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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Nutr. Author manuscript; available in PMC Feb 1, 2010.
Published in final edited form as:
PMCID: PMC2646205
NIHMSID: NIHMS77546

Green tea drinking modestly reduces breast cancer risk4,5

Abstract

Green tea is a commonly consumed beverage in China. Epidemiological and animal data suggest tea and tea polyphenols may be preventive against various cancers, including breast cancer. Catechol-O-methyltransferase (COMT) catalyzes catechol estrogens and tea polyphenols. COMT rs4680 AA genotype leads to lower COMT activity which may affect the relationship between green tea consumption and breast cancer risk. We evaluated whether regular green tea consumption was associated with breast cancer risk among 3454 incident cases and 3474 controls aged 20–74 in a population-based case-control study conducted in Shanghai, China during 1996–2005. All participants were interviewed in person about green tea consumption habits including age of initiation, duration of use, brew strength and quantity of tea. Odds ratios (OR) and 95% confidence intervals (CI) were calculated for green tea consumption measures and adjusted for age and other confounding factors. Compared with never drinkers, regular drinking of green tea was associated with a slight decreased risk for breast cancer (OR=0.88, 95% CI: 0.79–0.98). Among premenopausal women, reduced risk was observed for years of green tea drinking (p for trend, 0.02) and a dose response relationship with the amount of tea consumed per month was also observed 18 (p for trend, 0.046). No modifying effect of COMT rs4680 genotypes on the association of green tea intake with breast cancer risk was observed. Drinking green tea may be weakly associated with a decreased risk of breast cancer.

Introduction

Tea is a commonly consumed beverage worldwide. Although all tea, black, green and oolong, originates from the same plant, Camellia sinesis, the properties of these teas differ due to processing (1, 2). As a result, green tea, which is processed to prevent fermentation and oxidation, has much higher levels of some antioxidant polyphenols than black tea (3). Epidemiological and animal data suggest tea and tea polyphenols may be preventive against various cancers (46). These polyphenols and other components have been described as having anti-oxidant (7), pro-oxidant (8), tumor inhibitory(9, 10), anti-apoptotic (11), anti-angiogenesis (12, 13), anti-estrogenic (14, 15), epigenetic(16) and other potentially chemopreventive properties (5, 1720). In chemical carcinogen-induced cancers studies, green tea has been shown to inhibit or delay tumor formation and burden (21, 22).

Although tea has been extensively investigated in in vitro and in vivo studies, few epidemiologic studies have evaluated the relationship between green tea and breast cancer risk (2328), and results from these studies are inconsistent (2933). In general, the cohort studies, all based in Japan, report no statistically significant association (34, 35) and the case-control studies (3638), based on Asian-American or Chinese populations, all report an inverse relationship between green tea and breast cancer risk, although sometimes only among a subgroup (39). Previous studies have not evaluated the relationship between green tea consumption and pre- and post-menopausal breast cancer.

Estrogens, estrone, and estradiol are catabolized to catechol estrogens, estrogen metabolites, such as 4-hydroxyestrone and 4-hydroxyestrone, shown to be involved in breast carcinogenesis (40, 41). COMT catalyzes the O-methylation of these carcinogenic estrogens to methoxyestradiols and methoxyestrones. COMT also catalyzes the O-methylation of tea polyphenols. In the COMT gene, a G to A transition results in an amino acid change (Val-> Met) at codon 108 of soluble COMT and codon 158 of membrane-bound COMT (42). Studies have shown that the AA (Met/Met) genotype of rs4680 in the COMT gene is associated with 2- to 4-fold decreased enzyme activity (4345). Although a recent meta-analysis (46) did not find any evidence that COMT genotypes are independently associated with breast cancer risk, including among Caucasian, Asian, pre-menopausal, and post-menopausal women, one previous study has reported green and black tea consumption is protective for breast cancer only among the AA COMT genotype of rs4680 (47).

In this study, we evaluated green tea consumption habits and risk of breast cancer in a large population-based study of breast cancer from Shanghai, China. The study was designed to evaluate the green tea and breast cancer hypothesis. We further evaluated whether the relationship between green tea consumption and breast cancer risk was modified by COMT rs4680 genotype.

Materials and Methods

Participant Recruitment

The Shanghai Breast Cancer Study is a large population-based case control study conducted in Shanghai, China during August 1996-March 1998 (Phase I) and April 2002-February 2005 (Phase II). Detailed study methods have been published elsewhere (48, 49). Briefly, cases were identified primarily through the Shanghai Cancer Registry and diagnoses were confirmed by pathology review supplemented by medical record review. All incident breast cancer cases, newly diagnosed during the study period, and meeting the following criteria were eligible for this study: aged 25–70 y, resident of urban Shanghai, no previous history of any cancer, and alive at the time of interview. Controls had inclusion criteria identical to those of the cases with the exception of a breast cancer diagnosis. Controls were randomly selected and were frequency matched on age (5 year intervals) to the expected age distribution of the cases in a 1:1 ratio. Controls were selected using the Shanghai Resident Registry, a population-based registry containing address and demographic information for all residents of urban Shanghai. A total of 1,455 (response rate: 91.1%) and 1,999 (83.7%) cases and 1,556 (90.3%) and 1,918 (70.4%) controls were recruited in Phase I and II respectively, resulting in a total of 3,454 cases and 3,474 controls. The study protocols were approved by the Institutional Review Boards of all institutes involved in the study.

Data Collection

Information on demographic characteristics, personal and family history of cancer, other diseases, and behavioral and dietary habits and green tea consumption were collected by in-person interviews conducted by trained staff in the participant’s home. The food frequency questionnaire was designed to capture usual intake of 76 food items in the five years prior to diagnosis and over 85% of foods commonly consumed in Shanghai. Detailed information on the validated food frequency questionnaire has been previously published (50). With few exceptions, the questionnaires from both phases were identical. All participants were asked whether they drank tea regularly which was defined as at least twice per week for at least 3 mo continuously. If yes, they were also asked about the type of tea they usually consumed (green, black, oolong, or other), the age they started drinking green tea regularly, the total number of years they had consumed green tea, the type of brew they preferred (light, moderate, heavy), how often they changed the tea leaves, and the amount of tea leaves they usually consumed per month or per year. Among those that consumed tea regularly, over 92% only drank green tea. Thus, it is not possible to evaluate other types of tea consumption in this study and the analysis is limited to those that did not drink tea regularly and those who consumed green tea regularly.

Laboratory Methods

Genotyping assays for the COMT rs4680 polymorphism was conducted for cases and controls recruited in Phase I in which blood samples were collected from 1193 (82%) cases and 1310 (84%) controls. A detailed description of the methods to determine COMT genotype have been previously published (51). In brief, Genomic DNA was extracted from blood samples with the Puregene DNA Purification Kit (Gentra Systems, Minneapolis, MN) following the protocol of the manufacturer. The COMT rs4680 genotyping were performed using PCR-RFLP. PCR primers, restriction enzymes, and length of the resulting fragments are published elsewhere(52). The PCR was done in a Biometra T Gradient Thermocycler. Each 25 mL of PCR mixture contained 10 ng DNA, 1x PCR buffer with 1.5 mmol/L MgCl2, 0.16 mmol/L each of deoxynucleotide triphosphate, 0.4 µmol/L of each primer, and 1 unit of HotstarTaq DNA polymerase (Qiagen, Valencia, CA). The reaction mixture was initially denatured at 95°C for 15 minutes followed by 35 cycles of 94°C for 45 seconds, 59°C to 62°C for 45 seconds, and 72°C for 45 seconds. The PCR was completed by a final extension cycle at 72°C for 8 minutes. Each PCR product (10 µL) was digested with restriction enzymes (New England BioLabs, Beverly, MA) at 37°C for 3 hours. The DNA fragments were then separated and visualized by electrophoresis on 1.5% to 3% agarose gel containing ethidium bromide. The laboratory staff was blinded to the identity of the participants. Quality control samples were included in genotyping assays. Each 96-well plate contained one water, two DNA, two blinded quality control DNA, and two unblinded quality control DNA samples. The consistency rate between the quality control and study samples was 96.2%. Excluding a few Phase I participants for whom sufficient DNA was not available or for whom the genotyping assay failed, genotyping data were obtained from 1116 cases and 1191 controls in the green tea analysis.

Data Analysis

Odds ratios (OR) were used to measure the association of breast cancer risk with green tea consumption habits. Unconditional logistic regression models were used to obtain maximum likelihood estimates of the odds ratios and their 95 percent confidence intervals (95% CI), after adjusting for potential confounding variables. Risk factors previously identified as having an independent association with breast cancer in this population were controlled in all models. These included breast cancer in a first degree relative, history of fibroadenoma, age at menarche, age at first live birth, age at menopause, waist-to-hip ratio, physical activity, and number of parity. Models also controlled for age, enrollment period, education, total fruit and vegetable intake and total calorie intake. Age at diagnosis or interview was included as a continuous variable throughout. Tests for trend were performed by entering the midpoint of each category as a continuous variable in the model. Stratified analyses were used to evaluate potential effect modification. Tests for multiplicative interaction were done by including multiplicative variables in the logistic model and performing the likelihood ratio test. All statistical tests were based on two-sided probabilities (α=0.05) using SAS, version 9.1 (SAS Institute, Inc., Cary, NC).

Results

In Table 1, comparisons between cases and controls by study phase are presented for select demographic factors, established breast cancer risk factors, and dietary factors. In general, risk factors were comparable between the two study phases. In both phases and in comparison to controls, cases were more likely to have higher educational attainment, a positive family history of breast cancer in a first degree relative, a personal history of breast fibroadenoma, a higher waist-to-hip ratio, an older age at first live birth and less likely to be physically active in the past 10 y.

Table 1
Characteristics of breast cancer cases and controls. Shanghai Breast Cancer Study, Phases I and II1.

In Table 2, comparisons among controls between those who consumed or did not consume green tea regularly are presented. In comparison to those that never drank green tea regularly, regular tea consumers were younger and of higher educational attainment, had higher household income, had a younger age at menarche, were older at first live birth, were more likely to be pre-menopausal, and had higher daily intakes of energy, fruits and vegetables, and fat.

Table 2
Characteristics of controls by green tea drinking use. Shanghai Breast Cancer Study, Phases I and II1.

In Table 3, the associations between green tea drinking habits and breast cancer risk are presented for the total study population and stratified by menopausal status. Compared with never drinkers, regular drinking of green tea was associated with a statistically significant 12% decreased risk for breast cancer (OR=0.88, 95% CI: 0.79–0.98). The pattern was similar for both pre- or post-menopausal women, although the result was no longer statistically significant among post-menopausal women (OR=0.88; 95% CI: 0.74–1.04). Among pre-menopausal women, there was no relationship between age drinking began and breast cancer. However, among post-menopausal women and in the total study population, an older age of initiation was associated with decreased risk of breast cancer (OR=0.75; 95% CI: 0.57–0.99; OR=0.80; 95%CI: 0.65–0.98, for age of initiation of 41 y or more vs never drinkers, respectively). Years of drinking were associated with decreased risk among pre-menopausal women (p for trend, 0.02) while only green tea drinking for less than 6 y was associated with a significantly reduced risk among postmenopausal women (OR=0.61; 95% CI: 0.43–0.87). Three separate measures of usual dose of tea consumption were evaluated. The amount of dry tea leaves consumed per month showed a trend towards a decreased risk of breast cancer among pre-menopausal women (p for trend, 0.046) although not in the highest level of intake. Among post-menopausal women, only women with either the lowest or second greatest intake of tea leaves were at a statistically significantly reduced risk of breast cancer (p <0.05). Compared to never drinkers, a preference for heavy brew was associated with a borderline reduced risk for breast cancer in the total study population (p for trend, 0.02) and, particularly, for pre-menopausal women (p for trend, 0.01). Risk was also inversely related to frequency of leaf changes in pre-menopausal women (p for trend, 0.03). We evaluated soy and folate intakes as effect modifiers of the green tea and breast cancer association and did not find any evidence that either of these factors is an effect modifier in this population (Data not shown).

Table 3
Associations between green tea drinking habits and breast cancer risk. Shanghai Breast Cancer Study, Phases I and II.

In Table 4, the associations of green tea drinking and breast cancer risk are presented stratified by COMT rs4680 genotype. Although the relationship between green tea drinking and breast cancer risk was somewhat weakened in this smaller subset of participants, COMT genotype did not appear to modify the associations. We also evaluated COMT genotype, green tea intake, and breast cancer risk stratified by menopausal status (data not shown). COMT genotype did not modify the association between green tea drinking and breast cancer risk either among pre- or post-menopausal women.

Table 4
Associations between green tea drinking habits and breast cancer risk stratified by COMT genotype. Shanghai Breast Cancer Study, Phase I.

Discussion

In this study, we found that risk for breast cancer was weakly inversely associated with regular green tea drinking. Among pre-menopausal women, this relationship appeared to be related to years of drinking, and measures of frequency and amount. Among post-menopausal women, the relationship was stronger with recent use and lower amounts of intake. Among both pre- and post-menopausal women, there was a possible U-shaped relationship between amount of tea leaves consumed per month and risk of breast cancer. The relationship between green tea intake and breast cancer risk did not vary according to COMT rs4680 genotype.

Black tea has been evaluated in several previous studies (5358) and most, including a meta-analysis (59), found no relationship between tea and breast cancer. Few studies have reported on use of green tea and risk for breast cancer. Three cohort studies, all conducted in Japan where green tea consumption is highly prevalent, did not find a statistically significant relationship between breast cancer and green tea (60, 61). However, these studies may have been limited by a small number of cases, inability to comprehensively control for confounders, and a small unexposed reference group. Nonetheless, in two meta-analyses of these studies, the summary ORs, although not statistically significant, were 0.85 and 0.89 which are consistent with our finding of a weak association (62, 63). Three case-control studies, a population-based study of Asian Americans in Los Angeles (64), a population-based study of Chinese living in Singapore(65), and a hospital-based study in Southeast China (66), have evaluated green tea drinking and breast cancer risk. All three studies found inverse associations with at least one measure of tea drinking, although the Singapore study only observed an association among a subset of women defined by genotype (67). Most recently, the hospital-based Chinese study found all measures of green tea drinking were associated with reduced breast cancer risk, and many were associated in a dose-dependent manner(68). These findings are in contrast to our observation of a weak association between green tea consumption and breast cancer risk in a large population-based study. There are several notable differences between our and previous studies including the measurement, definition and prevalence of tea consumption. The Japanese studies, as mentioned previously, had virtually no abstainers from tea consumption and very high consumption patterns based on the number of cups per day(69, 70). In contrast to our study, consumers in two of the previous case-control studies, reported much higher prevalence of any green tea consumption, but much lower average intake of tea, with most controls consuming less than daily or weekly(71, 72). In our study, nearly one-third of the population consumes tea at least twice a week. These other studies reported only one measure of tea intake, cups, mL, or frequency. Only our study and the previous study in a Chinese population (73) had multiple measurements of tea intake and only our study included strength of brew. Over half of the hospital-based controls in the previous Chinese study reported drinking green tea and most also consumed the tea at least daily, a prevalence that is much higher but a frequency that is similar to our study. However, even though the prevalence was much higher than in our study, the amount of dried tea leaves consumed per year was substantially less than the amount of tea leaves reported by our population-based controls in another part of China.

Several plausible mechanisms have been proposed for green tea’s possible chemopreventive properties. Green tea components, among other activities, affect cell cycle arrest(74), have anti-oxidant properties(75), down regulate telomerase(76), inhibit vascular endothelial growth factor(7779), suppress cellular proliferation(80, 81), upregulate or maintain intercellular gap junction communication(82, 83), and increase apoptosis(84). In addition, green tea has also been described as having anti-estrogenic properties. As a hormone-dependent cancer, estrogen plays a critical role in breast carcinogenesis. In vitro studies found that tea polyphenols inhibit aromatase, the key enzyme converting androgens to estrone or estradiol (85). Furthermore, consumption of green tea was associated with reduced levels of estrogens, estrone and estradiol, among pre- and post-menopausal women(8688). No previous study has evaluated the association between green tea and breast cancer by menopausal status. Our study suggests the association between green tea drinking and breast cancer risk may differ by menopausal status and the inverse association with measures of longer duration and larger dose may be more pronounced among pre-menopausal women, while only recent and mild use is associated with decreased risk among post-menopausal women. It is also possible that we did not observe consistent relationships among post-menopausal women because the sample size was smaller than for pre-menopausal women.

There are also plausible mechanisms by which COMT and green tea may interact to affect breast cancer risk. On the one hand, low COMT activity may lead to elevated levels of catechol estrogens, but, on the other hand, low activity may also lead to a slower metabolism of tea polyphenols which have anti-estrogenic effects. A population-based study of Asian Americans is the only previous study to evaluate tea consumption, including green tea, COMT rs4680 genotype, and breast cancer risk (89). They found the inverse relationship they initially observed between tea intake and breast cancer (90) was primarily limited to those individuals with at least one low activity allele (91). We, however, found no evidence COMT rs4680 genotype affects the relationship between green tea consumption and breast cancer risk in a Chinese population. Based on the findings in our population, it appears the effect of green tea is independent of the rate of O-methylation of both the tea polyphenols and of catechol estrogens or that the anti-carcinogenic effects of green tea, including anti-estrogenic effects, are much greater than any small genetic differences in COMT activity.

As in all case-control studies, recall bias is a potential concern. However, nearly all regular drinkers reported daily use of green tea indicating that, in this population, green tea drinking is a frequent habit which may facilitate recall for both cases and controls. Interestingly, the proportion of controls reporting regular green tea consumption remained constant in both study phases (31%) which may also indicate green tea consumption patterns are relatively stable among women in Shanghai. Although we controlled for several potential confounders, it is possible that residual confounding remained. It is also possible that some of the associations we observed were due solely to chance or were a result of multiple comparisons. This study has several strengths. This is a population-based study with a large sample size and high response rates. We were able to evaluate several measures of green tea consumption including age of initiation, duration of use, brew strength, and quantity of tea leaves. We were also able to evaluate both pre-and post-menopausal breast cancer risk related to green tea drinking.

In summary, this population-based case-control study found a weak independent role for green tea consumption in breast cancer risk. The relationship may differ by menopausal status. Future population-based or cohort studies in populations with frequent and long-term green tea consumption are needed to further investigate green tea’s potential role in breast carcinogenesis and to elucidate the part menopausal status may play in this role.

Acknowledgment

The authors wish to express their gratitude to Dr. Fan Jin for her contributions in coordinating data collection in Shanghai.

This research was supported by grant R01CA64277 from the US National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or National Institutes of Health.

Footnotes

Author disclosures: Martha J. Shrubsole, Wei Lu, Zhi Chen, Xiao Ou Shu, Ying Zheng, Qi Dai, Qiuyin Cai, Kai Gu, Zhi Xian Ruan, Yu-Tang Gao, and Wei Zheng, no conflicts of interest

Abbreviations used: COMT, catechol-O-methyltransferase; OR, Odds Ratio

Publisher's Disclaimer: This is an un-copyedited author manuscript that has been accepted for publication in (the Journal of Nutrition), copyright © American Society for Nutrition (ASN). This manuscript may not be duplicated or reproduced, other than for personal use or within the rule of ‘Fair Use of Copyrighted Materials’ (section 107, Title 17, US Code) without permission of the copyright owner, the ASN. The final copyedited article, which is the version of record, can be found at http://www.nutrition.org/publications/. The ASN disclaims any responsibility or liability for errors or omissions in this version of the manuscript or in any version derived from it by the National Institutes of Health or other parties.

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