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J Nutr. Feb 2009; 139(2): 310–316.
PMCID: PMC2646205
NIHMSID: NIHMS77546

Drinking Green Tea Modestly Reduces Breast Cancer Risk1,2

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. The 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 y 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% CI were calculated for green tea consumption measures and adjusted for age and other confounding factors. Compared with nondrinkers, regular drinking of green tea was associated with a slightly 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–trend = 0.02) and a dose-response relationship with the amount of tea consumed per month was also observed (P-trend = 0.046). COMT rs4680 genotypes did not have a modifying effect on the association of green tea intake with breast cancer risk. 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 sinensis, 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 (1). Epidemiological and animal data suggest that tea and tea polyphenols may be preventive against various cancers (35). These polyphenols and other components have been described as having antioxidant (5), pro-oxidant (6), tumor inhibitory (7,8), antiapoptotic (9), antiangiogenesis (10,11), antiestrogenic (12,13), epigenetic (14), and other potentially chemopreventive properties (1,3,4,15,16). In chemical carcinogen-induced cancer studies, green tea has been shown to inhibit or delay tumor formation and burden (7,17).

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 (1823) and results from these studies are inconsistent (1923). In general, the cohort studies, all based in Japan, report no significant association (19,21) and the case-control studies (20,22,23), 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 (22). Previous studies have not evaluated the relationship between green tea consumption and pre- and postmenopausal 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 (24,25). Catechol-O-methyltransferase (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 (26). Studies have shown that the AA (Met/Met) genotype of rs4680 in the COMT gene is associated with 50–75% decreased enzyme activity (2628). Although a recent meta-analysis (29) did not find any evidence that COMT genotypes are independently associated with breast cancer risk, including among Caucasian, Asian, premenopausal, and postmenopausal women, one previous study has reported green and black tea consumption is protective for breast cancer only among the AA COMT genotype of rs4680 (30).

In this study, we evaluated green tea consumption habits and risk of breast cancer in a large population-based study of breast cancer in 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 the 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 (31,32). Briefly, cases were identified primarily through the Shanghai Cancer Registry and diagnoses were confirmed by pathology reviews supplemented by medical record reviews. 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, and no previous history of any cancer. 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-y 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 1455 (response rate, 91.1%) and 1999 (83.7%) cases and 1556 (90.3%) and 1918 (70.4%) controls were recruited in Phase I and II, respectively, resulting in a total of 3454 cases and 3474 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, behavioral and dietary habits, and green tea consumption were collected by in-person interviews conducted by trained staff in the participant's home. The FFQ was designed to capture usual intake of 76 food items in the 5 y prior to diagnosis and over 85% of foods commonly consumed in Shanghai. Detailed information on the validated FFQ has been previously published (33). 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 at which 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% drank only green tea. Thus, it is not possible to evaluate other types of tea consumption in this study and the analysis was limited to those who 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 (29). In brief, genomic DNA was extracted from blood samples with the Puregene DNA Purification kit (Gentra Systems) following the protocol of the manufacturer. The COMT rs4680 genotyping was performed using PCR-restriction fragment length polymorphism. PCR primers, restriction enzymes, and length of the resulting fragments are published elsewhere (29). The PCR was conducted in a Biometra T Gradient Thermocycler. Each 25 mL of PCR mixture contained 10 ng DNA, 1× 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). The reaction mixture was initially denatured at 95°C for 15 min followed by 35 cycles of 94°C for 45 s, 59–62°C for 45 s, and 72°C for 45 s. The PCR was completed by a final extension cycle at 72°C for 8 min. Each PCR product (10 μL) was digested with restriction enzymes (New England BioLabs) at 37°C for 3 h. The DNA fragments were then separated and visualized by electrophoresis on 1.5–3% agarose gel containing ethidium bromide. The laboratory staff was unaware of the identity of the participants. Quality control samples were included in genotyping assays. Each 96-well plate contained 1 water, 2 DNA, 2 blinded quality control DNA, and 2 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 OR and their 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: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 2-sided probabilities (α = 0.05) using SAS version 9.1 (SAS Institute).

Results

Comparisons between cases and controls by study phase are presented for select demographic factors, established breast cancer risk factors, and dietary factors (Table 1). In general, risk factors were comparable between the 2 study phases. In both phases and compared with 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:hip ratio, an older age at first live birth, and were less likely to be physically active in the past 10 y.

TABLE 1
Characteristics of breast cancer cases and controls in the Shanghai Breast Cancer Study, Phases I and II1

Comparisons among controls between those who consumed or did not consume green tea regularly are presented (Table 2). Compared with those who never drank green tea regularly, regular tea consumers were younger, had a higher educational attainment, had higher household income, were a younger age at menarche, were older at first live birth, were more likely to be premenopausal, and had higher daily intakes of energy, fruits and vegetables, and fat.

TABLE 2
Characteristics of controls by green tea drinking use in the Shanghai Breast Cancer Study, Phases I and II1

The associations between green tea drinking habits and breast cancer risk are presented for the total study population and stratified by menopausal status (Table 3). Compared with nondrinkers, regular drinking of green tea was associated with a significant, 12% lower risk for breast cancer (OR, 0.88; 95% CI, 0.79–0.98). The pattern was similar for both pre- or postmenopausal women, although the result was no longer significant among postmenopausal women (OR, 0.88; 95% CI, 0.74–1.04). Among premenopausal women, there was no relationship between age drinking began and breast cancer. However, among postmenopausal 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 vs. nondrinkers, respectively). Years of drinking was associated with decreased risk among premenopausal women (P-trend = 0.02), whereas only green tea drinking for <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 toward a decreased risk of breast cancer among premenopausal women (P-trend = 0.046) although not in the highest level of intake. Among postmenopausal women, only women with either the lowest or 2nd greatest intake of tea leaves were at a reduced risk of breast cancer (P < 0.05). Compared with nondrinkers, a preference for heavy brew was associated with a borderline reduced risk for breast cancer in the total study population (P-trend = 0.02) and, particularly, for premenopausal women (P-trend = 0.01). Risk was also inversely related to frequency of leaf changes in premenopausal women (P-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 was an effect modifier in this population (data not shown).

TABLE 3
Associations between green tea drinking habits and breast cancer risk in the Shanghai Breast Cancer Study, Phases I and II

The associations of green tea drinking and breast cancer risk are presented stratified by COMT rs4680 genotype (Table 4). 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 postmenopausal women.

TABLE 4
Associations between green tea drinking habits and breast cancer risk stratified by COMT genotype in the 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 premenopausal women, this relationship appeared to be related to years of drinking and measures of frequency and amount. Among postmenopausal women, the relationship was stronger with recent use and lower amounts of intake. Among both pre- and postmenopausal 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 (3439) and most, including a meta-analysis (34), found no relationship between tea and breast cancer. Few studies have reported on the 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 significant relationship between breast cancer and green tea (19,21). 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 2 meta-analyses of these studies, the summary OR, although not significant, were 0.85 and 0.89, which are consistent with our finding of a weak association (34,40). Three case-control studies, a population-based study of Asian Americans in Los Angeles (20), a population-based study of Chinese living in Singapore (22), and a hospital-based study in Southeast China (23), evaluated green tea drinking and breast cancer risk. All 3 studies found inverse associations with at least 1 measure of tea drinking, although the Singapore study only observed an association among a subset of women defined by genotype (22). 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 (23). 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 (18,21). In contrast to our study, consumers in 2 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 (20,22). In our study, nearly one-third of the population consumed tea at least twice a week. These other studies reported only 1 measure of tea intake: cups, milliliters, or frequency. Only our study and the previous study in a Chinese population (23) had multiple measurements of tea intake and only our study included strength of brew. Over one-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 (41), have antioxidant properties (5), downregulate telomerase (42), inhibit vascular endothelial growth factor (18,43,44), suppress cellular proliferation (18,41), upregulate or maintain intercellular gap junction communication (45,46), and increase apoptosis (41). In addition, green tea has also been described as having antiestrogenic 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 (12). Furthermore, consumption of green tea was associated with reduced levels of estrogens, estrone, and estradiol among pre- and postmenopausal women (13,47,48). 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 premenopausal women, whereas only recent and mild use is associated with decreased risk among postmenopausal women. It is also possible that we did not observe consistent relationships among postmenopausal women because the sample size was smaller than for premenopausal 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 antiestrogenic 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 (30). The authors found the inverse relationship they initially observed between tea intake and breast cancer (20) was primarily limited to those individuals with at least 1 low activity allele (30). We, however, found no evidence that the COMT rs4680 genotype affected 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 anticarcinogenic effects of green tea, including antiestrogenic 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 that 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 postmenopausal 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.

Acknowledgments

The authors thank Dr. Fan Jin for her contributions in coordinating data collection in Shanghai.

Notes

1Supported 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 NIH.

2Author disclosures: M. J. Shrubsole, W. Lu, Z. Chen, X. O. Shu, Y. Zheng, Q. Dai, Q. Cai, K. Gu, Z. X. Ruan, Y-T Gao, and W. Zheng, no conflicts of interest.

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