Soy may be getting bad rap in relation to breast cancer

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. Soy has a complicated relationship with breast cancer. Many breast cancer cells have receptors for estrogens. Receptors are like a lock, and estrogen is like a key. When the key enters the lock, and fits it, breast cancer cells start to multiply.

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.

Keep in mind that these findings refer to soy foods, not soy supplements. Supplements can have as much as 80 mg or more of isoflavones in each pill. That's more than the highest average daily intake of soy isoflavones in the study. And whole foods, unlike supplements, contain other nutrients that may interact beneficially with the isoflavones.

Here's what I tell my patients in this confusing situation: I don't think there is good evidence that women without breast cancer should eat more soy foods in order to protect against breast cancer. I also think breast cancer survivors can eat soy foods in moderation. An occasional serving is probably fine -- tofu every once in a while to replace red meat, for example, or soy milk instead of cow's milk.

n Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

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