We've all heard the claims that "soy is bad for you" or that "soy causes cancer" or that "men shouldn't eat soy because it mimics female hormones". Let's set the record straight once and for all with this definitive article, written by Susan Levin. My thanks to Susan for this excellent piece (and for contributing to my book, Vegan Vitality, a few years back)!
Susan Levin, M.S., R.D., C.S.S.D., is director of nutrition education for the Physicians Committee for Responsible Medicine, a Washington, D.C.-based nonprofit organization dedicated to promoting preventive medicine, especially better nutrition, and higher standards in research.
As director of nutrition education for the Physicians Committee, Ms. Levin researches and writes about the connection between plant-based diets and a reduced risk of chronic diseases. Through her work, she also addresses the need for nutrition guidelines that reflect the benefits of plant-based dietary patterns. In addition, Ms. Levin assists in teaching nutrition and health classes to participants in clinical studies exploring the links between diet and various medical conditions.
Ms. Levin received her Master of Science in Nutrition and her registered dietitian training at Bastyr University in Seattle. She also earned a specialty certification in sports dietetics from the Academy of Nutrition and Dietetics. She received her bachelor’s degree in journalism and mass communications from the University of North Carolina at Chapel Hill.
Take it away, Susan!
I’m not exactly sure how soy got its bad rap. It’s one of those nutrition mysteries, like when protein became an elusive, hard-to-find nutrient or when milk became synonymous with bone health. I guess hammering away at untruths for long enough can actually be effective. I could insert an allusion to U.S. elections here, but I’ll refrain(-ish). Regardless of how the misinformation started or even how it was spread, the evidence does not support reason to fear soy products. So let’s break it down by categories and follow the trail of science.
Cancer
Let’s start with one of the biggest misconceptions – soy products increase your risk for cancer, specifically breast cancer. Soybeans and many other foods contain isoflavones, natural compounds whose chemical structure is vaguely similar to estrogens. Some people have called these compounds “phytoestrogens” (“phyto” comes from the Greek word for “plant”).
In 2008, researchers combined the results of eight prior studies on the relationship between soy products and breast cancer, finding that women who had the most soy—in the form of soy milk, tofu, etc.—were 29 percent less likely to develop breast cancer, compared with women who ate little or no soy products.[1] In 2014, researchers looked again, this time combining the results of 35 prior studies. Again, soy had a preventive effect, cutting breast cancer risk by 41 percent.[2]
So soy products appear to reduce the likelihood that breast cancer will strike. But what about women who have had cancer already? Some women who have been treated for breast cancer avoid soy products, on the theory that soybeans contain estrogens that can make cancer grow. However, in 2012, researchers analyzed the results from 9,514 breast cancer survivors. It turned out that women consuming the most soy products had roughly 30 percent less likelihood that their cancer would recur, compared with women who consumed little or no soy.[3]
Male Hormones
Soy products have no adverse effects on men and may help prevent cancer in men. A meta-analysis based on more than 50 treatment groups, showed that neither soy products nor isoflavone supplements from soy affect testosterone levels in men.[4] An analysis of 14 studies published in the American Journal of Clinical Nutrition showed that increased intake of soy resulted in a 26 percent reduction in prostate cancer risk.[5] Researchers found a 30 percent risk reduction with nonfermented soy products such as soymilk and tofu.
Thyroid Health
Clinical studies show that soy products do not cause hypothyroidism.[6] However, soy isoflavones may take up some of the iodine that the body would normally use to make thyroid hormone.[7] The same is true of fiber supplements and some medications. In theory, then, people who consume soy might need slightly more iodine in their diets. Soy products can also reduce the absorption of medicines used to treat hypothyroidism.[6] People who use these medicines should check with their health care providers to see if their doses need to be adjusted.
Summary
Soy products are in no way essential. They are very handy for replacing meat, milk, and many other foods, and, if anything, soy products reduce the risk that cancer will ever start and, in women who have been treated for breast cancer already, reduce the risk that cancer will come back. They do not appear to have adverse effects on the thyroid gland, but may reduce the absorption of thyroid medications. The benefits of soy products appear to relate to traditional soy products, not to concentrated soy proteins. To read more about soy and its benefits, including for bone and heart health, click here.
References
1. Wu AH, Yu, MC, Tseng CC, Pike MC. Epidemiology of soy exposures and breast cancer risk. Br J Cancer. 2008;98(1):9-14.
2. Chen M, Rao Y, Zheng Y, et al. Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies. PLoS One. 2014;9(2):e89288.
3. Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012;96(1):123-132.
4. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94:997-1007.
5. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr. 2009;89:1155-1163.
6. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16:249-258.
7. Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action. Biochem Pharmacol. 1997;54:1087-1096.
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