Soy Isoflavones are Safe and Effective for Preventing Breast Cancer

by Jonathan V. Wright, M.D.

Soy isoflavones are phytoestrogens—naturally occurring estrogen-like compounds derived from plants. They are sold as a supplement for bone and heart health, and menopausal symptoms, and do not have the harmful side effects of some estrogens.

But some studies indicating that breast cancer cells in mice were stimulated by soy isoflavones cast a shadow on the supplement’s image. That image is about to change, thanks to several new studies.

According to results of the Osteoporosis Prevention Using Soy (OPUS) study published in the Journal of Nutrition, May, 2009, postmenopausal women who were given two different doses of soy isoflavones did not experience an increase in the density of breast tissue. Instead, there was a decrease in breast tissue density.

Since breast density is a general marker of breast cancer this is great news. The findings reassure users that soy isoflavones don’t act like hormone replacement medication on breast density.

The multi-site, double-blinded trial randomly assigned 406 postmenopausal women to a daily dose of 80 or 120 mg of isoflavones or a placebo for two years. According to the researchers, these doses are equivalent to the amounts of isoflavones provided in a daily serving of two to four cups of soymilk.

The average age of the 358 women who completed the trial was 55. At the start of the trial all the women had a similar body mass index (BMI), but the average breast density varied.

Over time, the change in breast density was significant, with breast density decreasing by 1.6% per year across all groups who took the soy supplements. The researchers concluded that the results offer some reassurance to those who have been concerned about adverse effects of soy supplementation on breast cell proliferation.1

Recent Studies

Two recent studies indicate that a high intake of soy during childhood and adolescence may reduce a woman’s risk of breast cancer later in life

The first one, published in the June ’09 issue of the American Journal of Clinical Nutrition, assessed the dietary intake of 73,223 Chinese women who participated in the Shanghai Women’s Health Study. It is a well-known fact that women who live in countries where soy is an integral part of the diet have a lower incidence of some types of cancer. And China happens to have the world’s lowest incidence and mortality from breast cancer.

A seven-year follow-up documented 592 incidences of breast cancer. Adolescent intakes of soy foods were associated with a 43 percent reduction in premenopausal breast cancer risk, while high intakes of soy protein and isoflavones were associated with 59 and 56 percent reductions in the risk of breast cancer before menopause.2

A second study interviewed 597 American women of Chinese, Japanese, and Filipino descent who had breast cancer, and 966 healthy women between the ages of 20 and 55 about their adolescent and adult diets. Additionally, the mothers of 255 participants who were living in the U.S. were asked about their daughter’s early intake of soy.

When Asian women migrate to the United States, their breast cancer risk typically increases over several generations. But the study found that soy intake during childhood, adolescence, and adult life was associated with decreased breast cancer risk.

The researchers determined that women with the highest soy intake during childhood had a 58 percent lower risk of breast cancer as adults than the women with the lowest soy intake during childhood. High soy intake during adolescence and adulthood was associated with a 20 to 25% reduction.3


Soy isoflavones do not increase breast density, and women who consume a high amount of soy foods consistently during adolescence and adulthood have a substantially reduced risk of breast cancer.

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  1. Maskarinec, G., Verheus, M., Steinberg, F.M., Amato, P., Cramer, M.K., Lewis, R.D., Murray, M.J., Young, R.L., Wong, W.W. “Various doses of soy isoflavones do not modify mammographic density in postmenopausal women” Journal of Nutrition, May 2009, Volume 139, Pages 981-986.
  2. Lee SA, Shu XO, Li H., Yang G., Cai H., Wen W., Ji BT, Gao J, Gao YT, Zheng W. “Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study,”American Journal of Clinical Nutrition, June 2009, Volume 89, Number 6, Pages 1920-1926.
  3. Korde LA, Wu AH, Fears T, Nomura AM, West DW, Kolonel LN, Pike MC, Hoover RN, Ziegler RG. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev. 2009 Apr;18(4):1050-9.