Consuming tofu and other soy-based foods significantly lowers levels of a class of estrogens normally associated with breast cancer risk in postmenopausal women, according to a new study published in the September issue of the American Association for Cancer Research (AACR) journal, Cancer Epidemiology, Biomarkers & Prevention.
The study found a link between soy-rich diets consumed by Asian women in Singapore and reduced levels of an estrogen called estrone, the predominant form of estrogen in women following menopause. High estrogen levels have been shown to increase the risk for breast cancer among postmenopausal women.
Specifically, the study found that estrone levels were about 15 percent lower among women who consumed the highest amounts of soy protein. No other easily modifiable lifestyle factors analyzed by the scientists yielded such a dramatic hormone reduction.
"Results from this study support the hypothesis that high soy intake may reduce the risk of breast cancer by lowering endogenous estrogen levels, particularly estrone," said Anna H. Wu, the study's lead investigator and professor of preventive medicine at Keck School of Medicine of the University of Southern California, Los Angeles, Calif.
Also participating in the study, titled "Soy Intake and Other Lifestyle Determinants of Serum Estrogen Levels among Postmenopausal Chinese Women in Singapore," were Mimi C. Yu and Frank Z. Stanczyk, both at USC; and Adeline Seow and Hin-Peng Lee, with the Department of Community, Occupational and Family Medicine at the National University of Singapore.
Historically, breast cancer rates among Asians in Japan and China have been significantly lower than their female counterparts in the West. At one time, low-risk Asian women had one-sixth the breast cancer rate compared to high-risk whites in the United States and other parts of the western world. Reasons for this difference have remained largely unknown. However, Asians are clearly as "genetically susceptible," since Asian-American women have roughly the same breast cancer incidence as their white American neighbors.
Moreover, from the 1970s to the 1990s, breast cancer incidence more than doubled in Singapore and Japan. While earlier age at menarche, increasing numbers of women without children and delay in childbearing may offer a partial explanation, changes in other lifestyle practices are likely to play a role.
"Aside from answering some basic questions about soy consumption and breast cancer, this study may provide some insight into the underlying increase in breast cancer in Asia," said Dr. Stanczyk, a co-investigator and professor of research in obstetrics/gynecology at the Keck School of Medicine at USC.
Study participants included 144 healthy postmenopausal Chinese women in Singapore currently enrolled in a population-based prospective investigation of diet and cancer risk. Information on diet and other lifestyle factors was obtained from a structured questionnaire administered through direct interviews.
Each of the 144 postmenopausal women, ranging in age from 50-74 years, was asked to estimate her usual eating frequencies and portion sizes for 165 food and beverage items consumed during a year. The questionnaire also requested information on demographics, lifetime use of tobacco, menstrual and reproductive history, medical history, and family history of cancer.
The Chinese population in Singapore (and elsewhere in Asia) is particularly suited for studies on the effects of soy-based foods because this food has been a staple in the traditional Asian diet. Six kinds of soy products (plain tofu, taupok, taukwa, foopei, foojook and tofu far) and soybean drink were included in the questionnaire.
In addition, as part of a Singapore Food Composition Database, levels of daidzein, genistein and glycitein were measured in the main types of soy foods consumed in Singapore, allowing the researchers to calculate intake of total isoflavones among individual subjects. Isoflavones, the main constituent of soybeans, are believed to be responsible for anti-cancer effects observed in an accumulating number of human and animal studies.
"However, the effect of soy on the breast is controversial," said Dr. Wu. "There are some in vitro studies of breast cancer cells – animal studies, as well as short-term soy intervention studies in women – suggesting that soy isoflavines may have stimulatory effects." Added Dr. Mimi Yu, principal investigator of the Singapore Chinese Heaslth Study and a professor of preventive medicine at the Keck School of Medicine at USC: "Though our study is suggestive, more work needs to be done before any specific dietary recommendations can be made about consuming soy proteins to protect against breast cancer."
Blood sample analyses not only showed lower estrone levels among those consuming the highest quantities of soy protein, they also showed similar patterns when correlated to consumption of isoflavonoids. However, estrone levels did not decline in a linear manner with increasing soy intake; an apparent reduction was only seen among those in the top 25 percent of soy protein consumers.
The study also showed that hormone levels remained unaffected by other dietary and lifestyle choices. These included consumption of alcohol, coffee, tea, fat, fiber and various micronutrients, including vitamins A, C and E, along with calcium and carotenoids. Physical activity also did not significantly influence serum hormone levels. Among the study's other findings was an association between increased estrogen levels and women with a high body mass index (BMI), a measure of weight that accounts for height.
"There is a suggestion that weight change (particularly weight increase) has a profound influence on breast cancer rates in Asian-American women," said Hin-Peng Lee, co-principal investigator in the Singapore Chinese Health Study. "The same may now be happening to their relatives on the Asian side of the Pacific." The study also may open new avenues for basic research to determine how soy proteins work to reduce estrone levels on the molecular level. The scientific team hypothesized that isoflavones may inhibit certain enzymes responsible for estrogen production and metabolism. "Our findings of a reduction of estrone levels in association with soy intake may represent a reduction in the production and/or an increase in the elimination of estrone," said Dr. Stanczyk. "Future studies may offer new insights into this mechanism."
Founded in 1907, the American Association for Cancer Research (AACR) is a professional society of more than 19,000 laboratory and clinical scientists engaged in cancer research in the United States, Canada, and more than 60 other countries. AACR's mission is to accelerate the prevention and cure of cancer through research, education, communication and advocacy. Its scholarly activities include the publication of five journals (Cancer Research, Journal of Clinical Oncology, Molecular Cancer Research; Cancer Epidemiology, Biomarkers & Prevention; and Molecular Cancer Therapeutics). AACR's annual meeting has more than 15,000 participants and features presentations of new and significant discoveries in the cancer field. AACR's 10 specialty meetings cover all of the important areas of basic, translational and clinical cancer research.
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