Iwasaki M, Inoue M, Sasazuki S, et al. Green tea drinking and subsequent risk of breast cancer in a population-based cohort of Japanese women. Breast Cancer Res. 2010;12(5):R88.
This is a prospective study that followed 53,793 women in Japan. In this group of women, 581 cases of breast cancer were newly diagnosed within the first 5 years. After the initial 5-year follow-up survey in 1995 to 1998, 350 additional cases were newly diagnosed in 43,639 women during 9.5 years’ follow-up. With questionnaires administered at baseline, the frequency of total green tea drinking was assessed. At 5-year follow-ups, and the frequency of consumption of Sencha and Bancha/Genmaicha green tea was assessed and correlated to breast cancer incidence over the course of 13.6 years of follow-up.
This prospective trial found no statistically significant difference in the incidence of breast cancer in women who consumed more than 5 to 10 cups of green tea (1 cup = 4 oz. tea) per day versus those who consumed less than one cup per day over an average follow-up of 13.6 years. Compared with women who drank less than 1 cup of green tea per week, the adjusted hazard ratio (HR) for women who drank 5 or more cups per day was 1.12 (95% confidence interval [CI] 0.81–1.56; P for trend = 0.60) in the baseline data. Similarly, compared with women who drank less than 1 cup of Sencha or Bancha/Genmaicha per week, adjusted HRs for women who drank 10 or more cups per day were 1.02 (95% CI 0.55–1.89; P for trend = 0.48) for Sencha and 0.86 (0.34–2.17; P for trend = 0.66) for Bancha/Genmaicha. Neither hormone receptor status of the breast cancer nor menopausal status influenced these results.
This large-scale, population-based prospective cohort study in Japan, along with 2 other Japanese cohort studies,1,2 refutes the results of several prospective trials that have previously shown a statistically significant reduction in the risk of breast cancer in women consuming large quantities of green tea.3,4,5 The results of this study are also at odds with smaller prospective trials that demonstrate statistically significant increases in disease-free survival in heavy green tea consumers previously treated for early stage breast cancer. Additionally, a prospective trial out of China demonstrated statistically significant reduction in premenopausal breast cancer risk in women who consumed large amounts of green tea beginning in their 20s.
Thus, while the results of this study are at odds with some previous studies, there are several reasons to give this latest study serious consideration. The study was prospective in design, a strong study design. The study had the advantage of having a wide variance in regular green tea consumption—from less than 1 cup daily to more than 10 cups daily. Approximately 12% of women drank less than 1 cup per week of green tea, and 27% drank 5 or more cups per day in the baseline data. In the 5-year follow-up data, 22% and 30% of women did not drink Sencha and Bancha/Genmaicha, while 5.2% and 2.5% drank 10 or more cups per day. The response rate of more than 80% to the questionnaire strengthened the statistical analysis. While self-reporting via a questionnaire may have led to inaccurate results, a validation study verified the validity of the questionnaire.6 Furthermore, these researchers utilized the same study design and methodology previously and, with this methodology, found a correlation between green tea consumption and lowered risk of distal gastric cancer and advanced prostate cancer. This would indicate a reliable methodology. The study did not control for drinking methods, which may have affected total polyphenol availability. However, the investigators conducted a nested case-control study7 within this study and found no overall association between plasma tea polyphenols and the risk of breast cancer, thus decreasing the likelihood of the green tea preparation method influencing the results. In essence, this study makes a fairly strong case for the lack of benefit of green tea consumption by Japanese women living in Japan for the prevention of breast cancer.
It is important to recognize that other cohort trials have shown statistically significant benefit in other populations—namely Chinese women who started their green tea consumption before the age of 25 experiencing a decreased risk of premenopausal breast cancer. Furthermore, Japanese women living in the United States with a previous diagnosis of early-stage breast cancer and after definitive treatment benefit from the consumption of at least 8 cups of green tea daily in increasing their chance of disease-free survival. It may be that in a population of women already at relatively low risk for the development of breast cancer, frequent green tea consumption is not an impactful prevention strategy. The lack of benefit in this study may also be the result of the fact that total dietary polyphenol consumption is already fairly high through the standard Japanese diet. This may obfuscate the impact of the polyphenols from green tea, even when consumed in relatively high amounts.
It is possible that in women consuming a low-polyphenol diet, such as the typical Western diet, the polyphenols from green tea consumption could have greater preventative impact.
It is possible that in women consuming a low-polyphenol diet, such as the typical Western diet, the polyphenols from green tea consumption could have greater preventative impact. Additionally, higher-risk women, women with a previous history of breast cancer, women with genetic polymorphisms that affect polyphenol or folate metabolism, and perhaps younger women may yet benefit from the cancer-preventive effects of green tea.
The relatively low incidence of breast cancer in Japanese women (age-standardized rate per 100,000 in 2002 was 32.7 in Japan compared to 101.1 in United States) may require a larger sample size to determine smaller, but significant effects. Additionally, although this study controlled for a number of potentially confounding variables, other, unidentified factors may have confounded the results.
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