WEDNESDAY, May 30, 2012 (HealthDay News) -- Babies who are fed soy formula do as well as babies drinking cow's milk formula on tests of mental ability in the first year of life, a new study finds. But breast-fed babies score slightly higher than infants on either type of formula, the researchers say.
About 20 percent of formula-fed babies in the United States are on soy formula, often because their mothers cannot breast-feed and they are allergic to cow's milk formula.
"Our study is very important because it shows that the growth and development of children in the U.S. who are fed soy formula is the same as children who are fed milk formulas," said Thomas Badger, professor of pediatrics at the University of Arkansas for Medical Sciences and lead investigator of the study.
Badger and his colleagues have looked at the effects of soy components in animal studies and found negative and positive effects, such as less weight gain. In particular, they were concerned that one of the chemicals in soy that can act like estrogen could alter brain development and function in babies.
The researchers did find a difference in brain development between breast-fed babies and those on cow's milk or soy formula, but it was so small that it will probably not affect long-term ability, Badger said. "I don't think parents should be worried at all if their kids are on formula."
Another expert agreed.
"This study should be a great relief to people who have been using soy formula," said Dr. Ruth Lawrence, medical director of the Breastfeeding and Human Lactation Study Center at the University of Rochester Medical Center, who was not involved in the study.
"[But] you really have to see what the babies do when they have to learn to read and do social things," Lawrence said. Badger and his colleagues will test the babies in the study when they are 6 years old to see if there are differences in test scores or behavior later in life.
"I would expect that at 6 years you will see a wider spread between human milk and formula [groups]," Lawrence said. Previous research has found that people who were breast-fed do better all the way up through high school than those who were fed cow's milk formulas, she added.
The benefit of mother's milk probably comes both from nutritional differences as well as the act of breast-feeding, Lawrence said. "There are a lot of items in human milk, and the formula companies take all these synthetic chemicals and dump them into [cow's] milk and it is not the same," she said.
The study, published online May 28, appears in the June print issue of the journal Pediatrics.
Another study published in the same issue found that supplementing cow's milk formula with one of the nutrients found in mother's milk -- long-chain fatty acids -- was not associated with better mental scores in babies.
For that study, researchers at Yale University analyzed 12 previous studies involving about 1,800 infants that looked at the effects of supplementation with the fatty acids DHA (docosahexaenoic acid) and arachidonic acid, which are important for brain development. Cow's milk normally contains a different profile of fatty acids than human milk, Lawrence said.
The fact that this analysis did not find a benefit of fatty acid supplementation on mental development "poses an interesting question of whether formula companies should stop adding it, because it costs money," Lawrence said.
"We know it is probably nutritionally good," Lawrence said. Similar to the findings of Badger's study, there might be a difference in mental ability associated with supplementation in babies after 6 or 12 years, she added.
The study by Badger and his colleagues involved 131 breast-fed babies, 131 babies on cow's milk formula and 129 babies on soy formula.
Of the breast-fed babies, about half got only breast milk for their first year, along with solid food starting at about 6 months, and the other half were fed cow's milk formula, either exclusively or along with breast milk and solid food, after they were 6 months old. Most of the babies in the formula groups were fed breast milk for some of their first two months.
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.
To learn more about breast-feeding, visit the American Academy of Pediatrics Breastfeeding Initiatives.