The seesaw popularity contest over soy has constantly been an ongoing debate among foodie skeptics. Is soy good for me or will it increase my risk for chronic disease or cancer? The question of whether soy is the superman or kryptonite to your health is reviewed in nearly 2,000 papers published annually.

Several epidemiological studies have argued that the daily consumption of soy is associated with a lower incidence of chronic diseases including cardiovascular disease, type two diabetes, breast and prostate cancer as well as even reducing menopausal symptoms especially hot flashes. These studies have observed soy consumption in Asian populations where its intake is the highest. On average, these populations have about one to two servings of soy per day. Compared to the United States, Asia continued to demonstrate a lower incidence of cancer and researchers wanted to know why. They found that the isoflavones in soy play a key role in the suppression of tumor growth. When isoflavones, also known as phytoestrogens, pass through our digestive barriers unscathed and enter circulation, scientists believe that it interacts with human estrogen and as a result, decreases the development of cancers.

However, some studies have suggested that a daily intake of soy may actually heighten one’s risk for breast cancer by actually promoting tumor growth. Previous in-vitro studies looking at rodents found genistein, one of the primary isoflavones in soy increased the growth of estrogen receptor-positive in breast cancers leading to growth.  However  in these animal studies, rodents were exposed to high doses of isoflavones and process soy differently from people, and the same results have not been seen in people. Also, doses of isoflavones in the animal studies are much higher than in humans. In fact, in human studies, the estrogen effects of soy seem to either have no effect at all, or to reduce breast cancer risk (especially in Asian countries, where lifelong intake is higher than the US). This may be because the isoflavones can actually block the more potent natural estrogens in the blood.

One study recently published this past March in Cancer looked at more than 6,000 American and Canadian women with breast cancer and found that consumption of foods post-diagnosis containing soy’s main component, isoflavones, is associated with a 21% decrease risk of death. This decrease was only observed in women with hormone receptor-negative tumors as well as those who were not treated with endocrine therapy. A meta-analysis that included close to 10,000 breast cancer patients also found a positive correlation between consuming at least 10 mg of dietary isoflavones and a 25% reduced risk of breast cancer reoccurrence. The researchers believe that the consumption of isoflavones in their dietary form may even contribute to longevity.

So far, the evidence does not point to any dangers from eating soy in people, and the health benefits appear to outweigh any potential risk. In fact, there is growing evidence that eating traditional soy foods such as tofu, tempeh, edamame, miso, and soymilk may lower the risk of breast cancer, especially among Asian women. Soy is an excellent substitute for any meat protein, and the only legume that contains high amounts of omega-3 fatty acids. For years, many countries including the United States have recognized soy’s ability to lower cholesterol, lower rates of heart disease, and have provided health claims to support their growing consensus. The benefits of soy remain clear and continue to surprise researchers. It may be worth incorporating soy onto your plate on a regular basis.

Contributed by Bryan Stengel, Dietetic Intern, updated 8/23 by Madeline Basler

Dietary Isoflavone Intake and All-Cause Mortality in Breast Cancer Survivors: The Breast Cancer Family Registry. (n.d.). Retrieved June 15, 2017, from

Hooper, L., Kroon, P. A., Rimm, E. B., Cohn, J. S., Harvey, I., Le Cornu, K. A., … Cassidy, A. (2008). Flavonoids, flavonoid-rich foods, and cardiovascular risk: a meta-analysis of randomized controlled trials. The American Journal of Clinical Nutrition, 88(1), 38–50.

Messina, M., & Messina, V. (2010). The Role of Soy in Vegetarian Diets. Nutrients, 2(8), 855–888.

Nechuta, S. J., Caan, B. J., Chen, W. Y., Lu, W., Chen, Z., Kwan, M. L., … Shu, X. O. (2012). 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. The American Journal of Clinical Nutrition, 96(1), 123–132.

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