Does Eating Soy Increase Your Risk for Breast Cancer?

Here's the truth.

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Soy has a bad rap, which has scared many folks into avoiding this plant-based protein altogether. Many believe that soy is linked to breast cancer. However, the truth is, it’s just another myth that's raised fears about eating a healthful food. Here’s a look into where this myth came from and the truth about soy and breast cancer.

Where the Myth Began

The idea that soy can lead to breast cancer began with several animal studies conducted in the 1990s. These studies showed that the compound in soy (called isoflavones) stimulated the growth of tumors in mice implanted with estrogen-receptor positive breast cancer cells. It was then believed that women with an increased risk of developing breast cancer should avoid soy because it would increase their risk even more.

However, according to Sharon Palmer, RDN, The Plant-Powered Dietitian, “These studies have several important limitations and when this model is just slightly tweaked, isoflavones do not stimulate tumor growth. More importantly, epidemiologic evidence shows that post-diagnosis, soy intake improves the prognosis of breast cancer patients.”

Unfortunately, the idea that phytoestrogens were the same as estrogens spread quickly through the media and over the internet. Today, many folks still strongly believe in this myth and avoid soy at all costs.

Debunking the Myth

There is a lot of evidence that debunks the myth that eating soy leads to breast cancer. Some studies even show that soy may have a protective effect. Mark Messina, PhD, MS Adjunct Assoc. Professor, Dept. of Nutrition, School of Public Health, Loma Linda University and the Executive Director, Soy Nutrition Institute explains, “Asian observational studies show soy intake is associated with about a one-third reduction in the risk of developing breast cancer. However, several lines of evidence suggest that to derive protection against breast cancer, soy must be consumed during childhood and/or adolescence.”

In addition, Messina says that there are animal studies in support of this hypothesis as well as four retrospective studies, two conducted in China and two in the United States although all four involved women of Asian ethnicity.

So How Much Soy Should You Have?

There is no official recommendation, although for cholesterol reduction, 25 grams per day of soy protein was set as the maximum intake by the FDA. Palmer recommends sources like soy milk, edamame, tofu, and tempeh and personally includes it in her diet 1 to 2 times per day. “It is the highest quality protein source among plants,” explains Palmer. She also does not recommend highly concentrated sources of isolated soy, as in powders and bars.

Here is a run-down of the soy protein found in common soy-based foods.

3 ounces firm tofu = 6.3 grams soy protein

1 cup unsweetened soy milk = 7 grams soy protein

1/2 cup in-shell edamame = 8 grams soy protein

3 ounces tempeh = 16 grams soy protein

Bottom Line

There's plenty of evidence debunking the myth that connects soy to breast cancer, so you can start including soy in your healthy diet and feel good about it!

Toby Amidor, MS, RD, CDN, is a registered dietitian and consultant who specializes in food safety and culinary nutrition. She is the author of The Greek Yogurt Kitchen: More Than 130 Delicious, Healthy Recipes for Every Meal of the Day.

*This article was written and/or reviewed by an independent registered dietitian nutritionist.

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