Genetic variation in the enhancement of intelligence by breastfeeding.

It's the environment AND the genes...Caspi et al. provide a fascinating example of how genetic differences can moderate the effects of environmental influences on an individuals' health and behavior. The authors chose breastfeeding as the environmental exposure because the biological processes underlying its benefits for the developing brain are increasingly well understood. Here are some edited clips from the paper:

The predominant long-chain polyunsaturated fatty acids (LC-PUFAs) present in human milk, but not in cow's milk or most infant formulas, are docosahexaenoic acid (DHA; 22:6n-3) and arachidonic acid (AA or ARA; 20:4n-6). Substantial amounts of DHA and AA accumulate in the human brain during the first postnatal months, and infants who are breastfed have higher concentrations of DHA and AA than infants fed unsupplemented formulas. Evidence, in general, is consistent with the hypothesis that LC-PUFAs in breast milk may enhance cognitive development. In humans, children who are breastfed have higher IQs than children not fed breast milk, and this advantage persists into adulthood.

The authors chose to study people with two allelic variants of FADS2, which they termed C and G. FADS2 is an attractive candidate gene because of its role in the modification of dietary fatty acids. FADS2, located on chromosome 11q12.2, encodes the delta-6 desaturase that is the rate-limiting step on the metabolic pathway leading to AA and DHA production.

Examination of different cohorts of several thousand children from ongoing longitudinal studies in New Zealand, England and Wales revealed that breastfed children carrying the C allele showed a 6.4-IQ-point advantage relative to children not fed breast milk. GG homozygotes neither gained an advantage from breastfeeding nor suffered a disadvantage from not being fed breast milk. The authors ruled out alternative explanations of the finding involving gene–exposure correlation, intrauterine growth, social class, and maternal cognitive ability, as well as maternal genotype effects on breastfeeding and breast milk.

...the finding has implications for the public understanding of genetics. The pendulum of opinion surrounding nature versus nurture has swung back and forth, yielding global estimates of heritability versus "environmentality" that have overlooked the contribution of interactions between specific genes and specific experiences. To date, research on gene–environment interactions has been dominated by the search for genetic variants that increase disease susceptibility to environmental pathogens; for example, carriers of "short" 5-HTT alleles who encounter stressful life events are at risk of becoming depressed; carriers of "rapid" NAT2 alleles who eat red meat are at risk of developing colorectal cancer. However, genes are not only implicated in disease; here we have shown that a genetic variant (in FADS2) may also enhance a favorable response (increased IQ) to a salubrious exposure present throughout human ancestry (breastfeeding).