Isabel Casimiro, MD, PhD, is an endocrinologist at the University of Chicago in Illinois. As a physician-scientist in molecular biology, she uses her research on diabetes, lipid disorders, cardiovascular function, and more to provide comprehensive care to her patients. Her research findings have been published in several scientific and medical journals, including Cell Metabolism and the Journal of the Endocrine Society. Dr. Casimiro also has extensive experience providing gender-affirming hormone therapy and improving education regarding transgender medicine for endocrinology fellows. Her work with transgender patients has been published in the Journal of the Endocrine Society and Transgender Health. Dr. Casimiro also serves on graduate and medical school program committees and is a clinical instructor at the University of Chicago. Dr. Casimiro received her PhD in biomedical research from the Albert Einstein College of Medicine and her medical degree from the University of Washington. She completed her internal medicine residency and endocrinology fellowship through the Physician Scientist Development Program at the University of Chicago. She is board-certified in internal medicine.
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.
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.
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.
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:
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:
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:
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.