The Soy and Breast Cancer Controversy: Cause for Concern?

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 simply 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?

Soy’s Role in Fighting Disease

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 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.

What the Research Says

There are few human studies, and no studies have looked directly at breast cancer survivors who eat soy isoflavones. 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.

Mark Messina, PhD, editor of “The Soy Connection” newsletter and former nutrition researcher at the National Cancer Institute, has studied soy for 12 years. “For all we know, the breast cancer rate in Asian women would be even lower if they didn’t eat any soy--we really can’t tell from these studies,” he says. “Until there is a long-term study looking at breast cancer survivors and soy, it’s going to be a bit of an unresolved issue.”

Animal studies have shown that when animals are fed soy isoflavones, they grow fewer breast tumors. The soy was more protective if given when the animals were younger, suggesting that age may play a part in learning when isoflavones are most effective.

Barbour Warren, PhD, research associate at Cornell University’s Program on Breast Cancer and Environmental Risk Factors, believes the most interesting new study involves soy in the diets of adolescent Asian females. “The animal studies and this newer Asian study suggest that exposure to isoflavones during the time of breast development during puberty may lead to a breast that has less cell proliferation and therefore fewer cells at risk for becoming tumors.”

But “the exact amount of soy needed to show this effect on breast tissue is not yet clear,” adds Warren.

Test-tube studies looking at human breast cells have shown that isoflavones may act like either 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?

What the Experts Say

Valerie Kogut, MA, RD, nutrition coordinator at the University of Pittsburgh Physicians, Otolaryngology Department, and nutrition consultant for the American Institute for Cancer Research (AICR), frequently receives calls about soy and breast cancer through AICR’s nutrition hotline. “The topic is hot right now because there are two schools of thought on it,” she says.

Some researchers believe soy is protective against breast cancer and recommend up to 3 servings daily for the general public. Warren says that the benefit of soy on breast cancer prevention is “still up in the air--soy has been hyped too soon.”

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.

“We really don’t know the exact amount of soy to eat for a protective benefit with breast cancer,” Kogut says. Therefore, she does not offer specific recommendations but informs people of the current research on soy. She says that women with breast cancer or who are survivors should discuss their intake of soy with their doctor.

Messina says, “The issue remains unresolved.” However, in following recent studies, he believes that future research will lead in the direction of soy not being a problem for breast cancer survivors.

For now, Messina says that breast cancer survivors should not be fearful of eating 3 to 4 servings of soy weekly, especially if it helps them eat a more healthful, plant-based diet. But, “it may not be wise to eat large amounts of soy with the sole purpose of preventing breast cancer recurrence,” he adds.

Soy Foods vs. Soy Supplements

Kogut recommends avoiding soy supplements. “It’s best to eat the whole food, which comes with vitamins, minerals, and other protective substances that can benefit health,” she says.

Messina has a slightly different view. “I don’t always agree with the thinking that says ‘stay away from soy pills and powders, but food is OK.’ I don’t differentiate between pills and food, because many pills on the market have the same amount of isoflavones as one serving of soy.” He notes that several studies showing an estrogenlike effect from isoflavones used standard servings of soy food, not supplements.

Messina believes that isoflavones are the active ingredient in soy at this point in regard to cancer. “They are absorbed well from supplements. But if the goal is to shift toward a more vegetarian-based diet that includes soy foods, then it doesn’t make sense to use pills,” he adds.

Warren prefers food sources of soy. “Consumers have little idea what dose they are getting in soy supplements or how much to take. A reasonable amount of soy in the diet makes more sense. This may also give benefits not seen when only using isoflavones supplements,” he says.

For more information on this or other nutrition-related topics and to speak with a registered dietitian, call the American Institute for Cancer Research’s toll-free nutrition hotline at 1-800-843-8114, Monday through Friday, 9AM to 5PM (Eastern Standard Time).

Nutrient Content of Soy Foods

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.

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.