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


3 Star

Soy food consumption linked to prolonged survival in some breast cancer patients

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.


Does the news release adequately discuss the costs of the intervention?

Not Applicable

Although a report on this study did not necessarily require observations involving the $ sign, the release would have been more helpful had it noted that soy products are relatively inexpensive compared to other sources of protein and might also have mentioned the financial and social costs of breast cancer.

Does the news release adequately quantify the benefits of the treatment/test/product/procedure?

Not Applicable

We would have given the release full credit if it had included absolute numbers along with the relative risk reduction data. Absolute numbers would better clarify for readers the actual number of women in each patient group who survived longer. Overall, the reader is given a sense of the modest but significant finding with respect to reduced deaths.

The release would have been improved further if it had added the qualifier “some” to the first sentence of the release. Many readers skim or read only the first few sentences and thus might miss the important point that soy consumption was beneficial only for some women — those who are hormone receptor negative and those who were not treated with hormone therapy, which of course is not recommended for those who are hormone receptor negative.

Breast cancer survivors and their family members also would benefit from information about the relationship of the stage at diagnosis and death as related to soy consumption.

Does the news release adequately explain/quantify the harms of the intervention?


The release does a competent job of explaining the concerns about soy’s estrogenic properties and the fact that the study was designed to look at overall deaths from any cases among the patients, and not at quality of life or morbidity.

A careful reader would be able to discern in the release that gobbling up soy supplements is not encouraged by the results of this study or the scientists who conducted it; and that there is a difference between saying something is “not detrimental” in women treated with hormone therapy and that something is completely safe for all women with breast cancer.

We would have liked the release to include a statement noting that women with breast cancer should consult their physicians about their tumor’s potential sensitivity to estrogen and to discuss dietary soy with them. This is particularly important since the release ends by quoting an editorial that encourages wide consumption of soy.

Does the news release seem to grasp the quality of the evidence?

Not Satisfactory

The release doesn’t mention that this is an observational study and that such studies don’t prove cause and effect.

The release would have benefited from more details about the 6,235 women whose data make up the study, such as their ages, cancer stage, ethnicity, and race. Some details about the questionnaire/survey tool, and its strengths or weaknesses would also have been helpful for evaluating the evidence.

It would also have been good to know whether the women surveyed were hormone receptor negative and if so, did they consume soy before their diagnosis?  How much soy did they consume?

Does the news release commit disease-mongering?


No disease mongering here. The release also provides useful context.

Does the news release identify funding sources & disclose conflicts of interest?

Not Satisfactory

The study was funded by the National Cancer Institute; noting that would have strengthened the release.

Sophisticated health news consumers appreciate knowing whether a study was government funded or funded by a special interest such as the soybean industry. Readers should also be told that this research is part of a larger study that has been ongoing since 1995.

Does the news release compare the new approach with existing alternatives?

Not Satisfactory

There other dietary and lifestyle factors that affect cancer risk and survival. The release doesn’t mention any of them.

Does the news release establish the availability of the treatment/test/product/procedure?

Not Applicable

It would have been interesting and helpful for the release to note what kinds of soy foods the participants said they ate, and which ones were most popular.

Does the news release establish the true novelty of the approach?


The release did a creditable job of explaining what the study was designed to do and how it differed from past studies.

One of the key findings of the study was that soy consumption was not detrimental. During the 1990s, many doctors and breast cancer organizations were urging women not to consume soy products because they could be harmful. Survivors may be relieved to know this study, at least, did not find soy consumption harmful since soy products are now found in many foods. But as noted in the review summary, study results have been mixed and conflicting.

Does the news release include unjustifiable, sensational language, including in the quotes of researchers?

Not Satisfactory

The release includes a comment from a related editorial that should be out-of-bounds for an observational study:

“We now have evidence that soy foods not only prevent breast cancer but also benefit women who have breast cancer. Therefore, we can recommend women to consume soy foods because of soy’s many health benefits,” he wrote.

Total Score: 3 of 7 Satisfactory


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