I’ve been recommending soy foods since the 1970s. Like all legumes, soybeans are superb sources of fiber, essential fatty acids, and other key nutrients. They contain high quality, low-fat protein, provide relief from menopausal symptoms, help conserve bone mass, and reduce your risk of heart disease. Yet, every time I write about all of the great things soy provides, I am flooded with e-mail and letters from subscribers who are concerned about its possible risks.
It seems that we are bombarded on a daily basis with new information about soy—in some cases praising its benefits for preventing osteoporosis or hot flashes, and in other cases making it the scapegoat for everything from Alzheimer’s to early maturation in teenage girls. While a handful of the critical studies have some merit, many are based on questionable research or test tube science. These studies really lose their impact when compared to the hundreds of clinical, animal, and epidemiological studies that attest to soy’s health-protective benefits.
My goal today is to address two of the most common questions I’ve received regarding soy—how it affects breast cancer and thyroid function. The information I will be sharing with you today is based on my clinical practice and decades spent monitoring the research.
Question #1: Breast cancer runs in my family and I’ve read that soy can increase your risk for the disease. Should I stop eating soy?
Back in 2000, the New York Times ran an article by two researchers from the Food and Drug Administration (FDA) who claimed that soy isoflavones may “speed up” the proliferation of cancer cells that depend on estrogen for their growth. This article, paired with a few studies that found that the estrogen-like isoflavones in soy stimulated potentially cancerous cell growth in premenopausal women, sent many women into a panic.
I firmly believe that the panic has little basis. I don’t find these studies to be very convincing, especially when compared to the breadth and depth of studies I’ve read touting the cancer-shielding attributes of soy. Specifically, a multitude of studies have shown that soy may actually help to combat breast cancer in a number of different ways, including:
- Blocking estrogen receptor sites. Soy isoflavones are similar to your body’s own natural estrogens, only much weaker and less potent. Like estrogen, they can bind to estrogen receptor sites within the breast without exerting a dangerous estrogen-like influence. As a result, they compete with and block your own estrogen, thereby jamming these receptor sites and reducing the potential for your estrogen to stimulate the growth of cancer.
- Inhibiting tumor-induced angiogenesis (growth of new blood vessels). Because tumors can’t grow large enough to be clinically significant if angiogenesis is blocked, this finding is very encouraging.
- Preventing cellular damage that can trigger conversion of normal cells to cancerous ones. Researchers have found that women who ate a diet rich in soy products were 55 percent less likely to have abnormal breast tissue growth than women who consumed the least amount of soy. Researchers concluded that these findings may have significant implications in breast cancer prevention. Most telling to me are the long-term observances of real women in the real world and their experiences with soy. I have counseled women who had had breast cancer and were also consuming soy foods. Not only did they thrive on this diet, but they also remained cancer-free. Additionally, in countries such as Japan or China, where soy is a dietary staple, women are four to six times less likely than their American counterparts to suffer from breast cancer. And, researchers found that when Japanese women adopt a more Western, lower-soy diet, their breast cancer rate increases. My advice combines these facts with my own experience. I believe that it is fine for women with breast cancer to eat soy foods in moderation.
Question #2: I have hypothyroidism. Can I still enjoy soy?
Soy opponents refer to studies that show a correlation between dietary intake of isoflavones and thyroid disease for several species of animals. Additionally, some studies have found that animals fed soy isoflavones developed enlarged organs, particularly the pancreas and thyroid, as well as increased deposition of fat in the liver.
I believe that the concern over soy’s impact on thyroid function is unwarranted. In my experience, I have found soy to be well-tolerated by most women with thyroid disease and used quite safely. The lack of evidence surrounding this topic has led the FDA to reverse its earlier position that soy adversely affected the thyroid. Finally, and I believe, most compelling, is the fact that the American Foundation of Thyroid Patients has reviewed the current medical literature on soy and thyroid health and now recommends soy for all its members.
Based on the research as well as my experience, I see no reason for most women with a thyroid condition to avoid soy or soy products. The only exception I have made is for women who have inflammatory bowel disease and autoimmune thyroiditis, combined with a known allergy or sensitivity to soy. For this very small group of women, I recommend avoiding soy, as it may aggravate their condition.
Here’s what I want you to take away from this article. Based on the many positive studies currently available, I believe that soy can be used safely by the vast majority of women. I particularly recommend consuming whole soy foods. There is often a synergistic effect among the active elements that make the whole food more than the sum of its parts. And, in the case of soy, there are so many healthful components that it would be a shame to miss any of them.
If you find that soy foods cause digestive upset such as gas, bloating, or intestinal discomfort, I suggest that either you take a digestive enzyme specifically formulated for bean and vegetable consumption, or you take soy in the form of an isoflavone capsule. If you are allergic to soy, avoid consuming it entirely. If you are using soy for its therapeutic benefits, such as hot flashes, bone health, or heart health, a good rule of thumb is to take in 50–100 mg of soy isoflavones each day, either through soy foods or isoflavone capsules, or a combination of both.