Soy

Background

• This fact sheet focuses on the use of soy by adults for health purposes.
• Soybeans have long been cultivated in Asia. Since the 1950s, they have also been produced in other parts of the world, including the Americas.
• In addition to its food uses, soy is available in dietary supplements. Soy supplements may contain soy protein, isoflavones (compounds similar in structure to the female hormone estrogen), or other components.
• Soy products are promoted for menopausal symptoms, bone health, improving memory, high blood pressure, and high cholesterol levels.

How Much Do We Know?

• Although there have been many studies on soy products, there are still uncertainties about soy’s health effects.

What Have We Learned?

• Consuming soy protein in place of other proteins may lower levels of total cholesterol and LDL (“bad”) cholesterol to a small extent.
• Soy isoflavone supplements or soy protein may help to reduce the frequency and severity of menopausal hot flashes, but the effect may be small.
• Observational studies indicate that among Asian women, higher dietary intakes of soy during childhood and adolescence are associated with a lower risk of breast cancer later in life. The amounts of soy in Western diets may be too low for this association to be observed. Soy products in supplement form have not been shown to reduce breast cancer risk.
• Current evidence suggests that soy isoflavone mixtures probably have a beneficial effect on bone health in postmenopausal women, but the evidence is not entirely consistent.
• Soy protein may slightly reduce blood pressure in people with hypertension (high blood pressure).

What Do We Know About Safety?

• Except for people with soy allergies, soy is considered to be a safe food. In research studies, soy protein supplements and soy extracts rich in isoflavones have been used safely on a short-term basis; the safety of long-term use is uncertain.
• The most common side effects of soy are digestive upsets, such as constipation and diarrhea.
• Soy may alter thyroid function in people who are deficient in iodine.
• Current evidence indicates that it’s safe for women who have had breast cancer or who are at risk for breast cancer to eat soy foods. However, it’s uncertain whether soy isoflavone supplements are safe for these women.
• The use of soy in amounts greater than those commonly found in foods may be unsafe during pregnancy because estrogen-like substances from soy could be harmful to the fetus. Little is known about whether it is safe to use soy in amounts greater than those commonly found in foods while breastfeeding.

Keep in Mind

• Take charge of your health—talk with your health care providers about any complementary health approaches you use. Together, you can make shared, well-informed decisions.

For More Information

• Using Dietary Supplements Wisely
• Know the Science: How Medications and Supplements Can Interact
• Know the Science: How To Make Sense of a Scientific Journal Article

Key References