Soy formula could lead to changes in reproductive tissues. - GreenMedInfo Summary

Purpose: Chemicals with hormone-like activity, such as estrogenic isoflavones, may perturb human development. Infants exclusively fed soy-based formula are highly exposed to isoflavones, but their physiologic responses remain uncharacterized. Here, we compare estrogen-responsive postnatal development in infants exclusively fed soy formula, cow-milk formula and breast milk.

Methods: We enrolled 410 infants born in Philadelphia area hospitals, 2010-2013; 283 infants were exclusively fed soy formula (n=102), cow-milk formula (n=111), or breast milk (n=70) throughout the study (birth to 28 [boys] or 36 [girls] weeks). We repeatedly measured maturation index (MI) in vaginal and urethral epithelial cells using standard cytological methods, uterine volume and breast-bud diameter using ultrasound, serum estradiol and, in girls, follicle-stimulating hormone. We estimated MI, organ-growth and hormone trajectories by diet using mixed-effects regression splines.

Results: Maternal demographics did not differ between cow-milk-fed and soy-fed infants but did differ between formula-fed and breastfed infants. Vaginal cell MI trended higher (p=0.01) and uterine volume decreased more slowly (p=0.01) in soy-fed girls compared to cow-milk-fed girls; however, their trajectories of breast-bud diameter and hormone concentrations did not differ. We observed no significant differences between boys fed cow-milk formula versus soy formula; estradiol was not detectable. Breastfed infants differed from soy-formula-fed infants in vaginal cell MI, uterine volume, girls' estradiol and boys' breast-bud diameter trajectories.

Conclusions: Relative to cow-milk formula-fed girls, soy formula-fed girls demonstrated tissue and organ-level developmental trajectories consistent with response to exogenous estrogen exposure. Additional studies are needed to further evaluate the effects of soy on child development.