Apart from age, and a family history of breast cancer, many of the known risk factors for breast cancer relate to a woman’s reproductive history, and any use of oral contraceptives and hormone replacement therapy (HRT). The majority of breast cancers are oestrogen dependant and, more recently, the role of dietary factors and particularly the high consumption of phytoestrogens, which are very weak mimics of natural oestrogens, have been associated with a lower incidence of breast cancer.
Indeed, the incidence of breast cancer in Eastern countries such as Japan is approximately one-third that of Western countries and one of the major differences in diet between these populations is that the Japanese and the Chinese consume a traditional diet high in soy products that are known to be particularly rich in phytoestrogens. There are several types of phytoestrogens and the isoflavones present in high concentrations in soy products, as well as red clover, are the most widely studied. Whilst the weak oestrogenic effects of phytoestrogens have been promoted by herbal medicine as a natural alternative to conventional long-term HRT usage, there have been, for some time, mixed views on their use by breast cancer sufferers or breast cancer survivors.
Should breast cancer survivors use soy products?
The majority of breast cancers express oestrogen receptors (ER). Breast cancer patients can have a mix of oestrogen receptor negative (ER-) and ER+ cells, but, typically, one type of cell predominates and this determines treatment. The growth of ER+ breast cancer cells is stimulated by oestrogen, and by the oestrogen-like properties of soybean isoflavones and its action has therefore provided the basis for concern about soy consumption by breast cancer patients.
However the majority of evidence against soy use in breast cancer patients stems from in vivo studies of chemically induced models of mammary carcinogenesis, or in vitro using transgenic mice. Whilst such studies can provide useful information on the mechanisms by which compounds can affect carcinogenic pathways, care must be taken when interpreting results and relating them to humans. As such, the use of human studies is needed to gain any real insight into the link between soy consumption and the progression of breast cancer.
However, several large epidemiological studies have investigated the role of soy isoflavones on breast cancer risk with encouraging findings.
The most recent of such studies, published in the journal of Cancer Epidemiology and Biomarkers Prevention, used data derived from The Women’s Healthy Eating and Living (WHEL) Study and has reported no adverse effects of soy foods on breast cancer prognosis. The WHEL Study was a large phase III clinical trial that investigated whether changes in the proportion of dietary fruits, vegetables, fibre, and fat could reduce breast cancer recurrence and new primary breast cancer, as well as death from any cause, in early-stage breast cancer survivors aged between 18 and 70.
Three thousand and eighty-eight breast cancer survivors, diagnosed between 1991 and 2000 with early-stage breast cancer and participating in WHEL, were followed for a median of 7.3 years and isoflavone intakes were measured post-diagnosis by using a food frequency questionnaire. Women self-reported new outcome events semi-annually, which were then verified by medical records and/or death certificates. The study reported that as isoflavone intake increased, the risk of death decreased (p = 0.02). Furthermore, women with the highest levels of isoflavone intake (>16.3 mg isoflavones) had a 54% reduction in risk of death.
This is the third epidemiologic study to report no adverse effects of soy foods on breast cancer prognosis, and the authors go as far as suggesting that clinicians no longer need to advise against soy consumption for women with a diagnosis of breast cancer.
Given that some health professionals still remain confused and even recommend that breast cancer survivors should not consume soy or isoflavone containing supplements, these current findings will hopefully help to clarify the safely of these products for women considering their use.
Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce JP. 2011 Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 20:854-8.
Dr Nina Bailey is a nutritional scientist whose efforts are concentrated within the role of dietary health and nutritional intervention in disease, with particular emphasis being placed upon the role of essential fatty acids in conditions such as chronic fatigue syndrome and depression.
More information can be found on her website at www.drninabailey.com