Nutritionally, soy is a great, affordable protein option—so why does it have such a bad reputation? Here we explore what the science says about soy and cancer, heart disease and more.

Jessica Ball, M.S., R.D.
March 13, 2020

Soy is a versatile, affordable vegan protein option. Whether or not you follow a meat-free diet, you have probably seen (and purchased) a variety of soy products. In recent years, soy has become a controversial food when it comes to health. We are here to set the record straight about this popular legume.

Getty Images, Jordan Lye

Soybean Nutrition

Here is the nutrition for one serving (1/2 cup) of soybeans:

  • 188 calories
  • 17 g protein
  • 9 g fat
  • 14 g carbs
  • 5 g fiber
  • 252 mg calcium (19% Daily Value)
  • 211 mcg folate (53% DV)
  • 4.5 mg iron (25% DV)

Soybeans are gluten-free; however, some soy sauces are made with wheat and salt. Gluten may also be present in some processed soyfoods, like veggie burgers or other soy "meat" products.

Soy is also a great for vegetarians and vegans to include because it is high in protein, calcium and iron, all of which can be hard to find in plant foods.

Related: Healthy Tofu Recipes

Soy and Heart Health

Soy can be great for your heart, and there are a few reasons why. The type of protein in soyfoods helps lower blood cholesterol. This alone reduces your risk for heart disease, but the benefits don't end there. Additional components of soy, like isoflavones, lecithins and fiber, may help prevent hypertension, high blood sugar, inflammation, cardiovascular disease and obesity.

Soy and Cancer

Most experts agree that soyfoods are safe, and may even be beneficial, when it comes to cancer risk. The American Cancer Society recommends keeping soy in your diet, but not taking soy supplements.

Soy and Breast Cancer

Soy is at the center of talk around breast cancer risk, and the main reason is because of isoflavones. Isoflavones are compounds in soy that have weak estrogen-like activity, meaning they may trick the body into thinking they are estrogen.

When talking about isoflavones, age and life stage is important. Prior to menopause, women have high levels of estrogen. However, after menopause, women's estrogen levels go down. Having a high amount of isoflavones in your blood can trick the body into thinking estrogen levels are still high. Prolonged high levels of estrogen after menopause is a known risk factor for breast cancer.

On the other hand, soy provides other health benefits for women. It can alleviate hot flashes, has plentiful anti-carcinogenic antioxidants and can even promote fertility in women undergoing infertility treatment.

Along with age, ethnicity is an important consideration when thinking about soy and cancer risk. Several studies have found that soy can protect pre- and post-menopausal women of Asian descent from breast cancer. This is thought to be due to the high consumption of soy in Asian culture, in the form of tofu, miso, soy sauce and more.

Soy and Prostate Cancer

Isoflavones may also artificially increase testosterone levels, which brings up concerns about prostate cancer. In a 2018 review published in Nutrients, researchers actually found an association between eating soy and decreased risk of prostate cancer. Men should be eating soy too.

Pictured: Super Green Edamame Salad

Soy and Memory

One exciting finding about soy is that eating it may boost your memory and cognition. Virginia Tech published an overview of research that cited mixed results of eating soy, including one finding that it improved cognitive function, particularly memory, in post-menopausal women. This is likely due to compounds called glyceollins that are plentiful in soyfoods. Strong antioxidants, glyceollins can protect our neurons from damage that leads to cognitive decline.

Don't Miss! Healthy Miso Soup Recipes

Soy and Your Thyroid

Soy is rumored to have a negative effect on thyroid health by blocking the absorption of the nutrient iodine. This is thought to increase the amount of thyroid-stimulating hormone in the body, which can increase the risk of thyroid disease. For those on thyroid medications, this is important to note. People with hypothyroidism should avoid eating large amounts of foods that may contain goitrogens, and soy is one of those foods (learn more about what to eat and limit with hypothyroidism).

However, a 2019 review of research found that there was no evidence to support soy impacting thyroid hormones. Even further, they concluded that eating soy had no clinical significance (positive or negative) for people with a healthy thyroid.

Types of Soy Products

Soy products come in many shapes and sizes, and have a variety of uses. Some of the most popular include:

  • Tofu: Tofu is made from heating soymilk with a curdling agent and straining the solid pieces from the liquid. The longer the process goes on, the firmer the tofu. You can buy tofu in silken, firm and extra-firm textures.
  • Edamame (aka soybeans): These little guys are green soybeans, usually boiled or steamed in their pods. You see these often at sushi restaurants or in the freezer section of your grocery store.
  • Soy Sauce: Soy sauce is a mixture of fermented soy liquid, wheat, water and salt. Note: This is why soy sauce is not typically gluten-free, even though soy contains no gluten (see some options for gluten-free soy sauces here).
  • Miso: Miso is a paste made from soy that's been fermented using a mold called koji. The different colors of miso correspond with different types of soybeans.
  • Tempeh: Tempeh is a textured, nutty fermented soy loaf. Sometimes tempeh can be made using other grains, so that is something to be aware of if you follow a gluten-free diet.
  • Soymilk: This soy-based dairy alternative is made from cooked, ground and pressed soybeans. It is one of a few plant-based milks that has a comparable amount of protein to cow's milk, at 7 grams per cup.
  • Soy Flour: Soy flour is made from dried, ground soybeans. It is a high-protein, gluten-free alternative to wheat flour.

Bottom Line

Soy comes in many shapes and sizes and has many uses. Overall, the legume has some beneficial properties for your heart and brain. It should be reassuring that no studies have found that soy conclusively increases your risk for breast or prostate cancer. Nutritionally, soy is a great, affordable protein option for those who are plant-based and fits well into a balanced diet.

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