The story of soy and breast cancer is an excellent example of the research process. Pieces of information come in slowly, some studies contradict others, and then, overtime, as we learn more, we gain a clearer picture of the answer. But until that occurs, confusion and worry are common, as the subject in question—in this case soy—pops back and forth between the "good" category and the "bad."
Initially, soy was thought of as a wonder food. This happened because researchers looking for an explanation as to why women living in Asia had lower breast cancer rates than women living in the U.S. found that Asian women ate more soy products. Virtually overnight, soy became the best thing in breast cancer prevention. Tofu and soy milk were all the rage. All kinds of processed foods began to contain soy, and soy powders and supplements could be found everywhere. There was no question: Soy was GOOD!
But as scientists began to conduct laboratory studies to look for reasons why soy might decrease breast cancer risk, they found that when genistein, which is a type of isoflavone, was added to breast cancer cells they grew faster. Soon after, it was suggested that women who were taking tamoxifen and other anti-estrogens should avoid soy because soy acted like a weak estrogen, and could potentially counteract the tamoxifen or increase a woman's risk of recurrence. Suddenly, soy was now BAD!
That's why it was big news when a study conducted in Shanghai, China, published in December 2009 in the Journal of the American Medical Association found that women with breast cancer who ate soy had a decreased risk of having a cancer recurrence or dying of the disease.
The study was based on data from the Shanghai Breast Cancer Survival Study, which includes 5024 female breast cancer survivors. After following the women for four years, the researchers found that the higher a woman's soy protein or soy isoflavone intake, the lower her risk of having a breast cancer recurrence or dying from the disease. This was true regardless of whether women had ER-positive or ER-negative tumors, whether or not they were taking tamoxifen, and whether they were pre- or postmenopausal. (Although the researchers didn't look at this question, the findings would be expected to be the same for women on aromatase inhibitors.)
The study's authors pointed out that there are differences in the types of soy eaten by women in Asia and women in the U.S. In Asia, women are more likely to eat whole soy foods, like cooked soybeans, edamame, tofu, miso, and soy milk, whereas in the U.S. women tend to eat more processed foods that contain soy—and at much lower levels.
This is additional evidence that women in the U.S. with breast cancer can feel even better about adding whole soy foods to their diet. To be sure, we need more studies in women to confirm this finding. But a study of this size that has been so well done and that shows no negative effects clearly suggests that eating soy will not increase your risk of a recurrence and that, in fact, it might even reduce it!
What about soy letcithin? It is extracted from soybeans, but it is not part of the soy protein. As a result, it does not contain any isoflavones, which are the part of the soybean that acts as an estrogen. It is primarily used as an emulsifier to hold ingredients together. So, there is also no reason to worry that you are getting lots of isoflavones in your diet each time you chew gum or take a supplement.
The soy story also shows us why we can't over simplify the science. Soy is not a phytoestrogen but rather a "phytoSERM," more like tamoxifen than estrogen. It also reinforces that we cannot automatically extrapolate from studies on cells and rats to women. We need to do studies on women so that we know what happens in women's bodies.
This post is courtesy of the Dr. Susan Love Research Foundation, dedicated to eradicating breast cancer and improving the quality of women's health through innovative research, education and advocacy. To support this important cause and donate, visit www.dslrf.org.