In a recent study, researchers find that soy-based formula for newborns can cause differences in reproductive-system cells and tissues, compared to babies who used cow-milk formula or were breastfed.
The team say the differences, measured in the months after birth, were subtle and not a cause for alarm.
But they also say that further research is needed to see the long-term effects of using soy-based formulas.
Some mothers who don’t breastfeed may use soy formula as an alternative to cow-milk formula.
They use soy formula because they worry about milk allergies, lactose intolerance, or other feeding difficulties.
However, soy protein contains high amounts of genistein, an estrogen-like compound.
Genistein can alter the body’s endocrine system and potentially interfere with normal hormonal development.
Previous studies have shown that genistein causes abnormal reproductive development and function in rodents, but little is known about its effects on human infants.
In the study, the team compared infants fed with soy formula to those fed with cow-milk formula and breastfed infants.
They enrolled 410 infant-mother pairs, and 283 pairs completed the study. Of those, 102 infants exclusively fed on soy formula, 111 on cow-milk formula, and 70 on breast milk.
They were born in eight Philadelphia-area hospitals between 2010 and 2013.
The researchers assessed three things: a 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 seen in blood tests.
They found that vaginal cell MI was higher and uterine volume decreased more slowly in soy-fed girls, both of which suggest estrogen-like responses.
In addition, there were similar patterns in differences between soy-fed girls and breastfed girls.
The researchers explained that 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.
Their study found subtle effects in estrogen-responsive tissues in soy-fed infants, and it is unknown if these differences are linked to long-term health effects.
They say ideally the children in this study should be followed later into childhood and adolescence.
For mothers who prefer giving formula, the American Academy of Pediatrics does not recommend soy formula for preterm infants.
AAP states that soy formula is indicated for infants with hereditary disorders that make them unable to properly digest milk, such as galactosemia and the rare condition hereditary lactase deficiency.
It also recommends soy formula in situations in which a vegetarian diet is preferred.
The study is in the Journal of Clinical Endocrinology and Metabolism.
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