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Effect of isoflavone soy protein supplementation on endometrial thickness, hyperplasia, and endometrial cancer risk in postmenopausal women: a randomized controlled trial

Quaas, Alexander M. MD, PhD; Kono, Naoko MPH; Mack, Wendy J. PhD; Hodis, Howard N. MD; Felix, Juan C. MD; Paulson, Richard J. MD; Shoupe, Donna MD

doi: 10.1097/GME.0b013e3182804353
Original Articles

Objective This study aims to determine whether long-term isoflavone soy protein (ISP) supplementation affects endometrial thickness and rates of endometrial hyperplasia and cancer in postmenopausal women.

Methods In this randomized, double-blind, placebo-controlled trial, 350 postmenopausal women aged 45 to 92 years were randomized to a total daily dose of 154 mg of ISP or a milk protein–matched placebo for a 3-year period. Women with a surgically absent uterus were excluded from the analysis (final study population, N = 224). The main outcome measures were as follows: mean change in endometrial thickness on transvaginal ultrasound from baseline until up to 36 months of follow-up and the incidence of endometrial sampling, endometrial hyperplasia, and endometrial cancer.

Results A total of 666 visits among 224 participants were evaluated. Treatment groups did not significantly differ on the mean baseline or on-trial changes in endometrial thickness. Of the 103 placebo-treated participants, 7 (6.8%) underwent endometrial biopsy; 6 (85.7%) of these biopsies were benign. One woman in the placebo group was diagnosed with complex endometrial hyperplasia with atypia and underwent hysterectomy. The pathology result from this surgical operation was stage IB endometrial cancer. Of the 121 participants in the soy group, 9 (7.4%) underwent endometrial biopsy. The results were benign in all nine cases (100%). Although the rate of hyperplasia/malignancy was higher in the placebo group (14.3% vs 0%), the difference was not statistically significant.

Conclusions Three-year ISP supplementation has no effect on endometrial thickness or on the rates of endometrial hyperplasia and cancer in postmenopausal women.

In this randomized controlled trial, three year ISP supplementation was not associated with significant increases in endometrial thickness or in the rates of endometrial hyperplasia/cancer.

From the Departments of 1Ob/Gyn and 2Preventive Medicine, 3Atherosclerosis Research Unit, and Departments of 4Medicine and 5Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA.

Received October 14, 2012; revised and accepted November 28, 2012.

This paper was presented orally at the 60th annual Pacific Coast Reproductive Society meeting, Rancho Mirage, CA.

Clinical trial registration number: NCT00118846 (

Funding/support: This study was supported by National Institutes of Health grant U01AT-001653 from the National Center for Complementary and AlternativeMedicine, the Office of Dietary Supplements, and the Office of Research on Women’s Health. Solae LLC (St Louis, MO) provided the study products gratis.

Financial disclosure/conflicts of interest: None reported.

Address correspondence to: Alexander M. Quaas, MD, PhD, IRD 534, 2020 Zonal Avenue, Los Angeles, CA 90033.E-mail:

© 2013 by The North American Menopause Society.