WASHINGTON, Nov. 6 (Xinhua) -- Breastfeeding boosts infants' IQs, but only if the babies have a genetic variant that enhances their metabolism of breast milk, a Yale researcher and collaborators report Tuesday in Proceedings of the National Academy of Sciences.

The study included 1,037 children in New Zealand and 1,116 families with same sex twins in England and Wales. Those who were breastfed and had the genetic variant FADS2 had IQs that were 5.6 to 6.3 points higher than children who were breastfed but did not have the variant.

"It is this genetic variant in FADS2, a gene involved in the control of fatty acid pathways, that may help the children make better use of the breast milk and promote the brain development that is associated with a higher IQ score," said Julia Kim-Cohen, assistant professor of psychology and a member of the research team.

"Children who do not carry the 'helpful' genetic variant have normal average IQ scores," Kim-Cohen said. "Being breastfed for them is not associated with an IQ advantage."

The study included researchers from King's College, London, Duke University, and the University of Otago, New Zealand.

The intelligence quotient (IQ) has long been at the heart of debates about nature versus nurture. Twin studies document both strong genetic influences and no genetic environmental influences on IQ, particularly for young children.

This study looked at how long-chain polyunsaturated fatty acids(LC-PUFAS), which are present in human milk but not in cow's milk or most infant formulas, are metabolized. LC-PUFAS in breast milk, the authors said, is believed to enhance cognitive development because the fatty acids are required for efficient neurotransmission and are involved in neuronal growth and regeneration.