Despite theoretical concerns about brain development in babies fed soy protein-based formula, a new report says they do just as well as babies given milk-based formula.
Breastfed infants still have an advantage over both groups of formula-fed babies in terms of cognitive development.
"While we feel there are significant benefits to breastfeeding, parents who cannot or do not breastfeed should not feel guilty or worried that their children will have adverse growth and development outcomes," said report author Thomas Badger, director of the Arkansas Children's Nutrition Center. (Badger is also a member of the science advisory board for the Soy Nutrition Institute.)
In the study, published Monday in the journal Pediatrics, researchers tracked nearly 400 babies at three-month intervals over their first year of life. They tested the babies' progress using infant development and expressive communication scales, adjusting for such factors as the mother's age, IQ and socioeconomic status.
Babies in all three groups fell within the normal range for brain development. However, the breastfed babies outscored both groups of formula-fed babies in their mental development and their ability to combine cognitive function and physical movement.
"Breast-fed infants test better in some -- not all -- standardized behavioral tests than formula-fed infants, although the differences are very small," Badger summarized in an email.
But the reasons why are not fully understood.
"We do not know if this is due to factors in the breast milk that are missing in formula, or if this is because the [breastfeeding] moms have higher IQ and the kids live in higher socioeconomic-status homes," Badger said.
Moreover, "when you're breastfeeding a baby, there's a physiological response, which involves the release of oxytocin," said Dr. Lane Strathearn, an assistant professor of pediatrics at Baylor College of Medicine, explaining how mother-baby bonding might influence early development. "Oxytocin does have a pro-social effect."
Despite the differences the study revealed, the authors say it should come as welcome news to parents who feed their babies soy-based formulas -- a growing population.
The American Academy of Pediatrics estimates that soy protein-based formula now makes up nearly one-quarter of the formula market in the United States. The academy has issued guidelines reiterating that breast milk is best, but stating that soy-based formulas may be appropriate for babies who come from strict vegan families or who can't tolerate lactose.
Soy-based formulas are "supposed to be more easily digested, although the evidence for all of this is not really strong," said Strathearn. "But infants who are having difficulties with digestion of milk-based formulas seem to respond better to the soy-based formulas."
Some studies have questioned the safety of soy isoflavones, suggesting they might negatively affect sexual and neurobehavioral development -- concerns that thus far appear to be unfounded. However, most studies showing the safety of soy-based formulas have focused on growth; the authors of the new study say theirs is the first to focus specifically on behavioral development.
And the experts stress that when it comes to safety, the takeaway from the new study is positive.
"People can be reassured that there is not a major disadvantage versus other formulas," said Dr. Michael Katz, senior vice president for research and global programs for the March of Dimes, adding, however, that breastfeeding is best.
In the United States, breastfeeding rates still lag behind public health goals. The American Academy of Pediatrics recommends that babies be breastfed exclusively for the first six months of life and that breastfeeding should continue for at least the first year.
However, recent estimates suggest that just 43 percent of mothers in the U.S. were still breastfeeding at six months, and only 23 percent were breastfeeding at one year.
"What we should be saying is that breastfeeding is the gold standard," said Strathearn.