How much impact do confounding variables have on the cognitive benefits of breastfeeding? This much.

Do the purported benefits of breastfeeding really exist or are they merely artifacts of wealth, health and privilege. A new paper dramatically demonstrates what happens to the benefits of breastfeeding when you take confounding variables into account.

The paper is To what extent does confounding explain the association between breastfeeding duration and cognitive development up to age 14? Findings from the UK Millennium Cohort Study by Peyrera-Ellis et al.

The authors start with the assumption that breastfeeding improves children’s cognition but acknowledge what critics (including me) repeatedly argue:

A systematic review found that on average breastfed infants scored 3.44 points higher in standardised intelligence tests than their non-breastfed peers, however, a causal relationship is still debated. It is argued that improved cognitive outcomes could be explained by other characteristics of the women who breastfeed their babies, principally socioeconomic position (SEP) and maternal intelligence.

The authors provide one of the best illustrations I ever seen of the many confounding variables that impact breastfeeding research.

It shows that maternal characteristics have important effects on cognition and ALSO have important effects on breastfeeding duration. Therefore any research that seeks to quantify the impact breastfeeding has on child cognition MUST take them into account or the results will be invalid.

This authors looked at data from 7,855 singletons born in 2000–2002 and followed up to age 14 years within the UK Millennium Cohort Study. What did they authors find?

A picture — or in this case two pictures — is worth a thousand words.

This is the impact of breastfeeding on standardized cognitive verbal scores:

This is the impact of breastfeeding on standardized cognitive spatial scores:

These charts illustrate four points:

1. The purported impact of breastfeeding on cognition steadily melts away as ever more adjustments are made for confounding variables

The top set of measurements depict the uncorrected data. If you were to only look at that you might think that breastfeeding has a significant impact on cognition

But when you read the charts from top to bottom as successive corrections are made for confounding variables in models 1-4, you can see that the magnitude of the effect drops dramatically until it is less than 0.03 standard deviations meaning that the difference is very small.

How do the authors spin the result that impact of breastfeeding is actually small?

Adjustment for SEP explained approximately half of the initially observed associations. Further adjustment for maternal cognitive measures failed to completely remove the remaining associations at ages 7, 11 and 14. The fully-adjusted coefficients where there is evidence of an effect of breastfeeding on verbal cognitive scores varied between 0.08 (age 7; <2 months vs never breastfed) to 0.26 SD (age 14; ≥12 months vs never breastfed). For spatial scores, the coefficients varied between 0.08 (age 7; <2 months vs never breastfed) to 0.19 SD (ages 7 and 11; 4 to <6 months vs never breastfed). This suggests that while the association in this population is not completely due to confounding, the effect of breastfeeding on cognitive development is modest in this population.

2. The remaining small impact of breastfeeding on cognition may simply reflect residual confounding.

In their illustration of confounding variables the authors include paternal cognitive score but acknowledge that they could not adjust for it because they did not have that information.

We also considered the possibility that the remaining associations were explained by residual confounding produced by unmeasured confounders, such as paternal measures of cognitive ability or broader measures of maternal cognitive ability. This was assessed through the calculation of the E-values. In order to explain the aforementioned associations, any unmeasured confounder should be associated with both BF duration and cognitive scores with coefficients of at least 0.39 (to fully explain a coefficient of 0.10) or 0.68 (to fully explain a coefficient of 0.26). Therefore, while there is room for the associations to be further explained, it is unlikely that all the observed associations could be explained in full by additional adjustment.

But you don’t have to reduce the coefficient to zero in order for the impact to be nearly non-existent and paternal cognitive score as well as broader measures of maternal cognitive ability may account for most of the differences that the authors found.

3. If breastfeeding really leads to improvements in cognitive outcomes, we would expect there to be a dose-response relationship but that’s not what the charts show. There appears to be little relationship between the duration of breastfeeding and its purported impact.

4. The fact that the impact of breastfeeding appears to increase over childhood raises further questions and they recognize it.

The association at age 14 seems to be stronger than at other ages. The outcome was measured with a different instrument at age 14, which may contribute to the observed differences. However, these results seem to be in line with those of Kanazawa, who showed that the effect of BF on intelligence increased as children got older in the 1958 British Birth cohort… [T]hese findings could be due to chance or (less likely) to residual confounding and should be revisited in future studies.

The results suggests that it is another factor which could have a cumulative impact like socio-economic status (allowing access to successive better educational institutions for example), not breastfeeding, is responsible.

The authors conclude with the same assumption with which they started:

…[T]he role of breastfeeding on the child’s cognitive scores should not be underestimated. While a small increase in cognitive outcomes may not be clinically meaningful at the individual level, it has the potential to be influential at the population level…

The irony is that the role of breastfeeding on children’s cognitive development has been repeatedly over-estimated and, as the authors acknowledge is not clinically meaningful and therefore not a reason for an individual woman to choose to breastfeed.