Reviewed: Yang G, Shu X-O, Li H, et al. Prospective cohort study of green tea consumption and colorectal cancer risk in women. Cancer Epidemiol Biomarkers Prey. June 2007;16(6):1219-1223.
Tea (Camellia sinensis, Theaceae) is the most popular beverage in China, and green tea is the most commonly consumed form of tea in the city of Shanghai. Although green tea and its constituents have shown antioxidative, anti-inflammatory, and anticarcinogenic activities in many in vitro and animal studies,( n1, n2) the association of green tea consumption with positive health outcomes in epidemiological studies is inconsistent. The authors of the present study analyzed data from a large prospective cohort study in urban Shanghai with detailed and repeated assessment of tea consumption to test the hypothesis that green tea consumption may reduce the risk of colorectal cancer (CRC).
Green tea contains many polyphenolic compounds, mainly catechins, comprising 30% to 40% of the extractable solids of dried green tea leaves.( n1) These compounds, especially epigallocatechin-3-gallate (EGCG), are believed to mediate many of the chemopreventative effects of tea. In addition, tea catechins have strong antioxidant activity.
The authors report on the Shanghai Women's Health Study, which recruited 74,942 women aged 40 to 70 years between 1996 and 2000 from seven urban communities of Shanghai. All participants were surveyed for information on demographic characteristics, lifestyle and dietary habits, medical history, and family history of cancer, among other factors. After exclusions were made for various reasons, including subjects who drank black or oolong tea regularly and exclusively, a total of 69,710 women remained.
The study followed the participants for occurrence of cancer and other chronic diseases with biennial home visits with them or their next of kin if the participants were deceased. In addition, the authors searched the records from the population-based Shanghai Cancer Registry monthly to learn of any new cancer cases in the study participants.
Green tea consumption was measured at the baseline survey and reassessed 2 to 3 years later. Each participant was asked whether she drank tea regularly (at least 3 times per week lasting at least 6 months) and at what age she started drinking tea, followed by questions on the type and amount of tea consumed during the past year, as well as current status of tea consumption.
Person-years of follow-up were calculated for each participant from the date of the baseline survey to the date of cancer diagnosis, death, or date of last follow-up (June 30, 2004), whichever came first.
About 30% of the study participants reported drinking green tea regularly at baseline. Among green tea drinkers, the median monthly consumption was 100 g dry weight (range, 50 to 150 g). The median duration of lifetime green tea consumption was 16 years (range, 8 to 24 years). The authors report that compared with those who never or occasionally drank tea, regular tea drinkers were slightly younger, had higher household income and educational attainment, and tended to exercise regularly and consume a little more vegetables and fruits. Only slight differences were noted in body mass index, waist-to-hip ratio, intakes of total energy and red meat, and family history of CRC between tea drinkers and nondrinkers. Few participants ever smoked cigarettes, drank alcoholic beverages, or used aspirin and other nonsteroidal anti-inflammatory drugs or postmenopausal hormones regularly.
During 6 years of follow-up, 256 incident cases of CRC (150 colon and 106 rectal cancer cases) were documented. After adjustment for age, women who reported drinking green tea regularly had a relative risk for CRC of 0.63 (95% confidence interval [CI], 0.45-0.88) compared with nonregular tea drinkers. The authors further report that the risk of CRC tended to decrease further with increasing the amount of tea consumed (Ptrend=0.01) and with increasing the duration of lifetime tea consumption (Ptrend=0.006). The reduction in risk was most evident among the participants who consistently reported drinking tea regularly at both the baseline and follow-up surveys (relative risk, 0.43; 95% CI, 0.24-0.77). For those participants who quit drinking tea or started drinking tea after the baseline recruitment, the relative risks were 0.91 (95% CI, 0.53-1.56) and 0.81 (95% CI, 0.50-1.30), respectively.
The inverse association between green tea consumption and the risk of CRC was independent of known risk factors for the disease (e.g., physical activity, body mass index, waist-to-hip ratio, red meat consumption, and vegetable and fruit intake).
The authors conclude that regular consumption of green tea may reduce the risk for CRC in women. These findings are consistent with data from in vitro and in vivo experiments, indicating that tea may serve as an effective chemopreventive agent.
PHOTO (COLOR): Tea Camellia sinensis Photo © 2007 Steven Foster
1. Yang CS, Maliakal P, Meng X. Inhibition of carcinogenesis by tea. Annu Rev Pharmacol Toxicol. 2002;42:25-54.
2. Lambert JD, Hong J, Yang GY, Liao J, Yang CS. Inhibition of carcinogenesis by polyphenols: evidence from laboratory investigations. Am J Clin Nutr. 2005;81:284-915.
By Shari Henson