At one time, soy seemed to be just the ticket for women: heart-healthy, good for bones and helpful for hot flashes. And then there was the low rate of breast cancer in soy-consuming countries. But as so often with "miracle foods," closer study has dampened some of the enthusiasm.
Early research indicated that soy protein could lower LDL (bad) cholesterol, but later studies were so unimpressive that the American Heart Association asked that food companies no longer be allowed to label soy products as helpful in preventing heart disease. It's still unclear whether soy does much for bones or hot flashes. And although some studies suggest that it may protect against breast cancer, other research has found that isoflavones (a component of soy that binds to estrogen receptors) can spur the growth of breast cancer cells in test tubes and animals. There's also some concern that soy's estrogenic activity may interfere with tamoxifen, a drug used to prevent recurrence in women with estrogen-sensitive breast cancer. As a result, some clinicians advise patients with breast cancer to limit their consumption of soy or avoid it altogether.
But now, a large study, presented in April at the annual meeting of the American Association for Cancer Research, shows no relationship between soy food consumption and increased risk of recurrence or death among women diagnosed with breast cancer.
Given the concerning past studies, what can we make of the recent study?
The investigators, led by researchers at Vanderbilt University, used data from four investigations constituting the After Breast Cancer Pooling Project. Together, these studies involve more than 18,000 breast cancer survivors in the U.S. and China, ranging in age from 20 to 83. Soy isoflavone intake was assessed by questionnaires 13 months after the diagnosis.
In the U.S., the average daily intake was 3.2 milligrams — about the amount in one-quarter cup of soymilk. The Chinese women had an average daily intake of 45.9 mg (the amount in two to three servings of tofu). In analyzing the data, investigators took into account many other potential risk factors, including age at the time of diagnosis, the stage of the cancer, past radiation therapy or chemotherapy, body mass index, smoking and whether the cancer was hormone receptor-positive or negative.
After about nine years, there was little difference in risk of recurrence or death when researchers compared women who ate the most soy isoflavones (more than 23 mg per day, on average) and those who ate the least (0.5 mg per day or less, on average). There were trends that suggested soy-related reductions in the risk for recurrence (15 percent) and mortality (9 percent), but these weren't statistically significant, meaning that they could have simply been due to chance.
This analysis drew from four prospective cohort investigations — observational studies that follow groups of people (cohorts) over time to examine the health impact of a particular factor, such as a dietary or lifestyle habit. Unlike a randomized trial, which is the gold standard for medical evidence, an observational study doesn't prove cause and effect, but it can be a good way to identify possible causes. A randomized trial, with its carefully controlled conditions, does a better job of correcting for extraneous factors.
Finally, keep in mind that these findings refer to soy foods, not soy supplements, which can have different kinds and amounts of isoflavones — sometimes as much as 60 to 80 mg or more in each pill, which is more than the highest average daily intake of soy isoflavones in the Vanderbilt-led study. And whole foods, unlike supplements, contain many other nutrients that may interact synergistically with the isoflavones.
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There's no agreed-upon "moderate intake" of soy for breast cancer survivors, but an occasional serving is probably fine — tofu every so often to replace red meat, for example. But there's no reason to start eating soy if you dislike it.Copyright © 2014, Los Angeles Times