The Guinea Pig Generation

Soy isoflavones may accelerate progression of breast cancer

Women with estrogen-responsive breast cancer who consume soy protein supplements containing isoflavones to alleviate the side effects of menopause may be accelerating progression of their cancer, changing it from a treatable subtype to a more aggressive, less treatable form of the disease, new research suggests.
The study, published in the journal Molecular Nutrition and Food Research, raises troubling questions about the safety and the potential health consequences associated with long-term use of dietary supplements containing soy isoflavones, such as the phytoestrogen genistein.
Estradiol and all three genistein concentrations stimulated tumor growth in mice, but when the estradiol implants and higher-dose dietary genistein (750 ppm) were removed, the tumors regressed completely.

I remember when that stuff was considered miracle food for middle aged women.