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New research from the U.S. National Institutes of Health shows that the biochemicals in green tea change a women's estrogen metabolism, revealing at least one of its mechanisms for reducing the risk of breast cancer.
The study comes from the NIH's National Cancer Institute, and was led by Dr. Barbara Fuhrman. The researchers tested the levels of urinary estrogens and metabolites among 181 healthy Japanese American women from California and Hawaii. Of the group, 72 of the women were postmenopausal. The remainder of the group was premenopausal.
The data was compiled using a combination of urinary testing along with personal interviews with each women. The woman's intake of not only green tea, but black tea, coffee (decaffeinated or not) and soda (decaffeinated or not) was also queried and recorded and measured, and the results were adjusted with respect to caffeine consumption. Considerations such as soy consumption, body mass index, age and others were also made and adjusted.
The research found that those postmenopausal women who drank green tea daily had 20% less urinary estrone and 40% less urinary estradiol levels, when compared to those levels of women who drank green tea less than one time per week.
These estrogen levels followed their categorization with regard to the estrogen metabolism pathway involved. This allowed the researchers to determine that these urinary estrogen differences were related to their estrogen metabolism and their future risk of breast cancer.
The primary estrogen pathway connected with breast cancer is the 16-hydroxylated estrogens. As for the 16-hydroxylated estrogen pathway, both estradiol and estrone markers were 40% lower among those women who drank green tea at least one time daily compared to those women who drank less than one cup of green tea a week.
Levels of caffeine consumption did not change these dynamics among the women. And black tea consumption did not produce these decreases in estrogen metabolites.
Furthermore, estrogen levels of premenopausal women did not respond to green tea consumption. This did not surprise the researchers, as previous research has found that postmenopausal women respond differently to medications such as tamoxifen and aromatase inhibitors.
Interestingly, the research also found that the average urinary estrogen levels of the entire postmenopausal Japanese-American group was about half of the levels that were found in a recent study of postmenopausal women from New York who were primarily Caucasian. The researchers could not determine the reason for the difference – stating that it could be related to differences in diet, lifestyle factors or others.
While other studies have shown some differences in urinary estrogen levels and green tea drinking among women, this is the first study that analyzed a broad range of estrogen metabolites among peri- and postmenopausal women.
Postmenopause follows one year after the stage of menopause, when a woman's ovaries halt egg production. During this period, estrogen and progesterone production is reduced.