Professor Richie Poulton
This ground-breaking research is the latest feather in the cap of the world-leading Dunedin Multidisciplinary Study.
The landmark finding once more places the study (which has followed the health and development of around 1000 children born in Dunedin in 1972 - 73) at the forefront of international efforts to understand gene-environment interactions.
Study director Professor Richie Poulton says the relationship between breastfeeding and IQ has been a hot topic for quite some time. "An association between the two has been found by a number of studies, but no one had been able to say with any certainty why this is."
Indeed, some researchers even doubt that the IQ benefit comes from breastfeeding itself. Instead, they believe it is related to the type of mother who tends to breastfeed.
Through a joint New Zealand and United Kingdom effort with collaborators Professors Terrie Moffitt and Avshalom Caspi, the study team has now shown that the "breastfeeding effect" appears to have a nutritional basis and is moderated by a gene called FADS2.
FADS2 is involved in the processing of omega-3 and omega-6 fatty acids, which have been shown to be important nutrients for brain development. Human milk contains significant amounts of omega-3.
The researchers looked at the gene, the environment (breastfeeding) and the outcome (IQ) in two groups of people: the 1000 Dunedin-born study members and another study of 2200 British children born in 1994 - 95.
The IQ of all study members was known from childhood testing and the mothers of study members were asked about breastfeeding practices.
DNA analysis revealed the breastfed children who showed an IQ advantage possessed a certain version of FADS2, known as the C version. People inherit two copies of any given gene, one from each parent. In the case of FADS2, children have either two C versions, two G versions, or a C and a G.
On average, the researchers found an IQ gain of 6 - 7 points for breastfed children carrying one or two copies of the C version. Ninety per cent of the children studied had at least one C copy.
Children with two G versions were neither advantaged nor disadvantaged by breastfeeding - their IQ remained in the healthy normal range, says Poulton.
Cautious scientist that he is, Poulton is reluctant to use the word "settle", but says the findings will definitely "clarify" the breastfeeding-IQ debate.
Some researchers have suggested the association between breastfeeding and IQ arises from women from higher social classes breastfeeding their children more than those from lower social classes.
"We tested for this and found the effect was the same in children from high and low social-class homes and in children with higher-IQ and lower-IQ mothers. This made no difference. It all came down to whether the breastfed child had at least one C copy."
Poulton says, however, people shouldn't blow the implications of the finding out of proportion, or get obsessively focused on fatty acids.
"IQ differences in society clearly cannot be put down solely to breastfeeding. People vary for all sorts of reasons. What we have here is a small but significant piece in the complex puzzle of cognitive development.
"Also, it's unlikely that FADS2 is the only gene involved. Breast milk contains many different nutrients and many genes are likely to have a role in processing them."
For Poulton, the far greater importance of the finding is that it is another nail in the coffin for the old nature versus nurture world view.
"What this reinforces is that genes are not a blueprint; rather, they help to shape how our bodies and brains respond to our environment. The environment can be what we eat or what we experience."
It's exciting new territory, and a pleasant change of pace, to have identified a gene working positively with an enhanced environment, he says.
Until now, research on gene-environment interaction - including the Dunedin study's own landmark findings - has been dominated by searches for genes that increase susceptibility to diseases. This includes the study's discovery that people who carry the "short" serotonin genotypes, and who also suffer stressful life events, are at a higher risk of depression.
"This new finding is a nice bookend to our previous work in this area. It reflects our overarching goal to understand all the negative and positive factors that shape us into who we are," he says.
Furthermore, the paper highlights a new direction for the Dunedin Multidisciplinary Health and Development Research Unit's approach to collaboration.
"By bringing together our cohort and the United Kingdom one, we could internally replicate the results in two separate groups from distant parts of world. This kind of multiple-cohort approach takes the robustness of such research to the next level."
Within New Zealand, the unit is also expanding its reach as a key partner in the newly-launched National Centre for Lifecourse Research. The umbrella centre, which Poulton co-directs, is focused on translating health and development research in concrete ways that directly benefit policymaking, practitioners and the public's understanding of the science.
"Our 1000 study members' amazingly generous gift of their time over three-plus decades has led to this breastfeeding discovery along with many other important findings. We owe it to them to ensure all this good science can get out there and make a difference in the real world."
FUNDING
Health Research Council
National Institute of Mental Health (US)
Medical Research Council (UK)