Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and commentary

Abstract

There has been considerable investigation of the potential for soyfoods to reduce risk of cancer, and in particular cancer of the breast. Most interest in this relationship is because soyfoods are essentially a unique dietary source of isoflavones, compounds which bind to estrogen receptors and exhibit weak estrogen-like effects under certain experimental conditions. In recent years the relationship between soyfoods and breast cancer has become controversial because of concernsbased mostly on in vitro and rodent data -that isoflavones may stimulate the growth of existing estrogen-sensitive breast tumors. This controversy carries considerable public health significance because of the increasing popularity of soyfoods and the commercial availability of isoflavone supplements. In this analysis and commentary we attempt to outline current concerns regarding the estrogen-like effects of isoflavones in the breast focusing primarily on the clinical trial data and place these concerns in the context of recent evidence regarding estrogen therapy use in postmenopausal women. Overall, there is little clinical evidence to suggest that isoflavones will increase breast cancer risk in healthy women or worsen the prognosis of breast cancer patients. Although relatively limited research has been conducted, and the clinical trials often involved small numbers of subjects, there is no evidence that isoflavone intake increases breast tissue density in pre-or postmenopausal women or increases breast cell proliferation in postmenopausal women with or without a history of breast cancer. The epidemiologic data are generally consistent with the clinical data, showing no indication of increased risk. Furthermore, these clinical and epidemiologic data are consistent with what appears to be a low overall breast cancer risk associated with pharmacologic unopposed estrogen exposure in postmenopausal women. While more research is required to definitively allay concerns, the existing data should provide some degree of assurance that isoflavone exposure at levels consistent with historical Asian soyfood intake does not result in adverse stimulatory effects on breast tissue.

Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy

“…Until now, clinical research has been limited, and there is little clinical evidence to suggest that intake of soy isoflavones increases the risk of breast cancer among healthy women or worsens the prognosis for those who have breast cancer. 17 The results of the Shanghai Breast Cancer Study, conducted between 1996 and 1998, showed that soy intake was unrelated to disease-free breast cancer survival, but the authors of that study did not take endocrine therapy into consideration in their survival analyses. 18 A recent epidemiologic study involving 1954 female survivors of breast cancer also suggested a lower risk of breast cancer recurrence with high intake of daidzein and glycetin among both postmenopausal women and tamoxifen users.…”

Effect of dietary soy intake on breast cancer risk according to menopause and hormone receptor status

“…Soybeans are the predominant source of isoflavones. Because of their structural similarity to 17b-estradiol, soy isoflavones may bind directly to the estrogen receptors (ERs) and modulate their functions (Messina and Wood, 2008). Given the association between estrogen and breast cancer risk, it is biologically plausible that dietary soy intake may have an anticarcinogenic effect in the breast.…”

Phytochemicals in Soy and Their Health Effects

“…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 (35). Epidemiological evidence together with preclinical data from animal and in vitro studies strongly supported a correlation between soy isoflavone consumption and protection towards breast and prostate cancers (45)(46)(47). However, clinical studies assessing soy consumption and risk of breast cancer have yielded inconsistent results.…”