The Link Between Soybeans and Breast Cancer
Cancer is the second leading cause of death worldwide and breast cancer is one of the most common types of cancers. Breast cancer is more commonly seen in women than men. In particular, it is seen in women living with higher estrogen levels – for example, in women who had not had children or are taking an oral contraceptive pill containing estrogen hormone.
Breast cancer is currently treated using a multimodal approach. This normally involves a combination of surgery, chemotherapy, hormonal therapy or radiation. The type of treatment depends on the variant of breast cancer, the age and fitness of the patient and the prognosis, among other things. However there have been studies to suggest that appropriate diets could help lower the incidence of all cancers by up to 40% (1). Some foods show promise in preventing recurrence and improving prognosis. Soybeans have now been shown to be part of that appropriate diet particularly in relation to breast cancer.
There was some confusion previously regarding the effect of soybeans on estrogen, and therefore breast cancer. Clinical trials on premenopausal women showed that a high intake of soybeans reduced the amount of estrogen the body produces. Reduction in estrogen production meant that it was protective against breast cancer. However, cell studies done in the laboratory showed a contradictory increase in tumor growth with soy isoflavones. Isoflavones are plant-based compounds that are capable of exerting an estrogen-like effect. They are also known as phytoestrogens. Recently, more and more studies have shown promising, and more consistent, data.
A large study done in China followed women over several years and showed that those who had the highest intake of soy isoflavones had a decreased rate of breast cancer mortality (2). These findings were corroborated by another review that showed the risk was lowest in Asian women who consumed 20mg or more soy isoflavones daily (3). Even lower intakes of 10mg per day were proven effective in lowering the risk of recurrence after being diagnosed with breast cancer (4). Soy helps improve the prognosis in women with either estrogen receptor positive or estrogen receptor negative breast cancer and in those who use or do not use tamoxifen. (5) Another study found that both pre- and postmenopausal women who consumed tofu, had a 15% reduced risk of breast cancer with each additional serving per week (6).
Most of the published studies available are based on Asian populations. This is primarily because soy is widely consumed in many different forms in Asia. In my previous practice, the Malay and Indonesian populations loved tempeh, a form of fermented soybeans. A half-cup of tempeh contains roughly 36mg of soy isoflavones. The Chinese population believed soy to have the additional benefit of making skin glow and appear smoother. They preferred eating soybeans in the form of soymilk, which contains about 23mg of isoflavones in 8 fluid ounces., or as tofu, which has about 39mg isoflavones in 4 ounces (7). In particular, many of the Chinese who were vegetarian and practiced Buddhism, considered soy to be a staple food. The benefits of soy have been shown in patients who were at risk of, undergoing treatment of and recovering from breast cancer. The advantages are seen with pre- and post-menopausal women. The processing of soybeans certainly affects its potency as an anti-cancer ingredient. So, it is important to note that the most benefit comes from whole soy foods.