Consuming tofu and other soy-based foods significantly lowers levels of a class of oestrogens normally associated with breast cancer risk in postmenopausal women, according to new research 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 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.
Specifically, the study showed that estrone levels were about 15 per cent lower among women who consumed the highest amounts of soy protein. No other easily modifiable lifestyle factors analysed 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 oestrogen 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, California.
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. In a previous study by Wu, published in Carcinogenesis, research showed that Asian-American women were still at high breast cancer risk if they had not consumed soyfoods from an early age.
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 came from a structured questionnaire discussed in 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 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."
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 per cent 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, fibre 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 oestrogen 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 hypothesised that isoflavones may inhibit certain enzymes responsible for oestrogen 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."