One more piece of evidence hailing the health benefits of soy joined the already considerable volume this week when scientists suggested that consuming tofu and other soy-based foods can significantly lower levels of a class of oestrogens normally associated with breast cancer risk in post-menopausal women.
According to a new study carried out by researchers at the Keck School of Medicine of the University of Southern California, researchers found a link between soy-rich diets consumed by Asian women in Singapore and reduced levels of an oestrogen called estrone, the predominant form of oestrogen in women following menopause. High oestrogen levels have been shown to increase the risk for breast cancer among postmenopausal women.
The results of the study found that estrone levels were a massive 15 per cent lower among women who consumed the highest amounts of soy protein.
"Results from this study support the hypothesis that high soy intake may reduce the risk of breast cancer by lowering endogenous oestrogen levels, particularly estrone," said Anna H. Wu, the study's lead investigator and professor of preventive medicine at Keck School of Medicine.
The American Association for Cancer Research writes that 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 neighbours, write the scientists this week.
"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/gynaecology at the Keck School of Medicine at USC.
Study participants included 144 healthy post-menopausal 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 post-menopausal 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 isoflavones may have stimulatory effects." Dr Mimi Yu, principal investigator of the Singapore Chinese Health Study and a professor of preventive medicine at the Keck School of Medicine at USC, added: "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."