"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
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