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Isoflavones (IFs) from soy and other legumes have weak estrogenic properties. Isolated IFs are available as dietary supplements and advertised to alleviate symptoms of menopause. The present chapter provides an overview of the occurrence, the chemical structure of IFs and their metabolites, the market situation and reviews the current evidence on the efficacy and safety of IF-containing dietary supplements.The biological effectiveness of IFs is attributable to the activation of the estrogen receptor (ER). Studies on the influence of IFs on endogenous estrogen levels in women show inconsistent results. So far, the European Food Safety Authority (EFSA) has rejected all submitted health claims for IFs due to insufficient scientific evidence for any of the postulated health effects. Based on the results of their recent risk assessment, the EFSA concluded that the available human studies did not support the hypothesis of adverse effects of isolated IFs on the human mammary gland, uterus or thyroid in healthy postmenopausal women. However, the assessment does not allow a general statement on the safety of IF-containing dietary supplements. Studies in animal models are often not comparable with the complex interactions in humans due to differences in the metabolism of IFs, in the developmental stage at time of consumption and in the temporarily restricted uptake of IFs during certain stages of life., Conclusion: So far, for none of the advertised functions is unequivocal scientific evidence available. On the basis of available data, potential unwanted side effects cannot be fully excluded. This holds particularly true for women with undiagnosed diseases, especially for those with undetected precancerous lesions in the mammary gland.

Isolated isoflavones are frequently offered as dietary supplements for the alleviation of peri- and postmenopausal complaints. These mainly soy-based secondary plant compounds are marketed with the claim of having numerous beneficial effects such as protection against breast cancer, osteoporosis, and cardiovascular diseases. Currently, there is no conclusive evidence for most of these health impacts. In addition, there is a controversial ongoing discussion about the safety of these products. After a long-term intake of high isoflavone doses, adverse effects on the breast tissue, the endometrium, and the thyroid, the last one especially under iodine-deficient conditions, cannot be excluded. Owing to their estrogenic effects, isoflavones may promote the growth of estrogen-sensitive malignant cells. The risk assessment of isoflavones is especially focused on peri- and postmenopausal women because they are the target group for dietary supplements based on isolated isoflavones and have, anyway, a higher risk for breast cancer. Since long-term treatment with isolated isoflavones, especially at high doses, is considered critical, we recommend that patients consume isoflavone-based supplements only after advice and under medical supervision.

Chemically, isoflavones belong to the group of polyphenols. The most important food source is soy, which contains mainly genistein and daidzein in the form of glycosides. The absorption rate ranges from 20-55%. Isoflavones are selectively incorporated in certain tissues like the breast and ovaries. They are able to bind to the estrogen receptors alpha (ER-alpha) and beta (ER-beta). However, the binding affinity for genistein to ER-alpha is only 4%, the affinity to ER-beta is 87% compared to 17beta-estradiol. Thus, depending on the estradiol concentration, they exhibit weak estrogenic or antiestrogenic activity. Isoflavones can influence transcription and cell proliferation. They modulate enzyme activities as well as signal transduction, and have antioxidant properties. Epidemiological studies have shown that the prevalence of hot flashes is lower in women from countries with high dietary isoflavone intake such as Japan than in Western nations with low isoflavone intake. Results of clinical studies on the effects of soy products or isolated isoflavones on vasomotor symptoms are contradictory. Due to a strong placebo effect and a time-dependent reduction of hot flashes, phytoestrogens were seen to have no significant effect in most studies. The use of soy isoflavones could, however, be considered for women with intense disorders. Because of contradictory data from in vitro and animal studies, a definite safety assessment of isoflavone intake cannot be made to date. While moderate isoflavone consumption seems to be safe in the majority of the population, women with breast cancer should avoid long-term use of soy products or isolated isoflavones.