Soy Supplements Linked to Endometrial Hyperplasia in Some Women

July 09, 2004

Soy (phytoestrogen) supplements induce endometrial hyperplasia in a statistically significant but small group, according to the results of a five-year randomized trial published in the July issue of Fertility & Sterility.

"Some clinical studies on the effects of phytoestrogens on the endometrium in postmenopausal women have been carried out during the past few years, but their results are discordant," write Vittorio Unfer, MD, from the Obstetrics and Gynecology Centre in Rome, Italy, and colleagues.

In this double-blind study, 376 healthy postmenopausal women, all with intact uterus, were randomized to receive soy tablets containing 150 mg of isoflavones per day or placebo tablets for five years. The main outcome measure was endometrial histology from biopsies obtained at baseline, 30 months, and five years after beginning treatment.

Of the 376 women, 298 women completed the five-year study, and none had malignancy detected on biopsy. Endometrium was classified as atrophic or nonassessable in 70% of women receiving soy and in 81% of women receiving placebo. Endometrial hyperplasia occurred in 3.37% of women receiving soy and in none of the women receiving placebo (P < .05).

Although the Italian Ministry of Health recommends that isoflavone supplements not be given at dosages greater than 80 mg/day, the dosage in this study was 150 mg/day.

"Long-term treatment (up to 5 years) with soy phytoestrogens was associated with an increased occurrence of endometrial hyperplasia. These findings call into question the long-term safety of phytoestrogens with regard to the endometrium," the authors write. "We need more information on the effects of these extracts on the endometrium. However, phytoestrogenic supplements should be reconsidered, particularly in women at high risk for endometrial cancer."