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.

Led by Dr. Anna Wu of the Keck School of Medicine at the University of Southern California at Los Angeles the researchers found that levels of the hormone estrone were 15 percent lower among women who consumed the highest amounts of soy protein. The researchers published the results in the September issue of Cancer Epidemiology, Biomarkers & Prevention.

"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," Wu said in a press release.

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.

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.

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. And in recent years, breast cancer has been rising among Asian women. Some think this be due to adoption of Western diet and increasing number of women who are overweight.

"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.

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