New US researchers has found that infants who are fed soy-based formulas as newborns show differences in some reproductive-system cells and tissues, possibly due to exposure to estrogen-like compounds found in this type of milk.
Funded and led by the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health, the research looked at the effect of different milks on postnatal development of estrogen-responsive tissues and hormone levels in male and female infants.
The study was completed by 283 infant-mother pairs completed the study, with 102 of the infants exclusively fed on soy formula; 111 infants were exclusively fed on on cow-milk formula, and 70 on breast milk.
The researchers repeatedly measured the boys up to 28 weeks and the girls up to 36, looking at the maturational index (MI) based on epithelial cells from the children's urogenital tissue; ultrasound measurements of uterine, ovarian and testicular volume as well as breast-buds; and hormone concentrations using blood tests.
The results showed that the main differences found related to the girls, with the team discovering that compared to those who were fed cow-milk formula, girls fed soy formula showed developmental changes consistent with those found in response to estrogen exposure.
The team also found that vaginal cell MI was higher and uterine volume decreased more slowly in girls who fed soy formula.
However, no significant differences were found between boys fed cow-milk formula and those fed soy formula.
"Soy formula contains high concentrations of plant-based estrogen-like compounds, and because this formula is the sole food source for many babies in the first six months of life, it's important to understand the effects of exposure to such compounds during a critical period in development," explained Virginia A. Stallings, MD, senior author of the study.
Soy formula has long been used among women who don't breastfeed as an alternative to cow-milk formula, often by mothers concerned about milk allergies or lactose intolerance.
However, soy protein contains high amounts of genistein, an estrogen-like compound which can alter the body's endocrine system and potentially interfere with normal hormonal development, although little is known about its effects on infants.
The researchers did point out that the differences were subtle and not a cause for alarm, but they do suggest the need for further research to investigate possible long-term effects of exposure to soy-based formula.
"Modern soy formula has been used safely for decades. However, our observational study found subtle effects in estrogen-responsive tissues in soy-fed infants, and we don't know if these differences are associated with long-term health effects," said first author Margaret A. Adgent.
Stallings added that in the meantime, "For new and expectant mothers deciding on how to feed their infants, as always, we strongly support breastfeeding, as recommended by the American Academy of Pediatrics."
The AAP does not recommend soy formula for preterm infants, but it is suggested for infants with hereditary disorders that make them unable to properly digest milk, such as galactosemia, and "in situations in which a vegetarian diet is preferred."
The findings can be found published online in the Journal of Clinical Endocrinology and Metabolism.