By far, soy is the most controversial plant food. You may have heard the following: “If men eat soy they will get man-boobs” or “Soy causes breast cancer.” Do a quick web search and you will find hundreds of pages on the “dangers of soy.” There are over 10,000 peer-reviewed journal articles on soy making it one of the most researched foods. With this much research, one could make a case against soy by highlighting a handful of outlier studies. Dietary recommendations, however, are made based on a comprehensive review of the research. Let’s see what the research has to say on the following questions:
Is eating soy the same as taking estrogen?
Soybeans contain isoflavones, which are classified as phytoestrogens, or plant estrogens. However, unlike the hormone estrogen, isoflavones selectively bind to one type of estrogen receptor. Depending on what type of receptor dominates in a given tissue, isoflavones can have estrogen-like effects in some tissues, anti-estrogen effects in other tissues, or no effects at all. Conclusions about the health effects of soy cannot be made based on how estrogen behaves in the body.
In two case studies, men consuming extremely high amounts of soy (14-20 servings per day) developed tender, enlarged breast tissue. In contrast, evidence from randomized controlled trials indicates that soyfoods (up to six servings per day) do not affect testosterone or estrogen levels and isoflavone consumption does not affect sperm concentration, count or motility.
Does eating soy increase risk for breast cancer?
Because hormone therapy increases risk of postmenopausal breast cancer, it was thought that women with estrogen sensitive tumors and those at increased risk of breast cancer should not consume soy. However, research indicates that soyfoods do not have harmful effects on breast tissue. In fact, two large studies found that soyfood intake is associated with the improved prognosis of breast cancer patients, including reductions in risk of death and disease recurrence. The position of the American Cancer Society is that breast cancer patients can safely consume up to three servings of soyfoods daily.
Does soy negatively affect thyroid function?
The evidence clearly shows that soyfoods do not adversely affect thyroid function in healthy people. For those with hypothyroidism on medication, changes in soy intake may require small changes in thyroid medication since soy protein affects medication absorption.
Discussions about the safety of soy continue to appear in the scientific and popular media. Research to date indicates that soyfoods can be safely incorporated into the diets of essentially all individuals.
- Soyfoods have been consumed in Asia for at least 1,500 years.
- Soy protein is considered equivalent to animal protein – it is highly digestible and its amino acid pattern closely matches human requirements.
- Moderate amounts of traditional soy foods can reduce the risk of prostate cancer and can lower LDL cholesterol.
- Young girls who consume soy have a lower lifetime risk of breast cancer.
- The calcium in fortified soymilk is absorbed as easily as calcium from cow’s milk.
- Only about one out of 2,500 American adults is allergic to soy protein.
For more information:
“Isoflavones” and “Safety of Soyfoods” RD Resources for Consumers. Vegetarian Nutrition Dietetic Practice Group http://vegetariannutrition.net