Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation

Abstract

Isoflavones are secondary plant constituents of certain foods and feeds such as soy, linseeds, and red clover. Furthermore, isoflavone-containing preparations are marketed as food supplements and so-called dietary food for special medical purposes to alleviate health complaints of peri- and postmenopausal women. Based on the bioactivity of isoflavones, especially their hormonal properties, there is an ongoing discussion regarding their potential adverse effects on human health. This review evaluates and summarises the evidence from interventional and observational studies addressing potential unintended effects of isoflavones on the female breast in healthy women as well as in breast cancer patients and on the thyroid hormone system. In addition, evidence from animal and in vitro studies considered relevant in this context was taken into account along with their strengths and limitations. Key factors influencing the biological effects of isoflavones, e.g., bioavailability, plasma and tissue concentrations, metabolism, temporality (pre- vs. postmenopausal women), and duration of isoflavone exposure, were also addressed. Final conclusions on the safety of isoflavones are guided by the aim of precautionary consumer protection.Electronic supplementary materialThe online version of this article (10.1007/s00204-018-2279-8) contains supplementary material, which is available to authorized users.

The isoflavones genistein and daidzein increase hepatic concentration of thyroid hormones and affect cholesterol metabolism in middle-aged male rats

“…However, genistein exerts similar estrogenic biological effect on female reproductive system independently of administration route [58] and inhibitory effect on TPO activity was confirmed upon oral IF administration [4,5]. These biological effects in target tissues are primarily related to aglycone form and not to genistein metabolites [59]. However, for a number of endpoints conjugation does not always decrease the biological activity of flavonoids [60].…”

Effect of Plasma-Activated Water on the Microbial Decontamination and Food Quality of Thin Sheets of Bean Curd

“…Isoflavones are a group of secondary metabolites found in soybean, which play a key role in lowering cholesterol levels and preventing cancer, osteoporosis, and cardiovascular disease [26][27][28]. Therefore, the total isoflavone content is an important food quality determinant of soybean products.…”

Are the cardioprotective effects of the phytoestrogen resveratrol sex-dependent?

“…However, this report points out that isoflavone administration needs to be carefully monitored in certain risk groups of people such as those with iodine deficiency and hypothyroidism, which is common in post-menopausal women (Huser et al 2018). There are also potential adverse cardiac outcomes from administering soy isoflavone genistein with other drugs, such as a tyrosine kinase inhibitor due to inhibition of signalling required for normal functioning of cardiomyocytes (Harvey and Leinwand 2015).…”

“…Soy isoflavones also have anti-cancer properties as shown by reduced prevalence of cancer in populations regularly consuming soy rich diet possibly through early stimulation of cell differentiation (Applegate et al 2018). A recent report on safety evaluation of isoflavones based on in vivo studies to date found that overall, the evidence is inconclusive regarding adverse effects of long term isoflavone consumption (Huser et al 2018).…”