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Phytoestrogens: What Are They, and Are They Safe for Women?

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Phytoestrogens are a group of naturally occurring, plant-derived compounds that have either a weak estrogenic, or anti-estrogenic effect. They are often called “dietary estrogens”, and are used widely by women to treat the symptoms of menopause. Their popularity has increased after several large women’s health trials alluded to health risks in women taking hormone replacement therapy for menopausal symptoms.

Phytoestrogens can be divided into three groups:

Flavonoids, such as genistein, naringenin, and kaempferol

Coumestans, such as coumestrol

Lignans, such as enterodiol, and enterolactone.

The most widely known and studied phytoestrogens are the isoflavones found in red clover and soy: genistein, formononentin, biochanin, and daidzein.

Urologists have known, or rather assumed, that diets high in soy, such as in Asia, have a protective effect on men against development of prostate cancer. However, in women there has been conflicting evidence. Some studies show a protective effect against breast cancer, while some in vitro studies differ on whether they hinder or potentiate tumor cell growth. Some phytoestrogens are weakly estrogenic and bind the estrogen receptor, but are approximately 1,000 times weaker than estradiol. Other phytoestrogens exhibit a blocking effect on the estrogen receptor. The isoflavones in soy show mainly an agonist effect, namely, a weak estrogen effect.

Does this mean that soy compounds can lead to “side effects” such as endometrial hyperplasia or cancer? This of course is not known.

More intrigue: genistein antagonizes the inhibitory effect of tamoxifen on breast cancer cell growth in vivo. This raises more questions than answers.

Recently, a group of researchers from Austria set out to clarify the safely profile of phytoestrogens by analyzing all known studies published in English. One hundred seventy four randomized controlled trials comparing phytoestrogens to placebos were identified, however in 82, no side effects were discussed. Mean treatment duration was short at approximately six months.

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