Even the tiniest premature babies should get breast milk while they are in a hospital's intensive care unit because it appears to boost their mental development, a study said on Monday.
A second study on breast feeding found that it does not raise the risk that children will develop tooth decay later in life, as some earlier research had suggested.
Both reports were published in the October issue of Pediatrics, the journal of the American Academy of Pediatrics.
The premature infant study involved 773 US babies born extremely underweight - less than 1kg - between 1999 and 2002.
It found that babies in the group given breast milk got higher scores on a test measuring their overall intelligence at 30 months of age, with the highest scores showing up among the children who had received the most breast milk as infants.
Premature infants are fed intravenously in the hospital.
"These findings strongly suggest that, whenever possible, preterm infants should routinely be given breast milk during their stay in the intensive care unit," said Dr Duane Alexander, director of the National Institute of Child Health and Human Development, which backed the study.
The study on tooth decay came from the School of Medicine and Dentistry at Rochester University in New York. Its authors said it was prompted by limited evidence and earlier research that breast feeding - especially when it is prolonged - could raise the risk of getting cavities among young children.
But a review of a US government database on child health involving more than 1 500 youngsters found no such association.
There is "no evidence to suggest that breast feeding or its duration are independent risk factors for early childhood caries (cavities)... or decayed and filled surface on primary teeth," the study concluded.
Health experts recommend that children be breast fed for at least the first year of life and beyond, for as long as mutually desired by mother and child, the study said.