Women who ate soy-containing foods in childhood may have a lower risk of breast cancer than women who did not, according to a new study.

March 24, 2009

By Mara Betsch
MONDAY, March 23, 2009 (Health.com) — Women who ate soy-containing foods in childhood may have a lower risk of breast cancer than women who did not, according to a new study.

“This is just one of a number of studies suggesting that there are exposures early in life that may determine [breast cancer] risk in adulthood,” says Larissa Korde, MD, a staff clinician at the National Cancer Institute and the lead author of the study in the journal Cancer Epidemiology, Biomarkers and Prevention.

The researchers compared 597 breast cancer patients of Chinese, Japanese, and Filipino descent living in the United States to 966 breast cancer–free women with a similar ethnic background. A childhood diet high in soy—1.5 servings or more a week—was associated with a 58% reduction in breast cancer risk. The same intake in the adolescent and adult years was associated with a 20% to 25% reduction in risk.

“The study is one of the best because it’s a very diverse sample of different kinds of Asian-American women. You get to see a good range of soy exposure that you can’t see in white women,” says Marilyn Tseng, PhD, an associate member at the Fox Chase Cancer Center in Philadelphia.

Soy can be found in tofu, soy milk, and edamame (ripe soybeans often served in Japanese restaurants), as well as other foods.

In general, Asian women have a lower risk of breast cancer than women in Western nations. But when Asian women immigrate to the United States, their breast cancer risk rises over several generations.

"Diet, we do know, plays a role in the development of breast cancer. It has been thought for some time that the Western diet—with more animal fat, red meat, and processed foods—may be more contributive," says Julia Smith, MD, PhD, the director of the breast cancer screening and prevention program at New York University’s Cancer Institute.

In the study, researchers interviewed participants about diet and cultural practices. If the women had mothers living in the U.S., the mothers were interviewed too.

Rich in protein and low in saturated fat, soy can lower cholesterol, produce smaller fat cells, and reduce blood pressure. Despite this, there is some controversy over the link between soy and breast cancer. Some studies show a diet high in soy lowers the risk of breast cancer, while others show no effect.

Soy contains chemicals called isoflavones, which act like a weaker version of the hormone estrogen. While estrogen seems to raise breast cancer risk by encouraging the growth of breast tissue, isoflavones are thought to slow cell growth.

However, different types of soy may have different effects on the body. Foods like tofu, soy milk, and tempeh may lower risk, but soy supplements may actually encourage the growth of estrogen-sensitive breast tumors, according to some research.

“A lot of studies [on soy] have had conflicting results. It is very difficult to get a good measure on one particular substance in your diet,” says Dr. Korde. It’s no easy task to figure out whether something in the diet, or another lifestyle factor, is responsible for a reduction in cancer risk, she adds.

The researchers did take into account Asian cultural practices—such as neighborhood ethnicity and the type of grocery store frequented—that may be associated with lower breast cancer risk.

But don't stock up on soy milk just yet. Though promising, the study relied on women’s memories of soy intake in childhood, a method prone to error. Experts say more research is needed before drawing any firm conclusions on the link.

"What we can say at this point is that both one's basic genetic blueprint, and one's environment, interact to affect risk,” says Dr. Smith. “I suggest women not smoke, use alcohol in moderation, and eat a diet rich in fresh fruits, vegetables, and whole grains.”

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