Soy Isoflavones Side Effects

Isoflavones are compounds derived from plants that have estrogen-like properties, according to doctors at the Linus Pauling Institute Micronutrient Information Center. The richest source of isoflavones comes from soybeans and soy products. Although adding soy to a nutritional plan has a number of benefits, soy isoflavones carry some risks of side effects.

Soybeans generally are safe, but soy isoflavones can have serious side effects.

Breast Cancer

Although studies sometimes are conflicting, researchers at the Linus Pauling Institute say soy isoflavones can increase the production of estrogen receptor breast cancer cells. High intakes of the compound can interfere with the effectiveness of some cancer treatments, such as tamoxifin. As research continues, women with breast cancer are advised to stick with moderate dietary consumption of soy and limit excessive supplementation.

Thyroid Disorders

Researchers at the Weston A. Price Foundation say soy isoflavones have a direct correlation with increased thyroid disease. Soy isoflavones can cause goiter and hypothyroidism by interfering with thyroid hormone synthesis. Thyroid dysfunction typically can be avoided as long as sufficient levels of iodine are added to the product, which is a protocol followed by most commercial producers of soy products. Even with sufficient levels of iodine, however, high doses of soy supplements can lead to thyroid cancer as well.

Digestive Disorders

Soy isoflavones can trigger food allergies in some people, with side effects including itching, a runny nose, stomach irritations and digestive disorders. Doctors at the Mayo Clinic say soy supplements have been associated with bloating, cramping and other stomach problems. Intestinal side effects vary and can cause infants to have serious diarrhea when fed soy-based formula. Constipation and changes in stool quality also can be side effects of soy isoflavones.


Products made with soy isoflavones can cause migraine headaches in some people, the Mayo Clinic says. People with allergic reactions can have low-grade headaches as a result of increased heart rate or heart palpitations.

Unknown Factors

Doctors at Penn State University say too little research is available on the ultimate consequences of taking high levels of soy isoflavone supplements. Although soy isoflavones that occur naturally in soybeans, tofu and tempeh generally are considered safe for healthy adults, supplements can pose additional unknown dangers. Soy powders and pills often contain levels of isoflavones that do not fall within safe parameters. To prevent thyroid or cancer-causing side effects, consumers should limit soy consumption to 100 milligrams or less per day.

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