What Are the Dangers of Soy Isoflavones?

Soy, a member of the pea family, is native to the subtropical area of southeastern Asia. Soy contains fiber, protein, and isoflavones. According to the Mayo Clinic, soy isoflavones, sometimes called phytoestrogens, have estrogen-like effects in the body. Although soy has been a part of the diet in many countries for thousands of years, there are some dangers to its consumption.


According to researchers at Oregon State University's Linus Pauling Institute, consuming large amounts of soy isoflavones may stimulate the growth of tumors in estrogen-sensitive breast cancer in women with a history of this type of cancer 1. Researchers recommend that, due to this possibility, these women do not consume large quantities of soy.

Large soy intake is also discouraged for women with a history of other cancers sensitive to hormones, such as ovarian and uterine cancer. There is also some concern about the effects of soy isoflavones on other conditions, such as endometriosis, that are sensitive to hormones.

Thyroid Function

In animal and cell culture studies, soy isoflavones inhibit the activity of thyroid peroxidase, Oregon State University researchers note 1. Thyroid peroxidase is an enzyme that is necessary for the synthesis of thyroid hormone. Consuming adequate dietary iodine decreases this effect.

There is evidence that soy may also affect the levels of thyroid hormone in infants, the Mayo Clinic adds. Although rare, there have been cases of increased thyroid size that result in goiter, or enlarged neck.


Because there is evidence that soy affect the levels of thyroid hormone in infants, iodine has been added to soy formula to eliminate this potential danger. Parents considering feeding their child soy formula should ask their pediatrician for a recommendation of a formula with the optimal amounts of vitamins and minerals.

Research on children during their first year has found that lower bone mineral density results when soy formula is used in place of cow's milk formula, the Mayo Clinic reports.

Infant girls fed soy-based formula report greater use of asthma or allergy drugs as adults than women who were fed cow's milk formula as infants, the Pauling Institute adds.

Drug Interactions

Some studies suggest that a high intake of soy isoflavones can interfere with the anti-tumor effects of cancer drugs, such as tamoxifen, researchers at Oregon State University's Linus Pauling Institute advise 1. Until more is known about the interactions between these types of drugs and soy, they recommend that these patients avoid isoflavone extracts or soy protein supplements.

Breast-cancer survivors taking anticoagulant medications, such as warfarin and aspirin, should also avoid high intakes of soy protein, as it may interfere with the efficacy of these medications.

Thyroid-replacement drugs doses are also affected by the ingestion of soy. The dose necessary for adequate thyroid function increases when consumption of soy products is high.

Other Dangers

Consumption of soy protein has been associated with symptoms of intestinal and stomach difficulties, such as:

  • constipation
  • bloating
  • nausea,
  • changes in stool quality

If you experience intestinal irritation from cow's milk, you may also experience this irritation with soy.