In the largest study done to date, soy foods were not linked to a higher risk of recurring breast cancer in women who’ve had the disease.
Researchers followed more than 9,500 women (roughly half in the United States and half in China) who had been diagnosed with invasive breast cancer. After seven years, those who consumed at least 10 milligrams of soy isoflavones a day had no higher risk of dying–and had a 25 percent lower chance of recurrence–than those who averaged less than 4 mg of isoflavones a day. (You’d get about 25 mg of isoflavones in 4 oz. of extra-firm tofu or a cup of soy milk.)
But here’s why the study isn’t definitive:
Chinese vs. U.S. women. The evidence that large intakes of soy appear safe (or even beneficial) is based largely on the Chinese women. Only 10 percent of the U.S. women consumed at least 10 mg of soy isoflavones a day. (They averaged 3 mg.) in contrast, 90 percent of the Chinese women got at least 10 mg a day. (They averaged 46 mg.)
Could something about U.S. women (like their heavier weight) put them at greater risk from soy than Chinese women? The study couldn’t say.
Are soy eaters healthier? The soy eaters in both countries were more likely to exercise and eat cruciferous vegetables like broccoli than those who ate less soy. The U.S. soy eaters were also more likely to be normal weight, highly educated, and nonsmokers. (In China, the soy eaters were more likely to weigh more than those who ate less soy.)
The scientists “adjusted” for those factors, but couldn’t adjust for unknown differences between the women who ate more vs. less soy. And unknown differences could explain the lower risk of recurrence in soy eaters.
What to do:
It’s too early to know if soy foods lower the risk of breast cancer recurrance. However, it looks like women needn’t worry that typical intakes of soy boost the risk of recurrence.
Source: NutritionAction Health Letter September 2012