A specific gene has been linked to higher IQ in breastfed infants, finds a report based on two large studies.

U.S. researchers found that one version of the gene FADS2, called "C," raised IQ seven points in children who were breastfed, indicating that higher intelligence in babies who are breastfed is the result of the interaction between the gene and its environment, researchers say. The genetic version "C" was found in about 90 per cent of the children studied.

Those infants who had a different version of the FADS2 gene, called "G," accounted for about 10 per cent of the studies' participants. They had no increase in IQ when breastfed, according to the report's authors.

The report, based on two studies involving a total of 3,000 children in the U.K. and New Zealand, was authored by researchers at Duke University in Durham, N.C. Their findings are published this week in the Proceedings of the National Academy of Sciences.

The 1,037 children tracked in the New Zealand study were born between 1972 and 1973. Breastfeeding status in the New Zealand study was assessed at three years, and IQ was measured at seven, nine, 11 and 13 years.

The 2,232 children in the U.K. study, born between 1994 and 1995, were assessed for breastfeeding at two years and IQ was measured at age five.

Breast milk is high in fatty acids, unlike cow's milk or many baby formulas, says the report. It is the conversion of the fatty acids that is critical to IQ, the study found.

FADS2 was found to produce an enzyme that converts fatty acids into the polyunsaturated fatty acids docosahexaenoic acid (DHA) and arachidonic acid (AA). Both of these are thought to boost brain development.

They are now routinely added to baby formula.

The children in the studies were born before mass fatty-acid supplementation began, said the authors.

"Our findings support the idea that the nutritional content of breast milk accounts for the differences seen in human IQ," said Terrie Moffitt, a professor of psychological and brain sciences in Duke University's Institute for Genome Sciences and Policy, in a release. "But it's not a simple all-or-none connection: it depends to some extent on the genetic makeup of each infant."

The report finds that the difference in the children's ability to metabolize fatty acids, which is genetically determined, decides how breastfeeding will affect them from a cognitive perspective.