Abstract
Isoflavones possess both estrogenic and anti-estrogenic actions, and are hypothesized to protect against breast cancer. However, two intervention studies of markers of proliferation on breast tissue have raised concerns that soy isoflavones may have an estrogenic effect on breast tissue. Increased mammographic breast density is associated with an elevated risk of breast cancer, although the mechanism underlying this relationship has not been explained. Estrogens increase and anti-estrogens decrease breast density. Breast density may therefore serve as a biomarker of estrogenic or anti-estrogenic effects of a treatment on breast tissue. The effect of isoflavones on breast density is under investigation.