Background: Phytoestrogen exposure from soy formula feeding during infancy may disrupt reproductive system development, resulting in altered menstrual bleeding in adulthood. Methods: We investigated this relationship in a cohort of 1,696 young African American women using enrollment data from the Study of Environment, Lifestyle, & Fibroids (2010-2012). Questionnaire data on soy formula feeding were available for 1,553 participants, 89% based on mother's report. Menstrual bleeding outcomes including those indicative of heavy menstrual bleeding were ascertained by interview. We estimated relative risks (RRs) and 95% confidence intervals (CI) for associations between soy formula feeding and menstrual bleeding outcomes using log-binomial regression and log-multinomial regression, comparing participants ever fed and never fed soy formula. Results: We observed associations between soy formula feeding and variables indicating a history of heavy menstrual bleeding, including ever experiencing heavy, gushing-type bleeding (RR: 1.2, 95% CI: 1.0, 1.4), ever use of nonsteroidal anti-inflammatory drugs for heavy bleeding (RR: 1.3, 95% CI: 1.0, 1.6), and ever use of a contraceptive method for heavy bleeding (RR: 1.2, 95% CI, 0.9, 1.6). Among the subset of participants with menses in the past year who did not use medication that may alter menstrual flow (n = 762), our data suggested that soy formula feeding was associated with heavier flow and the adverse impact of menstrual bleeding on quality of life, but CIs were wide. Conclusions: Our data suggested that soy formula feeding is associated with heavy menstrual bleeding. Our results support the idea that infancy is a susceptible developmental window for female reproductive function.
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