In the endometrium, excess estrogen relative to progesterone produces a proliferative stimulus, which may result in endometrial thickening. As measured by transvaginal ultrasound (TVU), endometrial thickness can be a biomarker for the proliferative effects of estrogens, and opposing different influences of progesterone. The increase of endometrial thickness may be associated with increased risk of endometrial carcinomas. Not surprisingly, there is interest in understanding the impact of isoflavones on endometrial tissue. To address this issue, Chinese researchers conducted a meta-analysis of studies evaluating isoflavone intake and endometrial thickness in peri- and post-menopausal women. A total of 23 trials involving 2,167 women were included in their analysis. The results showed that isoflavones reduced significantly decreased endometrial thickness in North American studies (7 trials, 726 women) but there was also a non-significant increase in endometrial cancer in Asian studies (3 trials, 224 women). However, none of the three Asian studies actually intervened with isoflavones derived from soybeans. It is noteworthy that recently the European Food Safety Authority concluded that soybean isoflavones don’t adversely affect endometrial tissue in postmenopausal women.
Jie Liu, Feixiang Yuan, Jian Gao, Boer Shan, Yulan Ren, Huaying Wang and Ying Gao. Oral isoflavone supplementation on endometrial thickness: a meta-analysis of randomized placebo-controlled trials. Oncotarget 7: 17369, 2016.