The researchers tested the babies' mental and motor skills and language ability at 3, 6, 9 and 12 months of age.
The researchers found that the breast-fed babies scored better than babies who were fed cow's milk formula at 9 and 12 months and better than babies on soy milk at 6, 9 and 12 months. When the researchers divided the babies into groups based on their mental test scores, they found that breast-fed babies were more likely to be in the top 20th percentile and formula-fed infants were more likely to be in the bottom 20th percentile.
"[However], all these kids are in the top half of the normal range," Badger said. "I think this is such a small component, and genetics and environment will make the big difference."
He added that the solid food some of the babies started eating at around 6 months could also have affected their scores, but the group has not taken into account this aspect of their diet yet. The group did adjust for factors including mother's age, IQ and economic status.
The American Academy of Pediatrics recommends exclusive breast-feeding for the first 6 months followed by a combination of breast-feeding and solid foods until at least 1 year of age.
A mom who cannot breast-feed might achieve some of the benefits by holding her baby close to her chest when she is bottle-feeding so the infant can make eye contact with her and hear her heartbeat, Lawrence said.
Badger is on the Science Advisory Board of the Soy Nutrition Institute and has given input to the U.S. National Institutes of Health committee on soy formula. The Soy Nutrition Institute did not contribute to the current study.
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