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Cancer Prevention By Green Tea: Evidence from Epidemiological Studies

Green tea contains high concentrations of tea poly- phenols including (-)-epigallo- catechin-3-gallate (EGCG) at the highest concentration. Both tea extracts and EGCG have shown inhibitory effect against the development, progress, and growth of carcinogen-induced tumors in animal models at various organ sites including the oral-digestive tract, lung, prostate, mammary glands, and urinary bladder. Although epidemiological studies have provided inconclusive results on the effect of green tea consumption against the development of cancers in humans overall, the inverse association between high consumption of green tea and risk of oral-digestive tract cancers is more consistently observed in studies with adequate control for potential confounders. Green consump- tion was associated with statistically significantly reduced risk of esophageal cancer in men and women who did not consume ether alcohol or tobacco or (1). Biomarker studies showed that individuals with high levels of tea polyphenols in urine samples collected many years before cancer diagnosis experienced significantly reduced risk of esophageal, gastriC and colon cancers (2, 3). Randomized clinical trial showed that oral supplementation of green tea extract significantly reduced the size or progression of precan- cerous lesion of oral cavity in patients (4, 5). A randomized phase II clinical trial supported a protective role of green tea extract against the liver damage by aflatoxin exposure and hepatitis B, two established risk factors for liver cancer, t:ugges- ting a protective role of liver cancer (6). Epidemiological studies also have demonstrated an inverse, albeit moderate, association between green tea consumption and lung cancer. Intake of 2 cups of green tea per day would result in a statistically significant, approximately 20% decrease in the risk of developing lung cancer (7). This protective effect of green tea consumption on lung cancer was more pronounced in nonsmokers than in smokers (8). Although observational studies do not support a beneficial role of tea intake against the development of prostate cancer, a phase II clinical trials have demonstrated an inhibitory effect of green tea extract against the progression of prostate pre-malignant lesions to malignant tumors, and the protective effect lasted for at least 2 years after termination of green tea supplementation (9, 10). Evidence from epidemio- logical studies that examined the association between green tea consumption and risk of breast cancer was inconsistent. An inverse association was reported in case-control studies whereas a null association was found in prospective cohort studies (11). Given the important role of O-methylation by catechol-O-methyltransferase (COMT) in the conjugation reaction of tea catechins, several studies examined the modifying role of COMT geno'type on the green tea- breast cancer association and also produced inconsistent results (12. 13). There is no sufficient evidence that supports a protective role of green tea intake on the development of urinary bladder cancer. The difference between results from animal and human studies is likely to be due to (a) the relatively weak cancer preventive effect in humans because the lower quantities of green tea consumed by a healthy individual as compared to the doses used in animal studies, and (b) the confounding factors in the epidemiological studies that could reduce or even mask the true protective effect of green tea whereas in animal experimental studies the conditions are well controlled to maximize the likelihood to detect a protective effect. Future prospective observational studies with biomarkers of exposure and phase III clinical trials are required to provide definitive evidence for the hypothesized beneficial effect of tea consumption on cancer development in humans.

References cited


  1. Gao YT, McLaughlin JK, Blot WJ, et al. Reduced risk of esophageal cancer associated with green tea consumption. J Natl Cancer Inst 1994;86:855-8.
  2. Sun CL, Yuan J-M, Lee MJ, et al. Urinary tea polyphenols in relation to gastric and esophageal• cancers: a prospective study of men in Shanghai, China. Carcinogenesis 2002;23.T 497 -503.
  3. Yuan J-M, Gao YT, Yang CS, Yu MC. Urinary biomarkers of tea polyphenols and risk of colorectal cancer in the Shanghai Cohort Study. Int J Cancer 2007;120:1344-50.
  4. Li N, Sun Z, Han C, Chen J. The chemopreventive effects of tea on human oral precancerous mucosa lesions. Proceedings of the Society for Experimental Biology and Medicine 1999;220:218-24.
  5. Tsao AS, Liu D, Martin J, et al. Phase II randomized, placebo-controlled trial of green tea extract in patients with high-risk oral premalignant lesions.Cancer Prevention Research 2009;2:931-41.
  6. Luo H, Tang L, Tang M, et al. Phase Ha chemoprevention trial of green tea polyphenols in high-risk individuals of liver cancer: modulation of urinary excretion of green tea polyphenols and 8- hydroxydeoxyguanosine. Carcinogenesis 2006; 27: 262-8. '
  7. Tang N, Wu Y, Zhou B, Wang B, Yu R. Green tea, black tea consumption and risk of lung cancer: a meta- analysis. Lung Cancer 2009;65:274-83.
  8. Zhong L, Goldberg MS, Gao YT, et al. A population- based case-control study of lung cancer and green tea consumption among women living in Shanghai, China. Epidemiology 2001; 12:695-700.
  9. Bettuzzi S, Brausi M, Rizzi F, et al. Chemoprevention of human prostate cancer by oral administration of green tea catechins in volunteers with high-grade prostate intraepithelial neoplasia: a preliminary report from a one-year proof-of-principle study. Cancer Res 2006;66: 1234-40.
  10. Brausi M, Rizzi F, Bettuzzi S. Chemoprevention of human prostate cancer by . green tea catechins: two years later. A follow-up update. Eur Urol 2008; 54:472-3.
  11. Wu AH, Butler LM. Green tea and breast cancer. Mol Nutr Food Res 2011; 55:• 921-30.
  12. WuAH, Tseng CC, Van Den Berg 0, Yu MC. Tea intake, COMT genotype, and breast cancer, in Asian- American women. Cancer Res 2003;63:7526-9.
  13. Shrubsole MJ, Lu W, Chen Z, et al. Drinking green tea modestly reduces breast cancer risk. J Nutr 2009; 139:310-6


05 Aug, 2013

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