The nature-nurture debate is one of the most famous in biology, but its own nature has shifted substantially in recent years. We now know that genes and environment are not opposing agents that shape our lives separately, but partners walking hand-in-hand. More often than not, genes affect our bodies and behaviour by altering the ways in which we react to our environment.
Now, an international team of researchers have discovered a stark example of this gene-environment partnership. They found that breastfed children have higher IQ scores, but only if they have a certain version of a gene called FADS2.
The concept of IQ has been central to the nature-nurture debate for years, ever since studies in twins suggested that a large part of the variation in IQ scores could be explained through inherited genetic factors. Avshalom Caspi and Terrie Moffitt from King’s College London wanted to kill this tiresome debate finding a gene that affected IQ via the environment.
They chose to look at breastfeeding, as studies have mostly found that babies who drink their mothers’ milk have higher IQ scores, among other benefits. These higher scores persist into adulthood and across social classes. We also have a reasonable idea of how breastfeeding could affect brain development at a molecular level, and it involves fatty acids.
Why breast is best
Mum’s breast milk is undoubtedly the most nutritious food source for a baby and contains chemicals lacking in cow’s milk and the majority of formulas. Among these are fatty acids that are both long in size and name, including docosahexaenoic acid (DHA) and arachidonic acid (AA). Some studies suggest that these acids play a vital role in developing brains, helping nerve cells to send signals, grow new branches and repair damage.
Indeed, an infant’s brain builds up substantial amounts of both DHA and AA in its first months of life and much more so if it drinks breast milk than unsupplemented formulas. Animal studies support this idea too – those that are fed on diets that lack these fatty acids grow up to have faulty memories and senses, while those that are given supplements do better in a whole gamut of mental tasks.
In light of this evidence, Caspi and Moffitt reasoned that genes that control how fatty acids are processed are likely to link breastfeeding and IQ scores. They focused on one in particular, FADS2 (short for fatty acid desaturase 2), which controls an important step in the metabolism of both DHA and AA.
Along the gene is a specific spot that goes by the unwieldy name of rs174575. At this location, a person can have one of two bases (the basic units that make up DNA) – guanine (G) or cysteine (C). Caspi and Moffitt compared children with these two versions in two samples from Britain and New Zealand.
In both cases, they found that breastfed children scored about 7 points higher on IQ tests, but only if they had at least one copy of the ‘C’ version of FADS2. If they had two copies of the ‘G’ version, their performances were the same regardless of whether they drank mum’s milk or formulas.
What this means for mums and babies
Before concluding that FADS2 modified the effect of breastfeeding on IQ, the team were very careful to rule out alternative explanations.
People from advantaged socioeconomic groups are more likely to breastfeed and have higher IQs, but the effects of FADS2 applied across different social classes. Mums are also more likely to breastfeed if they have high IQs but again, FADS2 had the same effect in high-IQ and low-IQ mothers. Finally, FADS2 didn’t affect the likelihood that a mother would breastfeed her child, the babies’ weight at birth or the nutritional quality of the mother’s breast milk.
With all these caveats accounted for, Caspi and Moffitt felt safe to say that FADS2 affected the breastfeeding-IQ link by altering the baby’s ability to metabolise the fatty acids in its mother’s milk.
The finding is good news for advocates of breastfeeding, who have had a hard time of it between the advertising tactics of formula manufacturers and the insanely prudish attitude of Facebook moderators. The study’s results (and their consistency in two different countries) lends considerable weight to the idea that breast milk really does improve mental development because of its nutritional content.
Over 90% of people carry the ‘C’ version of FADS2, so the benefits of breastfeeding that accompany it would be strongly felt at the population level. It may also be no coincidence that the ‘C’ version is so common. Back in prehistoric times, when all children were breastfed, the effects of FADS2’s different guises would have been even more strongly felt, with the ‘C’ bearers enjoying an advantage in later life.
What this means for genetics
The study also has big implications for gene-hunters. The usual tactic for finding genes linked to physical traits or behaviours is to scan the entire genome for genes that have direct and prominent effects.
But if the team had used this tactic, they would never have billed FADS2 as an IQ-related gene (I’m avoiding using the phrase “a gene for IQ” because it’s trite and misleading). That’s because there are no significant differences between the IQ scores of people with the two FADS2 variants if you take breastfeeding out of the equation. The upshot is that geneticists can look to the environment for important clues when looking for genes that affect human behaviour and health.
For the foreseeable future, it looks like the dichotomy of nature and nurture is dying. It’s proving to be far more interesting to look at how the two interact, and good examples are springing up fast.
Some variants of the PTC gene make people very sensitive to bitter tastes and could affect their health by putting them off vegetables. Some people have a particularly ‘fast’ version the NAT2 gene that can activate cancer-causing chemicals in red meat – they may have higher risks of bowel cancer.
And the ‘obesity genes’ so beloved by the press could affect someone’s chances of putting on weight by making them more sensitive to food smells, making them less likely to feel full after meals or giving them a preference for sweet, fatty food.
A final caveat – what this study does not show…
Whether the concept of IQ is actually a measure of intelligence is a lengthy debate in itself, and one that I’m not going to get into here. In many ways, it’s an abstract concept. It correlates with certain things like academic achievement and job performance but it’s becoming increasingly doubtful that it’s a valid measure of ‘intelligence’.
I’d put solid money on some of the press coverage of this paper saying things like “breastfeeding makes babies smarter if they have certain genes.” That’s a misinterpretation, and Caspi and Moffitt are clear in both their paper and press release to only mention IQ and never ‘intelligence’ or ‘braininess’. It’s a cautious note that I’ve repeated in this article.
UPDATE: Yep, I was right. Reuters lead with Gene explains why breastfeeding makes kids smarter, while the Daily Mail proclaims that Breastfeeding is best for a brainy baby. Even Nature starts waxing on about intelligence. It wasn’t all bad though – see Roger Highfield at the Telegraph get it right.
More on genes and behaviour: Discovery of ‘fat gene’ highlights stigma against obese people
Reference: Caspi, Williams, Kim-Cohen, Craig, Milne, Poulton, Schalkwyk, Taylor, Werts & Moffitt. 2007. Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism. PNAS doi/10.1073/pnas.0704292104.