Soy protein isolate with isoflavones does not prevent estradiol-induced endometrial hyperplasia in postmenopausal...

To test the hypothesis that soy protein isolate (SPI) with isoflavones opposes the proliferative effects of exogenous estradiol (E2) on the endometrium after menopause.

Thirty-nine postmenopausal women were randomized to receive daily for 6 months either 0.5 mg E2 + placebo, 1.0 mg E2 + placebo, 0.5 mg E2 + 25 g SPI with 120 mg isoflavones, or 1.0 mg E2 + 25 g SPI with 120 mg isoflavones. Primary outcome measures were endometrial histology, ultrasound endometrial thickness, and Ki67 staining quantification, a marker of cellular proliferation. Secondary outcome measures were serum lipids and markers of bone resorption.

Endometrial hyperplasia, endometrial stromal and epithelial cellular proliferation, and sonographically measured endometrial thickness were similarly affected in all groups. SPI did not lessen the beneficial effects of E2 on lipids and markers of bone resorption.

In this pilot study, SPI with isoflavones did not protect the endometrium from E2-induced hyperplasia in postmenopausal women. If higher, long-term doses of isoflavone supplementation are found to be safe for postmenopausal women, then future studies combining E2 with isoflavones may be feasible as an alternative to traditional hormone replacement therapy.