Soy consumption and the risk of laparoscopically confirmed endometriosis in a prospective cohort study.

Abstract

OBJECTIVE: To investigate the association of soy and isoflavone intake with the risk of laparoscopically confirmed endometriosis. DESIGN: The Nurses' Health Study II, a prospective cohort study from 1991-2021. SUBJECTS: A total of 82,084 premenopausal participants aged 27-44 years in 1991. EXPOSURE: Soy and isoflavone intake was evaluated from 1991 and every 4 years using a Food Frequency Questionnaire. MAIN OUTCOME MEASURES: Self-reported laparoscopically confirmed endometriosis in biennial follow-up questionnaires. Cox proportional hazard models with age in months were used to calculate hazard ratios and 95% confidence intervals for laparoscopically confirmed endometriosis. Restricted cubic splines were used to examine the possibility of nonlinear relations between isoflavone intake and the risk of endometriosis. RESULTS: A total of 3,829 incident cases of laparoscopically confirmed endometriosis were reported over 1,038,888 person-years of follow-up (incidence rate = 369 per 100,000 person-years). Increasing soy intake by one serving per week was associated with an 8% lower risk of laparoscopically confirmed endometriosis (Hazard ratio, 0.92, 95% confidence interval, [0.87-0.98]). This association was present among participants without a concurrent report of infertility (hazard ratio, 0.92, 95% confidence interval, 0.86-0.99), although not among participants with a concurrent infertility diagnosis (hazard ratio, 0.97, 95% confidence interval, 0.83-1.13, and test for heterogeneity 1.00). There was evidence of a nonlinear inverse association of isoflavones intake with the risk of laparoscopically confirmed endometriosis, in which the inverse association between isoflavones and endometriosis was approximately linear up until an intake of 4 mg/d (approximately 95th percentile of intake), which plateaued thereafter. CONCLUSION: In a population with a modest intake of soy products, consistent with levels seen in other Western populations, soy intake is associated with a lower risk of laparoscopically confirmed endometriosis.