicatechin-3-gallate (ECG) and (−)-epicatechin (BOX 1). Tea catechins are characterized by the dihydroxyl or trihydroxyl substitutions on the B ring and the m-...

The results on their own are not definitive, but the promising findings should stimulate further research to clarify the potential benefits of micronutrient supplements.

Plants consumed by humans contain thousands of phenolic compounds. The effects of dietary polyphenols are of great current interest due to their antioxidative and possible anticarcinogenic activities. A popular belief is that dietary polyphenols are anticarcinogens because they are antioxidants, but direct evidence for this supposition is lacking. This chapter reviews the inhibition of tumorigenesis by phenolic acids and derivatives, tea and catechins, isoflavones and soy preparations, quercetin and other flavonoids, resveratrol, and lignans as well as the mechanisms involved based on studies in vivo and in vitro. Polyphenols may inhibit carcinogenesis by affecting the molecular events in the initiation, promotion, and progression stages. Isoflavones and lignans may influence tumor formation by affecting estrogen-related activities. The bioavailability of the dietary polyphenols is discussed extensively, because the tissue levels of the effective compounds determine the biological activity. Understanding the bioavailability and blood and tissue levels of polyphenols is also important in extrapolating results from studies in cell lines to animal models and humans. Epidemiological studies concerning polyphenol consumption and human cancer risk suggest the protective effects of certain food items and polyphenols, but more studies are needed for clear-cut conclusions. Perspectives on the application of dietary polyphenols for the prevention of human cancer and possible concerns on the consumption of excessive amounts of polyphenols are discussed.

Tea is one of the most popular beverages consumed worldwide. The relationship between tea consumption and human cancer incidence is an important concern. This topic has been studied in different populations by many investigators, but no clear-cut conclusion can be drawn. Whereas some studies have shown a protective effect of tea consumption against certain types of cancers, other studies have indicated an opposite effect. Our purpose is to provide a critical review of this topic, covering basic chemistry and biochemical activity of tea, epidemiologic investigations, and laboratory studies, as well as possible directions for future research. Studies have demonstrated either a lack of association between tea consumption and cancer incidence at specific organ sites or inconsistent results. On the other hand, many laboratory studies have demonstrated inhibitory effects of tea preparations and tea polyphenols against tumor formation and growth. This inhibitory activity is believed to be mainly due to the antioxidative and possible antiproliferative effects of polyphenolic compounds in green and black tea. These polyphenolics may also inhibit carcinogenesis by blocking the endogenous formation of N-nitroso compounds, suppressing the activation of carcinogens, and trapping of genotoxic agents. The effect of tea consumption on cancer is likely to depend on the causative factors of the specific cancer. Therefore, a protective effect observed on a certain cancer with a specific population may not be observable with a cancer of a different etiology. On the basis of this concept, we suggest future laboratory and epidemiologic studies to elucidate the relationship between tea consumption and human cancer risk.