Soy foods are rich in protein, fiber, and minerals like iron, calcium, magnesium, potassium, and folate. Soy contains phytoestrogens. These are plant molecules similar to estrogen, a hormone with a key role in reproductive, bone, brain, and thyroid health.
Estrogen is a hormone that affects reproductive, bone, brain, heart, and thyroid health.
Phytoestrogens are plant compounds that have a similar structure to estrogen. These compounds naturally occur in various foods, including soy foods, some nuts, seeds, fruits, and vegetables.
When you eat phytoestrogens and they enter your body, their estrogen-like structure allows them to bind to estrogen receptors. For this reason, phytoestrogens can mimic the effects of estrogen in the body, but often with weaker effects.
Isoflavones—the main phytoestrogens in soy foods—can either mimic or block estrogen when they bind to the receptors. This depends on which type of estrogen receptors they bind to.
Soy’s effects on health started to gain interest in the 1990s. Most test-tube and animal studies at this time showed soy’s negative effects on health. However, more recent human-based studies have found new evidence.
Eating Soy Is Linked to Reduced Risk of Cancer
Animal studies in the 1990s found that soy increased breast and prostate tumor growth. Current studies show different effects. The difference results from estrogen receptors.
Cells have different types of estrogen receptors: estrogen receptor (ER)α and estrogen receptor (ER)β. Estrogens bind to both receptors with the same strength. Isoflavones tend to bind to ERβ.
In rats, isoflavones tend to bind to ERα, which can cause tumor growth. In humans, they tend to bind to ERβ, which may have protective effects against breast and prostate cancer.
Some tissues, such as bones, have more ERβ. When estrogen levels decline (which happens during menopause), isoflavones can bind to ERβ and make up for some of the missing estrogen. This can help reduce symptoms related to low estrogen levels.
Other tissues, like breast and prostate tissues, have more ERα. Too much estrogen in these tissues can increase the risk of cancer. In menopause, isoflavones can bind to ERα more effectively due to lower estrogen levels. This may lessen estrogen’s effects in these tissues, which could explain the cancer-reducing effects of isoflavones.
Soy Isn’t Likely To Cause Hypothyroidism
Low thyroid levels cause a health condition called hypothyroidism. Isoflavones can block an enzyme needed for thyroid hormone production. This enzyme also needs iodine.
Isoflavones may cause hypothyroidism in people with low iodine intake. However, people who eat enough iodine are unlikely to experience negative effects on thyroid hormones.
Soy Isn’t Likely To Disrupt Male Reproductive Hormones
Several studies reported infertility and erectile dysfunction in males eating too much soy. However, larger studies have shown that eating soy and isoflavones didn’t affect reproductive hormone levels in people assigned male at birth.
Soy foods include soybeans, edamame, tofu, tempeh, soymilk, miso, soy sauce, and soybean oil.
Eating soy foods may offer several health benefits, such as:
- Provides protein, fiber, and minerals: Soybeans are a good source of protein, fiber, and minerals such as iron, calcium, magnesium, potassium, and folate. The high protein in soy foods can support protein intake, especially in people following vegetarian and vegan diets.
- May improve menopause symptoms: Isoflavones may improve menopause symptoms such as hot flashes and night sweats.
- Might lower cholesterol: Eating soy foods may lower blood cholesterol levels. A research review found that eating 25 grams of soy protein for six weeks decreased blood total and low-density lipoprotein (LDL) cholesterol.
- Might support bone health: Estrogen decline negatively affects bone health. Bones start to lose calcium and become more fragile. Isoflavone supplements may increase bone calcium storage.
Soy Food Servings
Soy foods can provide health benefits when eaten in moderation. Studies suggest eating one to two servings (about 25-50 grams) of soy foods isn’t likely to pose health risks.
One serving of the following common soy foods is:
- Edamame: 1/2 cup
- Tofu: 3-4 ounces
- Tempeh: 1/2 cup
- Soymilk: 1 cup
Taking isoflavone supplements can cause different effects than eating whole soy foods. It’s easy to get too much isoflavone from high-dose supplements. Isoflavones in soy foods are less available to the body.
Many factors influence how soy affects the body:
- Foods vs. supplements: Isoflavone or soy protein supplements have more isoflavones than soy foods. The health effects (positive or negative) are likely to be more noticeable at higher doses.
- Fermented vs. unfermented soy foods: Fermentation changes the form of isoflavones. Unfermented soy foods have isoflavones mostly in glycoside form. Fermented soy foods, such as tempeh or natto, contain mainly aglycone isoflavones. This affects isoflavone absorption and availability. Aglycone isoflavones may have stronger effects, but more research is needed.
- Individual response to soy: Some people have gut bacteria that convert isoflavones to equol. These people are called equol producers. Equol binds to ERβ, which is linked to the beneficial effects of soy. Equol producers may benefit from soy more than others. Other individual factors, such as age and sex, can also influence isoflavones’ effects.
- Amount of soy intake: People who consume soy in excess may experience side effects. Eating too many soy foods can cause bloating, diarrhea, constipation, and nausea.
- Hormone levels: Soy’s effects can vary based on estrogen levels. Eating soy may affect people with a uterus differently before and after menopause.
Eating soy foods provides protein, fiber, minerals, and isoflavones. Isoflavones have estrogen-like effects in the body and may improve menopause symptoms as well as bone and heart health. They are also linked to a reduced cancer risk. Moderate soy intake isn’t likely to cause cancer, hypothyroidism, or disrupt male reproductive hormones.
