Introduction

This fact sheet provides basic information about soy-uses, potential side effects, and resources for more information. Soy, a plant in the pea family, has been common in Asian diets for thousands of years. It is found in modern American diets as a food or food additive. Soybeans, the high-protein seeds of the soy plant, contain isoflavones-compounds similar to the female hormone estrogen. This fact sheet highlights what is known about soy when used by adults for health purposes.

Common Names—soy

Latin Names—Glycine max

What It Is Used For

People use soy products to prevent or treat a variety of health conditions, including high cholesterol levels, menopausal symptoms such as hot flashes, osteoporosis (a condition in which bones become thin and brittle), memory problems, high blood pressure, breast cancer, and prostate cancer.

How It Is Used

• Soy is available in dietary supplements, in forms such as tablets and capsules. Soy supplements may contain isoflavones or soy protein or both.
• Soybeans can be cooked and eaten or used to make tofu, soy milk, and other foods. Also, soy is sometimes used as an additive in various processed foods, including baked goods, cheese, and pasta.

What the Science Says

• Research suggests that daily intake of soy protein may slightly lower levels of LDL ("bad") cholesterol.
• Some studies suggest that soy isoflavone supplements may reduce hot flashes in women after menopause. However, the results have been inconsistent.
• There is not enough scientific evidence to determine whether soy supplements are effective for any other health uses.
• NCCAM is supporting ongoing studies of soy, including its effects on women's arteries and bones after menopause.

Side Effects and Cautions

• Soy is considered safe for most people when used as a food or when taken for short periods as a dietary supplement.
• Minor stomach and bowel problems such as nausea, bloating, and constipation are possible.
• Allergic reactions such as breathing problems and rash can occur in rare cases.
• The safety of long-term use of soy isoflavones has not been established. Evidence is mixed on whether using isoflavone supplements, over time, can increase the risk of endometrial hyperplasia (a thickening of the lining of the uterus that can lead to cancer). Studies show no effect of dietary soy on risk for endometrial hyperplasia.
• Soy's possible role in breast cancer risk is uncertain. Until more is known about soy's effect on estrogen levels, women who have or who are at increased risk of developing breast cancer or other hormone-sensitive conditions (such as ovarian or uterine cancer) should be particularly careful about using soy and should discuss it with their health care providers.
• Tell your health care providers about any complementary and alternative practices you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.

Sources

Balk E, Chung M, Chew P, et al. Effects of Soy on Health Outcomes. Evidence Report/Technology Assessment no. 126. Rockville, MD: Agency for Healthcare Research and Quality; 2005. AHRQ publication no. 05-E024-1.

Low Dog T. Menopause: a review of botanical dietary supplements. American Journal of Medicine. 2005;118(suppl 12B):98S-108S.

Sacks FM, Lichtenstein A, Van Horn L, et al. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation. 2006;113(7):1034-1044.

Soy. Natural Medicines Comprehensive Database Web site. Accessed on January 2, 2007.

Soy (Glycine max [L.] Merr.). Natural Standard Database Web site. Accessed on January 2, 2007.

NCCAM has provided this material for your information. It is not intended to substitute for the medical expertise and advice of your primary health care provider. We encourage you to discuss any decisions about treatment or care with your health care provider. The mention of any product, service, or therapy is not an endorsement by NCCAM.