Isoflavones are predominantly found in soy and soy products, foods that have been consumed safely for thousands of years in countries around the world. These compounds act like the hormone estrogen in the body; this raises questions as to whether they might cause unintended effects in the body by binding to estrogen receptors.
Upper Limit
An upper limit to the consumption of soy isoflavones has not been established, but the figure is likely to be high. In Western cultures, isoflavone intake is often low, sometimes below 2 mg a day, according to the Linus Pauling Institute. In Asian cultures, typical amounts consumed range between 25 mg to 50 mg per day with intake as high as 65 mg a day for some populations. Clinical trials, such as a 2009 study in "Age and Aging," have used doses as high as 100 mg per day for up to six months at a time.
Breast Cancer Survivors
Because of the estrogen-mimicking effects of isoflavones, their safety in breast cancer survivors may be of concern. In particular, research such as a 2001 study in the "Journal of Nutrition" which found that isoflavones caused breast cancer cells to grow and reproduce in the lab cast doubt on the wisdom of recommending soy consumption to breast cancer patients. More recent research has found evidence of a benefit for breast cancer survivors eating isoflavone-rich soy, including a 2009 study of more than 5,000 women published in the "Journal of the American Medical Association" that linked soy isoflavone consumption with a lower risk of cancer recurrence and a lower risk of death.
Infancy
Infant soy-based formulas are another area where safety is of concern, but no problems with growth or development have been noted in studies done since these formulas came onto the market in the 1960s. According to the American Academy of Pediatrics, soy-based infant formula that is high in isoflavones is no less safe than equivalent cow's milk-based formulas.
Drug Interactions
The estrogenic activity of isoflavones may interact with some drugs. Isoflavones may interfere with selective estrogen receptor modulators, such as tamoxifen, which are drugs used in the treatment of breast cancer. The anticoagulant warfarin is another drug that can interact with isoflavones. Isoflavones may also interfere with the hypothyroidism drug levothyroxine.
Considerations
In general, isoflavones at doses found in foods is unlikely to cause any safety issues. Individuals who are taking medication that may interact with isoflavones should consult a doctor before including these compounds in their diet. People who are allergic to soy should avoid taking isoflavone supplements derived from soy.
References
- American Academy of Pediatrics Committee on Nutrition: Soy protein-based formulas: recommendations for use in infant feeding
- The Linus Pauling Institute at Oregon State University: Isoflavones
- University of Maryland Medical Center: Soy
- "Age and Ageing": A preliminary study of the safety, feasibility and cognitive efficacy of soy isoflavone supplements in older men and women.
- "Journal of the American Medical Association": Soy food intake and breast cancer survival.
- "Journal of Nutrition": Physiological concentrations of dietary genistein dose-dependently stimulate growth of estrogen-dependent human breast cancer (MCF-7) tumors implanted in athymic nude mice.
