EFSA concluded that at levels typically found in supplements there was no human data to support the hypothesis of an increased risk of breast cancer, an effect on mammographic density nor an effect on proliferation marker Ki-67 expression.
No effect was found on endometrial thickness - the inner mucous membrane of the uterus - and histopathological tissue changes in the uterus with up to 30 months of 150 mg per day of soy isoflavones in supplements. After 60 months some non-malignant - not cancerous - histopathological changes were reported.
Thyroid hormones levels were not changed following intake of isoflavones from food supplements.
It concluded isolated isoflavones were “not without risk”.
The concern was that, according to toxicological studies, isoflavones at high doses impaired the functioning of the thyroid gland and could change mammary gland tissue.
“It cannot be ruled out that these estrogen-like effects could promote the development of breast cancer,” the BfR wrote at the time.
Isoflavones are a class of phytochemicals - compounds found only in plants.
They are found naturally in foods like soy, red clover and kudzu root.
The main isoflavones are genistein, daidzein, glycitein, formononetin, biochanin A and puerarin.
“As women during and after menopause are at increased risk of breast cancer, the long-term intake of food supplements with a high level of isoflavones is not without risk for this consumer group.”
The German report could not establish a safe dose for post-menopausal women taking food supplements containing isolated isoflavones over a long period of time and the issue was therefore referred to EFSA in 2008.
It requested that EFSA give advice on a safe intake level of isolated isoflavones from dietary supplements.
However, EFSA said it was not possible to derive a single health-based guidance value for the different preparations in post-menopausal women.
“However the doses used in the intervention studies and their duration could serve as guidance for the intake of food supplements.”
EFSA looked at 43 human studies and 62 animal studies in its opinion.
It said isoflavones from supplements accounted for the majority of intake - an assumption upon which its safety opinion was based.
There are no health claims approved for isoflavones in the EU, with 14 applications rejected many of which referred to menopause symptoms.
Supplements targeting peri- and post-menopausal women typically provide a daily dose of between 35–150 mg per day.
Background exposure from the diet in the general European population was estimated to be lower than one mg per day, while for consumers of soy-based foods this could be higher.
"The Panel considered that more data on the doses and duration of consumption should be generated as this would improve the available database on the safety of prolonged use of food supplements containing isoflavones. This assessment identified the need for a harmonised way of reporting the isoflavone content of food supplements."