TUESDAY, Jan. 13 (HealthDay News) -- Researchers say they boosted the brain function of female preterm babies by increasing the amount of an omega-3 fatty acid in either formula or breast milk.

The study, which appears in the Jan. 14 issue of the Journal of the American Medical Association, found that preterm baby girls fed a diet high in docosahexaenoic acid (DHA) improved their scores on a mental development test by five points. The intervention also led to an 80 percent reduction in the number of baby girls with significant mental delays, the researchers said.

But the intervention didn't confer the same benefits to baby boys, the team noted.

"We hypothesized that if the diets of preterm infants contained a concentration of DHA that was at a level the baby would have received if still in the womb, then we would improve the mental development of these preterm children," said the study's lead author, Maria Makrides, deputy director of the Women's and Children's Health Research Institute and professor of human nutrition at the University of Adelaide, Australia.

Makrides said the idea for the study came from previous research that found that giving low levels of DHA to preemies could improve their visual acuity. That study, however, was unclear whether there were mental benefits as well. Makrides and her colleagues hoped that by increasing the levels of DHA, they would see an improvement in neurocognitive functioning.

The study included 657 infants who were randomly assigned to receive a standard dose of DHA or a high dose. If mothers were breast-feeding, they were given capsules containing the omega-3 fatty acid. Otherwise, they were given fortified formula. Both treatments were given until the babies reached what would have been their expected delivery date.

To test the babies' brain function, the researchers used a test called the Bayley Mental Development Index, which assesses memory, problem solving, early number concepts and language.

When the researchers first tallied the data, they found virtually no difference. But, when they separated the groups by gender, a difference emerged. The baby girls' scores on the test went up by an average of five points. According to Makrides, that translates to a 55 percent reduction in the number of girls with a mild mental delay and, for those given high levels of DHA, an 80 percent reduction.

Why the treatment didn't help boys isn't clear.

"We can only speculate that there are differences in the metabolism of boys and girls that we do not yet understand," Makrides said. "The higher metabolic rate in boys may mean that they utilize much of the DHA they receive into energy. Also, boys may have a higher requirement for DHA. Clearly, this is an area of important research for the future."

Samantha Heller, a New York City-based registered dietician, said she also could not explain why there was such a difference between boys and girls in this study.

"What I can tell you is that DHA is really important for the development of the brain in the womb, and the eyes and visual acuity," Heller said. "There have been studies that show mid-pregnancy supplementation can improve children's outcomes."

"So, what pregnant women can do is focus on their diets, before getting pregnant and during pregnancy, and include foods that have omega-3 fatty acids, like low-mercury fish," she said. "Some examples are anchovies, herring, catfish, canned salmon, sardines, Pacific sole, tilapia, freshwater trout and whitefish."

And she had one more bit of advice: "If you're going to take supplements, talk to your ob/gyn first."

More information

To learn more about premature birth, visit the March of Dimes .