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Soy and Breast Cancer - Soy Supplements, Allergies, Healthy Diet

Know When to Avoid, and When to Benefit from Soy


Updated June 09, 2014

Reasons to be Shy About Soy
While soy may help relieve your hot flashes, researchers caution postmenopausal women against having too high a dose of soy, particularly in the form of supplements that contain high amounts of soy isoflavones. And if you've had estrogen-sensitive breast cancer, and are taking a selective estrogen receptor modulator, such as tamoxifen, or an aromatase inhibitor, such as Aromasin, it's a good idea to refrain from soy. The soy isoflavone genistein may counteract estrogen suppressors - and that would make your post-treatment medication less effective. After you've completed a full course of estrogen suppressors (usually 5 years) you can start including soy in your diet again, in modest amounts. If you still want the benefits of isoflavones, try dining on legumes, whole grains, and nuts. On the other hand, a good reason to avoid soy altogether is if you know that you're allergic to it. You should also skip soy if you have a thyroid disorder or goiter.

The Bottom Line on Soy and Breast Cancer
You may get the most benefits from consuming soy isoflavones such as genistein, if the isoflavones come from food – not from nutritional supplements. The American Cancer Society says that concentrated extracts of soy isoflavones may encourage tumor growth, and should be avoided. Women in the Japanese study who had the lowest rates of breast cancer had consumed soy from childhood, or at least from pre-puberty. Post-menopausal women should not overdo soy products, because the powerful isoflavones mimic natural estrogen, which fuels 80% of all cases of breast cancer. Adults who start a diet that includes 25 grams of soy foods daily will experience some benefit from soy isoflavones (lower cholesterol, better heart health) but will not gain the same protection from cancer as people who have eaten soy regularly over a lifetime.

The Soy and Breast Cancer Controversy

American Cancer Society. Soybean. Revised: 07/12/2007.

American Cancer Society. Soy May Counteract Tamoxifen Used by Breast Cancer Patients. Published: 06/20/2007.

U. S. Food and Drug Administration. Soy: Health Claims for Soy Protein, Questions About Other Components. FDA Consumer, May-June 2000. John Henkel.

J Clin Oncol. 2008 Apr 1;26(10):1677-83. Plasma isoflavone level and subsequent risk of breast cancer among Japanese women: a nested case-control study from the Japan Public Health Center-based prospective study group. Iwasaki M, et al.

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