A soy-rich nutrition is generally accepted as beneficial for health. Among the epidemiologically observed effects, a reduction of risk of hormone-sensitive cancer types such as breast or endometrial cancer and prostate cancer has regularly been observed and attributed to the fraction of isoflavones in soy.

Despite these encouraging findings, a debate on safety of isoflavones has been started following the observations of an increased breast cancer risk induced by hormone replacement therapy in the Women’s Health Initiative study (WHI). This cancer-enhancing effect had originally been attributed to estrogens, although today it is known that it was in fact the combination of estrogens with certain synthetic progesterones. Still, with the isoflavones labelled as “phyto-estrogens” and believed to act like estrogens, this class of natural compounds was suddenly suspected to have a potential for the induction of breast or endometrial cancer in postmenopausal women.

Correspondingly, the debate whether the prolonged intake of isoflavone-containing preparations is or is not safe is still ongoing. It is primarily based on data obtained from certain in vitro and in vivo studies, although the design of these studies has been criticised as overly artificial, and other experimental data obtained under more relevant conditions clearly contradict the findings of an increased cancer risk. More importantly, the clinical data speaks in favour of safety – with few exceptions which are discussed under the topic of safety. Even these exceptions do not stand up to closer scrutiny.

With the mass of evidence accumulated to date, soy preparations and isoflavone supplements must be regarded as a safe and beneficial dietary option for menopausal women in doses corresponding to the range of dietary intakes with soy rich nutrition (at least up to 120 mg of aglycones per day).