Soy isoflavones function in a similar manner to human estrogens. And according to Gertraud Maskarinec, M.D., Ph.D., and colleagues from the Cancer Research Center of Hawaii in Honolulu, the supplements "may protect against breast cancer as a result of their antiestrogenic activity or increase risk as a result of their estrogen-like properties." In order to determine whether there is a relationship between soy supplements and increased breast tissue density, the researchers turned to The Osteoporosis Prevention Using Soy (OPUS) study. This two-year, placebo-controlled, double-blind clinical trial was designed to investigate the effect of soy supplements on bone density.

All study participants were in early menopause. Some received a placebo, while others received one of two doses - either 80 or 120 milligrams a day - of isoflavone supplements. The doses were the equivalent of eating 300 to 500 grams of tofu a day or drinking 500 to 1,000 milliliters of soy milk - significantly more than the average isoflavone intake in Japan, where soy is widely consumed. Of the participants, 88 percent had mammograms taken at both the beginning and end of the two-year study. (There was a 12 percent dropout rate due to missing data.)

The study's findings, which were published in the Journal of Nutrition, found that the soy supplements had no effect on the density of breast tissue - long recognized as a risk factor for breast cancer - in postmenopausal women. As Dr. Maskarinec notes, the finding "offers some reassurance to those who have been concerned about adverse effects of soy supplementation on breast cell proliferation." However, he notes that the study found no evidence that the supplements were beneficial, although only post menopausal women were evaluated. Therefore, the positive effects of taking supplements earlier in life could not be ruled out. 

News Release: Breast density unaffected by soy supplements   www.medpagetoday.com  April 15, 2009