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Confused About Phytoestrogens?

Phytoestrogens are widely distributed in plants and are structurally similar to estrogens and can thus bind weakly to estrogen receptors. Estrogen is increasingly recognized as a cause of breast cancer. Elevated levels of naturally produced estrogen and hormonal therapy with estrogen for menopause are associated with an increased risk of breast cancer. There are three major classes of phytoestrogens:

  • Isoflavones, which are concentrated in soybeans and soy products but are also found in other legumes; Most isoflavones are from soy and unfermented soy has its own health problems.
  • Lignans, which are distributed in seeds, whole grains, berries, fruit, vegetables, and nuts.
  • Coumestans, which are found in broccoli and sprouts.

Although their medical importance has been less extensively studied, lignans occur in higher concentrations in U.S. and European diets than do isoflavones. The daily dietary intake of phytoestrogens in white U.S. women has been estimated to be <1 mg, with 80 percent from lignans, 20 percent from isoflavones, and <0.1 percent from coumestans.

However, phytoestrogens have an unclear role as the studies provide much confliciting evidence. In animal models and in vitro studies, phytoestrogens bind weakly to estrogen receptors and can either produce or inhibit estrogen effects. The activity of a phytoestrogen depends on its structure and metabolism, its concentration relative to that of naturally produced estrogen. The assumption that plant estrogens are protective came from comparisons between international studies. Historically, breast cancer rates in the United States have been four to seven times those in Asia, whereas isoflavone intake in the United States is <1 percent that in Asian populations, which reportedly ranges from 20 to 80 mg/d. However, high soy consumption is only one of many potentially protective lifestyle factors that distinguish Asian and Western women.

Now a study looking at 15,000 Dutch women in one of my favorite journals, the AJCN, shows that the isoflavones may have virtually no effect on breast cancer while the lignans reduced breast cancer by 30 percent. Freshly ground organic flaxseeds can provide a very high amount of protective lignans. So in conclusion, women searching for an alternative to synthetic estrogen therapy are wondering whether phytoestrogens might increase breast cancer risk and not substantially improve cardiovascular or bone health. If you review this comprehensive study you will find that the relevant research is very complicated, inconsistent and inconclusive. At present, it appears that scientific research does not support increasing phytoestrogen intake among U.S. women to Asian levels, nor does it suggest that the typical U.S. phytoestrogen intake is problematic for healthy women.

American Journal of Clinical Nutrition February 2004;79(2) 282-288

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