A new prospective Japanese study has reported that there is no link between drinking green tea and risk of breast cancer. Although many laboratory and animal studies have reported a protective effect of green tea against breast cancer, findings from population studies have been inconsistent and whether high green tea intake lowers breast cancer risk remains uncertain. The study initially included 53,793 women who were enrolled during the period 1990 to 1994 and followed for a median of 13.6 years. Green tea intake was determined upon enrollment and five years later. Approximately 12% of the women drank less than one cup per week while 27% drank five or more cups of green tea per day. The baseline questionnaire measured the frequency of total green tea consumption while the five-year follow-up questionnaire assessed intake of two specific types of green tea (Sencha and Bancha/Genmaicha). A total of 581 cases of breast cancer were diagnosed among the 53,793 women during the 13.6 follow-up period.
Women who drank at least five cups of green tea per day had the same risk of breast cancer as women who drank less than one cup per week in the baseline data. Similarly, the risk for women who drank 10 or more cups per day compared with women who drank less than one cup of Sencha or Bancha/Genmaicha were equivalent. The results held when calculated based on hormone receptor-defined breast cancer subtype (e.g., hormone receptor positive (ER+/PR+), triple negative) or menopausal status. The authors conclude that there is no evidence of an association between green tea drinking and risk of breast cancer in Japan.
Other studies find that green tea could be beneficial
This is a large, well-designed prospective study that determines the overall assocation between consumption of green tea and breast cancer risk in Japan as well as it can be measured. However, findings of other studies suggest that green tea drinking could be beneficial for certain women, including some already diagnosed with breast cancer:
- Green tea may increase the effectiveness of radiation treatment against breast cancer cells
- Green tea component polyphenol epigallocatechin-3 gallate (EGCG) may increase the effectiveness of Herceptin in Herceptin-resistant HER2+ breast cancer
- EGCG also downregulates the activity of key molecules that are highly expressed in tamoxifen-resistant breast cancer cells, suggesting that green tea may enhance tamoxifen treatment. Green tea may also protect against tamoxifen-induced liver injury
- Green tea appears to have a synergistic effect with mushrooms in inhibiting the adhesion, migration and invasion of triple negative breast cancer cells
- The impact of green tea on risk of breast cancer may be partially dependent on genetic factors that have varying distributions in different populations
- Green tea consumption may reduce levels of circulating estrogen, suggesting a potentially stronger protective effect among obese and overweight women than among those with lower levels of obesity such as the Japanese.