Soy isoflavones supplements do not decrease the risk of breast cancer. Isoflavones are components of soy foods that have anti-estrogen activity and were therefore thought to prevent breast cancer.
For the study, the researchers randomly assigned 98 women to receive a mixed soy isoflavones supplement or placebo. 6-months later the researchers examined levels of Ki-67 (protein marker of cancer cell growth) in certain breast cancer cells taken from the women. They found that there were no differences in Ki-67 levels between women who took the soy supplement and those who took the placebo. Rather, the level of Ki-67 increased from 1.71 to 2.18 in pre-menopausal women, suggesting a negative effect of the soy supplementation.
Lead researcher Seema A. Khan, M.D., professor of surgery at the Robert H. Lurie Comprehensive Cancer Center of Northwestern University, said, "Simply put, supplements are not food. Although soy-based foods appear to have a protective effect, we are not seeing the same effect with supplementation using isolated components of soy, so the continued testing of soy supplements is likely not worthwhile."
Even Dr. Khan stressed that this was a small finding but one that should suggest caution.
The study is published in 'Cancer Prevention Research'.