Soy benefits women with breast cancer? Release on observational study needed more caution

Our Review Summary

This news release describes results of a patient survey-based study that examined the association between dietary soy foods and death from any cause among thousands of American and Canadian women with breast cancer.

The release reports that women with breast cancer who ate the highest amounts of isoflavones (a primary active ingredient in soy foods) had a 21 percent lower risk of dying (from any cause, not just breast cancer) over a median nine year follow up period than women who ate the lowest amounts. It offers important details about the hormone-receptor status and treatment of the women to help readers sort out which women with breast cancer might experience the benefits.

It also noted the controversies that have emerged from previous research linking soy intake and breast cancer.

While the release clearly has its strengths, it would have been stronger had it explicitly stated that nothing in the study demonstrates that eating soy will absolutely prevent breast cancer or extend the life of patients already diagnosed. This was an “association” study and not one designed to determine cause and effect. And the release would have been more useful had it noted the weaknesses found in research based on self reporting through patient surveys of dietary habits.

To see how health journalists covered this research, see the related blog post.

Why This Matters

Previous laboratory and animal studies have offered mixed and conflicted results about the impact of soy’s active ingredients on the incidence of breast and other cancers, although epidemiological studies have shown a relatively lower risk of breast cancer in populations (such as those in Asia) with diets high in soy foods. Adding to the confusion, however, is  that soy isoflavones are also known to be estrogenic, increasing available estrogen to cells and potentially increasing the risk of breast cancer in places — like the U.S. and Canada — where soy consumption is very low overall. And even further adding to the confusion are data showing that some isoflavones bind to estrogen receptors much the way tamoxifen does in the treatment of  patients whose tumors are estrogen hormone receptor-positive.

Because soy is an increasingly high quality source of protein worldwide; because soybean products are a component in many processed foods; because breast cancer death rates overall have declined very little over the last 40 years; and because the U.S. is the number one producer of soybeans, questions about the safety and benefits (prevention and post-diagnosis) of dietary soy have been of concern to women with breast cancer, those with a family history of the disease, and women in general. Consequently, studies designed to clarify the situation are of high interest and high value.