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Soy Supplements Don’t Affect Breast Density

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Taking hormone replacement therapy (HRT), which contains estrogen, increases breast tissue density and denser breast tissue is linked to higher breast cancer risk. Because one compound in soy products -- isoflavone -- is similar to estrogen, there have been concerns that eating a lot of soy products might increase breast cancer risk by increasing breast density as HRT does. The study reviewed here found that postmenopausal women who took large amounts of isoflavone supplements had no change in breast density. The results suggest that a diet rich in soy doesn't affect breast cancer risk.

Breast density is the proportion of fatty tissue compared to non-fatty tissue in the breast. A denser breast has more non-fatty tissue in it than a less-dense breast. Breasts usually get a little less dense over time.

Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. You can eat soybeans in many forms, including tofu, the beans themselves (also known as edamame), soy milk, miso, and soy powder. Asian women typically eat much more soy than women in Western countries. Yet Asian women have lower rates of hormone-receptor-positive breast cancer than women in the United States.

In the study reviewed here, researchers looked at the records of more than 350 U.S. postmenopausal women participating in the OPUS (Osteoporosis Prevention Using Soy) study, which studied the effect of soy on bone health. In the OPUS study, women received either:

  • 80 mg. per day of isoflavone supplements OR
  • 120 mg. per day of isoflavone supplements OR
  • a placebo (sugar pill)

The amount of isoflavone supplements taken in the OPUS study is quite a bit higher than what you would typically get from regularly eating soy foods.

After 2 years, the women in the OPUS study had mammograms. The researchers compared the breast density of all the women and found that average breast density was the same for each group: the breast density of the women who took either dose of isoflavone supplements was no different than the breast density of the women who took the placebo.

So while this study suggests that soy doesn't increase breast cancer risk in postmenopausal women, it also doesn't show that soy offers any protection against breast cancer. More research, especially research that includes premenopausal women, is needed to see if eating a diet high in soy foods helps reduce breast cancer risk.

The relationship between soy and breast cancer is complicated by other factors. Most women living in Asia depend on soy as their main source of protein. They consume only small amounts of beef, chicken, and pork -- which means less animal fat and other possibly unhealthy substances (such as growth hormones and antibiotics) in these animal protein sources. Also, compared to the average woman in the United States, the average Asian woman:

  • eats more fresh vegetables
  • is closer to her ideal body weight
  • is more physically active
  • is less likely to consume significant amounts of alcohol

All of these other factors add up to produce a healthier lifestyle and a lower overall risk of breast cancer in Asian women living in Asia.

Visit the Breastcancer.org Lower Your Risk section to learn much more about changes you can make in your own diet and lifestyle to lower your breast cancer risk. You can learn more about foods that may keep you as healthy as you can be on the Foods to Consider page.

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