Researchers found no differences in behavioral development, such as language and other thinking-related skills, between infants fed soy formula or milk formula during the first year of life.
Breastfed babies, however, had a slight advantage.
"Although all three diet groups scored within the established norms in the behavioral testing, BF [breastfed] infants scored slightly better than formula-fed infants," researcher Aline Andres, PhD, of Arkansas Children's Nutrition Center in Little Rock, Ark., and colleagues write in Pediatrics.
The American Academy of Pediatrics recommends breast milk first as the ideal source of nutrition for infants, followed by milk and soy formulas as the second and third choices, respectively.
Researchers say about two-thirds of U.S. infants were breastfed as newborns in 2008, but nearly three-fourths of them were transitioned to baby formula by age 6 months. Overall, about 20 percent of infants in the U.S. are fed soy protein-based formulas during the first year of life.
Several studies have shown that the growth and physical development of infants fed soy formula is similar to that of infants fed milk-based formulas.
But researchers say concerns have been raised about the isoflavone content of soy protein-based formulas. Isoflavones are estrogen-like compounds found in plants, which some have suggested may have effects on the brain and nervous system development.
Small Differences, Breast Milk Best
In this study, researchers compared the development of 391 healthy infants fed breast milk, milk-based baby formula, or soy-based baby formula. The infants were followed for one year from birth and tested every three months.
The results showed no differences between soy formula and milk formula-fed infants on all developmental tests.
Breastfed infants, however, had an advantage over formula-fed infants in three different developmental areas:
- Mental development: Breastfed infants scored slightly higher on the Mental Development Index at 6 and 12 months compared with formula-fed infants.
- Psychomotor skills: Infants fed breast milk also had higher scores on motion skills tests at 6 months than infants fed soy formula.
- Language skills: Breastfed infants had slightly higher scores on the Preschool Language Scale-3 than milk-based formula-fed infants at 3 and 6 months.
Researchers say it is important to point out that differences between breastfed and formula-fed infants were quite small after adjusting for other factors like being small for gestational age, etc.
They say the infants involved in this study will also be followed until age 6 years to see if there are any long-term developmental differences.