Tuesday, 30 July 2013

Breast is best, but for IQ as well?


If there was something really wrong with breast milk, we would not have survived so far. We have about 4 million years of data on efficacy. However, modern life gives us options, and formula feed is one such option, hence the plethora of studies trying to determine whether breast or bottle are better in some way, including the development of intelligence.

After many years of claim and counter-claim I believed that the field had settled down. There was no effect on the child’s IQ once you had controlled for the mother’s IQ. Received wisdom was that intelligent mothers breastfed their children, but it was the genes already in them that boosted their intellects, not their mother’s milk.

Now a new study seems to suggest that breastfeeding has an independent additive effect, even after controlling for maternal variables.

Belfort et al. Infant Feeding and Childhood Cognition at Ages 3 and 7 YearsEffects of Breastfeeding Duration and Exclusivity. ONLINE FIRST JAMA Pediatr. 2013;():-. doi:10.1001/jamapediatrics.2013.455.

The authors say that once they have adjusted for socio-demographics, maternal intelligence, and home environment in linear regression, longer breastfeeding duration was associated with higher Peabody Picture Vocabulary Test score at age 3 years (0.21; 95% CI, 0.03-0.38 points per month breastfed) and with higher intelligence on the Kaufman Brief Intelligence Test at age 7 years (0.35; 0.16-0.53 verbal points per month breastfed; and 0.29; 0.05-0.54 nonverbal points per month breastfed). Breastfeeding duration was not associated with Wide Range Assessment of Memory and Learning scores. Beneficial effects of breastfeeding on the Wide Range Assessment of Visual Motor Abilities at age 3 years seemed greater for women who consumed 2 or more servings of fish per week (0.24; 0.00-0.47 points per month breastfed) compared with less than 2 servings of fish per week (−0.01; −0.22 to 0.20 points per month breastfed) (P = .16 for interaction).

As you can see, not only are breasts in the equation, but fish as well. You may remember my earlier dictum that researchers should discuss one thing at a time, scholars two things at a time, and only outstanding scientists should attempt three things at a time. Fish is good for the brain, so fishmongers say, but this is no reason to drag it in.

An eagle-eyed reader, Stuart Ritchie, points out that some of the lower ranges of the confidence limits are rather low, and this all becomes clear in Table 1, in which model 2, which includes the demographic variables, leads to a halving of the effect, and low confidence as judged from the bottom of the range, which is almost zero.


Seems like intelligent mums are a big part of the effect, and we are very possibly close to noise when that is factored out. To give the authors their due, they try to control for the fact that many of the lower ability mothers were lost to the complete data set. If only human subjects understood precisely how important behavioural science research is, they would be as gloriously in favour of official surveys as the are in North Korea. Anyway, the paper presents data suggesting that breastfeeding has an effect on intelligence, but it is not an overwhelming finding.

A wider perspective, almost amounting to a meta-analysis comes from Brion et al.

Int J Epidemiol. 2011 June; 40(3): 670–680. Published online 2011 February 24. doi:  10.1093/ije/dyr020

PMCID: PMC3147072

What are the causal effects of breastfeeding on IQ, obesity and blood pressure? Evidence from comparing high-income with middle-income cohorts

They compared Bristol, England with Pelotas, Brazil. They deserve a medal for this comparison alone. Pelotas is in Rio Grande do Sul just above Uruguay, and I mean no disrespect to its citizens and to my school friends who live there when I say that it is more flown over than flown into. The authors found that in wealthy Bristol higher socio-economic position was strongly associated with breastfeeding but not in poorer and probably more cheerful Pelotas. In Bristol breastfeeding was associated with lower BP, lower BMI and higher IQ, adjusted for confounders. In contrast, in Pelotas, breastfeeding was not strongly associated with BP or BMI but was associated with higher IQ. Crucially, only 3% of these Brazilian children were not breast fed. About a third of Bristolian mothers never start breastfeeding or get discouraged in the first month. Breast is Brazilian best. For that reason the variable was entered as numbers of months of breastfeeding. Trial data supported the conclusions inferred by the cross-cohort comparisons, which provided evidence for causal effects on IQ but not for BP or BMI. These researchers conclude that breastfeeding may have causal effects on IQ. They make the key point that comparing associations between populations with differing confounding structures can be used to improve causal inference in observational studies.

They had IQ data for 614 kids. They struggled to work out comparative socio-economic rankings. Table 2 is informative, in that to my eye the key variable is clear: maternal education is more important than any of the other factors, followed by paternal education.





I think that the mother’s milk effect only shows up when you control for the mother’s and father’s intelligence effect, which is the main factor. Have a look and see what you think.

Meanwhile I will think about these insights every time I fly over Pelotas. In summary, chose your parents carefully, and advise them that breast is probably best, if only for nutritional reasons.

In only a few years all these studies will seem quaint. Current epidemiological projects will have entire genomes for each child, and we will probably have first estimates of intelligence derived from the individual variations on the genetic code. There may be nothing in breast milk except exceptionally healthy, nutritious and human-friendly milk.


  1. For the first paper (Belfort et al.), it's been pointed out that only researchers who'd forgotten about genes would fail to control for paternal, as well as maternal, IQ. But fail they did.

    Since the confidence intervals for the breastfeeding effects are, as you note, so close to zero (at least for two of the three childhood IQ measures), it's not exactly a stretch to predict that they'd overlap zero if paternal IQ were added into the equation.