There is a debate raging that has respected nutrition experts on both sides of the battle lines. The topic is the role of soy in breast cancer. Does soy prevent breast cancer or promote it? Does soy have different effects in different people? Unfortunately, there are no clear-cut answers yet.
But as soy grabs the spotlight for preventing heart disease, improving bone health, and possibly protecting against other cancers, people want to know -- is there cause for concern?
Soybeans and foods made from soybeans may contain powerful ingredients that fight disease. Phytoestrogens are one of those powerhouse ingredients. They act like a weaker form of the female hormone estrogen. The specific name for phytoestrogens found in soy foods is isoflavones. They’re also called isoflavonoids. These hormone-like isoflavones may cause concern for people with breast cancer or those who are at risk for breast cancer.
Estrogen has both beneficial and harmful effects in the body. In women, estrogen protects the bones and heart and is vital for childbearing. However, years of exposure to high levels of estrogen may raise the risk for tumors in the breast and uterus.
Because the structure of isoflavones is very similar to estrogen, the body may recognize and use isoflavones as it would estrogen. But isoflavones can also block natural estrogen. It depends on how many isoflavones are present and where they are in the body.
There are few human studies, and no studies have looked directly at breast cancer survivors who eat soy isoflavones. And the results of the few human studies are conflicting. Many of them focused on the whole diet rather than just soy, used a small number of people, and observed women from Asian countries. They showed either a protective link from breast cancer or no link at all. No studies showed an increased risk for breast cancer.
Keep in mind that the results of Asian women cannot necessarily be applied to women in the United States. There are a number of differences between Asian and Western women. Many Asian women eat soy daily, starting at a very young age. Asian women also eat a diet with less fat and meat and more grains and vegetables. They are more physically active and have less body fat.
Test-tube studies looking at human breast cells have shown that isoflavones may act like either very weak estrogens or estrogen-antagonists (blocking natural estrogens from working). It seems to depend on how many isoflavones the cells are exposed to. At higher doses, isoflavones no longer act like estrogens and may block estrogen activity. However, breast cells in a test tube may not act the same as breast cells in a human body.
Questions that are being addressed in future research include:
What are the effects of isoflavones on the risk of getting breast cancer?
How do isoflavones affect breast growth in humans?
What role do isoflavones have on breast cancer survival?
What is the effect of isoflavones on other forms of estrogens used as drugs, such as tamoxifen and hormone replacement therapy?
Some researchers believe soy is protective against breast cancer and recommend up to three servings daily for the general public. But the benefit of soy on breast cancer prevention has not been proven and more study is needed before recommendations can be made.
Those with breast cancer or who have a family history of breast cancer should avoid soy until more is known, according to some experts. Others feel there is not enough evidence to restrict high-risk women from eating soy, especially if it helps them eat a more plant-based diet. The best advice at this time is that women with breast cancer or who are survivors should discuss their intake of soy with their doctor.
For now, most experts agree that breast cancer survivors should not be fearful of eating three to four servings of soy weekly, especially if it helps them eat a more healthy, plant-based diet. But, they should not take concentrated soy supplements or intentionally eat very high levels of soy in their daily diet.
Soybeans and most soy foods (tofu, tempeh, soy milk) are rich in high-quality protein and isoflavones, and low in saturated fat. Soy foods may also contain fiber, B vitamins, iron, calcium, and other minerals. The more a soy food is processed, however, the more isoflavones and fiber may be lost.
This chart lists a sampling of soy foods and their nutrient content.
Saturated fat (grams)
½ cup tofu, tempeh
1 cup soy milk
½ cup green soybeans (without pods)
1/4 cup defatted soy flour
3-ounce soy burger
Note: These are only average numbers. The exact amount will depend on how the food product is processed and if it is enriched with additional isoflavones or protein. Soy sauce and soybean oil do not contain any isoflavones.