Isoflavones and breast cancer risk

Are soy isoflavones healthy or risky for breast cancer patients and survivors? Many studies have demonstrated the anticancer properties of soy isoflavones, but its estrogen-like effect and conflicting data from in-vivo and in-vitro studies have raised concern have raised concern about isoflavones intake and the promotion or propagation of estrogen-sensitive cancers. Some health professionals are confused and even recommend that breast cancer survivors should not consume soy or isoflavones supplements.



One study that raised concern about the safety of genistein was an experimental with rodents carried about by Helferich and co-workers. They implanted estrogen-dependent tumours into ovariectomized athymic mice and found that dietary genistein was able to negate the inhibitory effect of tamoxifen on tumour growth. According to a review by Taylor et al, Asian epidemiological studies attribute reduced incidence of breast and prostate cancers to soy food and isoflavones consumption. Concerns are mainly based on in vitro and animal studies which suggest that genistein aglycone can stimulate tumor cell proliferation and growth in mice having deficient immune systems. But recent case-control study and epidemiological studies show an inverse correlation between genistein intake and breast cancer risk. Clinical studies in postmenopausal women confirm the breast and uterine safety of pure genistein administered for up to 3 years.

A recent population-based cohort study that attracted worldwide attention concluded "that among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and recurrence". Shue and co-workers found that women with the highest intake of soy protein (>15 g soy protein per day) had a 29 percent lower risk of death and 32 per cent lower risk of breast cancer recurrence compared to patients with the lowest intake (<5.3 g soy protein per day). This inverse relationship was evident among women with both estrogen receptor positive and estrogen receptor negative breast cancer. The association of soy isoflavones with mortality and breast cancer recurrence followed a dose-dependent pattern until soy isoflavones intake reached 40 mg per day, after which the association appears to level or even rebound. They suggest that soy isoflavones protect against breast cancer by competing with estrogens in the binding of estrogen receptor. Soyfood intake was associated with improved survival, also for women who took the anti-cancer drug tomaxifen. These results indicate that isoflavones do not reduce the efficacy of tamoxifen.

Most studies that demonstrated the protective effect of isoflavones looked at the intake of soy foods and not isoflavones supplements. Not enough studies have been done to determine whether or not high concentrations of isoflavones may encourage the growth of breast cancer. If you're taking soy supplements to treat menopausal symptoms, speak with your health professional about quantity of isoflavones that may be safe for you.

Taylor CK, Levy RM, Elliott JC, Burnett BP. The effect of genistein aglycone on cancer and cancer risk: a review of in vitro, preclinical, and clinical studies. Nutr Rev. 2009 Jul;67(7):398-415.
Helferich WG, Andrade JE, Hoagland MS. Phytoestrogens and breast cancer: a complex story. Inflammopharmacology. 2008 Oct;16(5):219-26.
Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43.