Suche
reference
Iwasaki M, Inoue M, Sasazuki S, et al. Green tea drinking and resulting breast cancer risk in a population-based cohort of Japanese women.Breast Cancer Res.2010;12(5):R88.
design
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 first 5-year follow-up in 1995 to 1998, 350 additional cases were newly diagnosed in 43,639 women during the 9.5-year follow-up. The frequency of overall green tea consumption was determined using questionnaires administered at baseline. At 5-year follow-up visits, the frequency of sencha and bancha/genmaicha green tea consumption was assessed and associated with breast cancer incidence over a period of 13.6 years.
Key findings
This prospective study 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 ounces of tea) per day compared to women who consumed less than one cup per day for an average follow-up period of 13.6 years. Compared to 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. Compared with women who drank less than 1 cup of sencha or bancha/genmaicha per week, the adjusted HRs for women who drank 10 or more cups per day for sencha were 1.02 (95% CI 0.55-1.89; P for trend = 0.48). and 0.86 (0.34–2.17; P for trend = 0.66) for Bancha/Genmaicha. Neither breast cancer hormone receptor status nor menopausal status influenced these results.
Effects on practice
This large-scale, population-based prospective cohort study in Japan, together with two other Japanese cohort studies,1.2refutes the results of several prospective studies that had previously shown a statistically significant reduction in breast cancer risk in women who consumed large amounts of green tea.3,4,5The results of this study are also at odds with smaller prospective studies showing statistically significant increases in disease-free survival in heavy green tea consumers previously treated for early-stage breast cancer. Additionally, a prospective study from China showed a statistically significant reduction in premenopausal breast cancer risk in women who consumed large amounts of green tea beginning in their 20s.
Although the results of this study contradict some previous studies, there are several reasons to seriously consider this latest study. The study was prospective, a strong study design. The study's advantage was that there were wide variations in regular green tea consumption - from less than 1 cup daily to more than 10 cups daily. About 12% of women drank less than 1 cup of green tea per week in baseline data, and 27% drank 5 or more cups per day. In the 5-year follow-up data, 22% and 30% of women, respectively, did not drink sencha and bancha/genmaicha, while 5.2% and 2.5%, respectively, drank 10 or more cups per day. The response rate of over 80% to the questionnaire supported the statistical analysis. While self-reporting of a questionnaire may have produced inaccurate results, the validity of the questionnaire was confirmed by a validation study.6In addition, these researchers previously used the same study design and methodology and, using this methodology, found an association between green tea consumption and a lower risk of distal gastric cancer and advanced prostate cancer. This would indicate a reliable methodology. The study did not control for drinking methods that may have affected overall polyphenol availability. However, the researchers conducted a nested case-control study7as part of this study and found no overall association between plasma tea polyphenols and breast cancer risk, reducing the likelihood that the green tea preparation method influenced the results. Essentially, this study makes a pretty strong argument that green tea consumption has no breast cancer prevention benefit for Japanese women living in Japan.
It is important to recognize that other cohort studies have shown statistically significant benefits in other populations - particularly in Chinese women who started consuming green tea before age 25 and who are at lower risk of breast cancer before menopause. In addition, Japanese women living in the United States who have already been diagnosed with early-stage breast cancer and have received definitive treatment benefit from consuming at least 8 cups of green tea per day, increasing their chance of disease-free survival. It may be that in a group of women who already have a relatively low risk of developing breast cancer, frequent consumption of green tea may not be an effective prevention strategy. The lack of benefit in this study may also be due to the fact that total dietary polyphenol consumption is already quite high in the standard Japanese diet. This can mask the effects of green tea polyphenols, even when consumed in relatively large quantities.
It is possible that the polyphenols from green tea consumption could have a greater preventive effect in women consuming a low polyphenol diet, such as the typical Western diet.
It is possible that the polyphenols from green tea consumption could have a greater preventive effect in women consuming a low polyphenol diet, such as the typical Western diet. Additionally, women at higher risk, women with a history of breast cancer, women with genetic polymorphisms that affect polyphenol or folate metabolism, and possibly younger women may benefit from green tea's cancer-preventing effects.
restrictions
The relatively low incidence of breast cancer in Japanese women (the age-standardized rate per 100,000 in 2002 was 32.7 in Japan compared to 101.1 in the United States) may require a larger sample to detect smaller but significant effects. Additionally, although this study controlled for a number of potentially confounding variables, other unidentified factors may also have confounded the results.
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