Soy, breast cancer link unclear

DEAR DOCTOR K: I’m a breast cancer survivor. Can soy foods such as tofu or soy milk increase my risk of a cancer recurrence?

DEAR READER: I wish I had a simple and reassuring answer.

A component of soy, isoflavone, binds to estrogen receptors just like estrogen does. That could stimulate the growth of breast cancer cells. It also could interfere with the effects of tamoxifen, a drug used to prevent recurrence in women with estrogen-sensitive breast cancer. As a result, some clinicians advise breast cancer patients to limit their consumption of soy or avoid it altogether.

On the other hand, some studies suggest that soy may protect against breast cancer. For example, Asian women have a much higher intake of soy (mainly because they eat a lot of tofu) than U.S. women, but have lower rates of breast cancer. Of course, there are many differences in the lifestyles and environment between Asian and U.S. women, so that statistic doesn’t tell us much.

The tides may be turning in favor of soy. Last year, at a scientific meeting, researchers presented evidence that showed no link between eating soy foods and increased risk of recurrence or death among women diagnosed with breast cancer. Like the studies mentioned above, this study compared women in Asia with women in the U.S. Unlike the studies above, however, this one focused specifically on women with breast cancer.

The study involved more than 18,000 breast cancer survivors in the United States and China. In the U.S., the average daily intake of soy isoflavones was 3.2 milligrams (mg). That’s about the amount in one-quarter cup of soy milk. The Chinese women had an average daily intake of 45.9 mg (the amount in two to three servings of tofu).

After nine years, there was little difference in risk of breast cancer recurrence or death between women who ate the most soy isoflavones and those who ate the least.