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Are soy isoflavones safe for long-term use?

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Some postmenopausal women take soy isoflavone supplements for possible health benefits including bone strength. However, some scientists caution against their use because they contain are phytoestrogens: estrogen-like substances. Thus, they might increase estrogen levels and stimulate thickening of the endometrium (uterine lining). A new study evaluated soy isoflavones in this regard. The findings were published online on July 7 in the journal Menopause.

The investigators conducted a study to evaluate the overall safety and potential endometrium-stimulating effects of soy isoflavone tablets consumed on a long-term basis by postmenopausal women. They also assessed whether soy isoflavones would cause thickening of the endometrium.
Healthy postmenopausal women (aged 45.8 to 65.0 years) were randomly assigned to receive either a placebo or one of two doses (80 or 120 mg daily/day) of soy isoflavones. The study group comprised 224 women; 208 were evaluated regarding compliance. Circulating hormone concentrations, adverse events, and endometrial thickness (measured via transvaginal ultrasound).

The investigators found that the average values for endometrial thickness decreased from baseline through the following 36 months. Statistical analysis that compared the groups found no differences in absolute, or percentage of change, of endometrial thickness or in circulating hormones at any time point. Compared to the 120 mg daily group, more adverse events involving the genitourinary system among the 80 mg daily group. The therapy had no effect on other oran systems. A statistical model predicting endometrial thickness response to treatment among compliant women across time points was significant; this indicated that estrogen exposure, plasma estrogen, and alcohol intake contributed significantly to the response. Neither the 80 mg dose nor the 120 mg dose exerted an effect on endometrial thickness over time.
The investigators concluded that their study verified the long-term overall safety of soy isoflavone supplement intake by postmenopausal women who display excellent compliance. They found no evidence of treatment effects on endometrial thickness, adverse events, or circulating hormone concentrations, including thyroid function, during the three-year period.