Soy Isoflavones Safe After 3 Years

Soy Intake Not Linked to Endometrial Cancer Risk
Soy Intake Not Linked to Endometrial Cancer Risk
Soy isoflavones did not appear to affect endometrial thickness and was not linked to adverse events after 3 years of treatment.

Consuming soy isoflavones had no effect on endometrial thickness, adverse events and circulating hormone levels after 3 years, according to a study presented at the North American Menopause Society (NAMS) 2014 Annual Meeting.

In the Soy Isoflavones for Reducing Bone Loss (SIRBL) study, researchers randomly assigned healthy postmenopausal women aged 45.8 to 65 years to placebo or 80-mg or 120-mg soy isoflavone tablets daily at two sites.

Intent-to-treat (n=224) and compliant (>95%; n=208) analyses were used to evaluate circulating hormone concentrations, adverse events and endometrial thickness as assessed by transvaginal ultrasound, according to the researchers.

Results showed declines in median values for endometrial thickness from baseline through 3 years, although no treatment differences between groups in absolute or percentage change in endometrial thickness were observed at any time point.

The researchers also found no differences in circulating hormones at any time point.

Compared with women taking 120 mg of soy isoflavones, those taking 80 mg experienced a greater number of genitourinary adverse events (P=.005), but no treatment effects were noted for other systems.

The model predicting endometrial thickness response-to-treatment in compliant women was significant across time points (P<.0001). This suggested that estrogen exposure (P=.0013), plasma 17 beta-estradiol (P=.0086) and alcohol intake significantly influenced response, the researchers wrote in an abstract.

No effect on endometrial thickness was noted for the 80-mg (P=.57) or 120-mg (P=.43) dose across time, they reported.

“Our [randomized, controlled trial] verified the long-term overall safety of consuming soy isoflavone tablets by postmenopausal women who displayed excellent compliance,” they wrote.

“We found no evidence of a treatment effect on endometrial thickness, adverse events, or circulating hormone concentrations, most notably thyroid function, during a three-year period.”


  1. Alekel DL et al. Abstract S-8. Presented at: North American Menopause Society (NAMS) 2014 Annual Meeting; Oct. 15-18, 2014; Washington, D.C.