Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood

Context A large body of evidence documents the role of phytoestrogens in in fluencing hormone-dependent states. Infants fed soy formula receive high le vels of phytoestrogens, in the form of soy isoflavones, during a stage of d evelopment at which permanent effects are theoretically possible. However, a paucity of data exists on the long-term effects of infant soy formulas. Objective To examine the association between infant exposure to soy formula and health in young adulthood, with an emphasis on reproductive health. Design, Setting, and Participants Retrospective cohort study conducted from March to August 1999 among adults aged 20 to 34 years who, as infants, par ticipated during 1965-1978 in controlled feeding studies conducted at the U niversity of Iowa, Iowa City (248 were fed soy formula and 563 were fed cow milk formula during infancy). Main Outcome Measures Self-reported pubertal maturation, menstrual and repr oductive history, height and usual weight, and current health, compared bas ed on type of formula exposure during infancy. Results No statistically significant differences were observed between grou ps in either women or men for more than 30 outcomes. However, women who had been fed soy formula reported slightly longer duration of menstrual bleedi ng (adjusted mean difference, 0.37 days; 95% confidence interval [CI], 0.06 -0.68), with no difference in severity of menstrual flow. They also reporte d greater discomfort with menstruation (unadjusted relative risk for extrem e discomfort vs no or mild pain, 1.77; 95% CI, 1.04-3.00). Conclusions Exposure to soy formula does not appear to lead to different ge neral health or reproductive outcomes than exposure to cow milk formula. Al though the few positive findings should be explored in future studies, our findings are reassuring about the safety of infant soy formula.