Animal studies are contradictory in their conclusions about the intake of soy isoflavones:
- Some studies showed that in an existing breast carcinoma (tumor of the mammary gland tissue), isoflavones may accelerate the growth of tumor cells.
- In studies on mice, the administration of isolated genistein in existing breast cancer led to increased spread of tumor tissue.
- In contrast, the combination of different phytoestrogens (isoflavones and lignans) in comparable amounts reduced tumor growth.
- Other studies found no risk.
- Some studies of former breast cancer patients even showed a reduced risk of tumor recurrence after isoflavone administration.
However, the data from animal studies cannot be readily extrapolated to humans.
National and international professional societies see the following as harmless:
- Women who have or have had breast cancer may consume soy in the form of food, 1-2 servings daily (for example, 1 serving is equivalent to 250 ml of soy milk or 100 g of tofu). The ingested amount of isoflavones from soy or soy products is between 25 and 50 mg.
- A drug breast cancer therapy with tamoxifen or so-called aromatase inhibitors is no reason to eliminate soy-containing foods from the menu.
The following can be said about the intake of isolated isoflavones from dietary supplements:
- The Federal Institute for Risk Assessment (BfR) and the European Food Safety Authority (EFSA) conclude that the dosage and duration of intake used in human studies, under which adverse effects were not observed, at least on the target organs studied (mammary gland, uterus and thyroid), should be regarded as guidance for the use of isolated isoflavones in food supplements in women without existing estrogen-dependent diseases in the post-menopausal period (postmenopause) to be considered sufficiently safe. For the mammary gland, this means:
- No increased risk of mammary carcinoma (breast cancer).
- No increased tissue density in mammography (X-ray examination of the breast).
- No effects on the expression (release) of the proliferation marker KI-67 (synonym: MIB1, proliferation marker for objectification and validation of grading; allows conclusions about growth behavior).
The amount of isoflavones from soy should be limited to a maximum of 100 mg per day and the duration of intake up to 10 months.
- In view of the insufficient data for perimenopausal women addressed by the EFSA, from the point of view of the BfR, in the case of use in the phase around menopause, the stated guidance values should also not be exceeded until further notice.
- Due to insufficient data, the intake of food supplements with isolated isoflavones is not recommended for the following individuals: Women,
