Study question: Is soy formula feeding during infancy associated with menstrual pain in reproductive-age women?
Summary answer: Our data suggest that soy formula feeding during infancy is associated with several indicators of severe menstrual pain in reproductive-age women.
What is known already: A prior study observed greater severity of menstrual pain in young women who as infants participated in feeding studies and were assigned to soy-based formula feeding.
Study design, size, duration: We used data from the Study of Environment, Lifestyle & Fibroids (SELF), a cohort of 1696 African-American women ages 23-35 years at enrollment.
Participants/materials, settings, methods: Data on infant soy formula feeding was ascertained by self-administered questionnaire for 1553 participants, with 89% of participants receiving assistance from their mothers. Information on menstrual pain indicators was collected by web- and telephone-interview. We estimated the relative risk (RR) and 95% confidence interval (CI) using log-binomial regression, or log-multinomial regression, adjusting for participant age and maternal education.
Main results and the role of chance: Women ever fed soy formula as infants were more likely than unexposed women to report ever use of hormonal contraception for menstrual pain (RR 1.4, CI: 1.1-1.9) and moderate/severe menstrual discomfort/pain with 'most periods', but not 'every period', during early adulthood (ages 18-22 when not using hormonal contraception) (RR 1.5, CI: 1.1-2.0).
Limitations, reasons for caution: We relied on retrospective recall to ascertain infant exposure to soy formula feeding and data on menstrual pain indicators.
Wider implications of the findings: Our observations add to the growing body of literature from animal and human studies on the reproductive health consequences of early-life exposure to soy formula.
Study funding/competing interest(s): This research was supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences and, in part, by funds allocated for health research by the American Recovery and Reinvestment Act. This research was also supported by grant K99NR017191 (KU). None of the authors has a conflict of interest.
Trial registration number: Not applicable.