On the occasion of the Menopause Congress in Vienna in December 2009 we gave a brief introduction into the latest scientific data on safety and benefits of isoflavones, with a focus on safety. The presentation took place on Friday, 4th of December (Schmidt 2009).

Safety of isoflavones has been under debate since the WHI study revealed potential cancer-promoting effects of hormone treatment in postmenopausal women. The hypothesis that isoflavones might likewise have cancer-promoting effects was seemingly confirmed in the model of xenotransplanted ovarectomized athymic mice (Allred et al. 2001). The clinical relevance of this model is, however, highly questionable, as already small changes in the study conditions – mainly the presence of small quantities of estrogen – transform the cancer-promoting into cancer-protective effects (Zhou et al. 2004).

Clinically, only one study has claimed potential proliferation-enhancing effects of high doses of isoflavones on the endometrium (Unfer et al. 2004). However, next to some potential flaws in the study design this publication contradicts the absolute lack of observations of hormonal effects in > 47 clinical trials with almost 2,000 women exposed to isoflavones for up to 3 years. From epidemiological studies a 16 % reduction of relative breast cancer risk has been calculated for each 10 mg of isoflavone intake.

On the efficacy side isoflavones exert an effect on hot flushes in a range of approximately 20 % above placebo, and a total reduction of approximately 50 % of hot flushes (Kurzer 2009). In addition, positive effects on bone and cardiovascular health may be expected. The risk-benefit ratio of isoflavones must be considered positive.

Download: Presentation slides. Menopause Congress, Vienna, December 4, 2009.

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Schmidt M: Safety and Benefits of isoflavones. J. Gynaekol. Endokrinol. 11(4): 37-38 (2009).

Allred CD, Ju YH, Allred KF, Chang J and Helferich WG: Dietary genistin stimulates growth of estrogen-dependent breast cancer tumors similar to that observed with genistein. Carcinogenesis 22(10):1667-1673 (2001).

Zhou JR, Yu L, Mai Z and Blackburn GL: Combined inhibition of estrogen-dependent human breast carcinoma by soy and tea bioactive components in mice. Int J Cancer 108(1): 8-14 (2004).

Unfer V, Casini ML, Costabile L, Mignosa M, Gerli S and Di Renzo GC. Endometrial effects of long-term treatment with phytoestrogens: a randomized, double-blind, placebo-controlled study. Fertil Steril 82(1):145-148 (2004).

Kurzer MS. Soy isoflavones reduce postmenopausal hot flush frequency and severity: Results of a systematic review and meta-analysis of randomized controlled trials. Symposium on Evaluating the Efficacy and Safety of Isoflavones for Postmenopausal Women, 13-14 May. Milan (Italy): Council for Responsible Nutrition (2009).