There is mixed evidence from correlational studies that breastfeeding impacts children’s development. Propensity score matching with large samples can be an effective tool to remove potential bias from observed confounders in correlational studies. The aim of this study was to investigate the impact of breastfeeding on children’s cognitive and noncognitive development at 3 and 5 years of age.
Participants included ∼8000 families from the Growing Up in Ireland longitudinal infant cohort, who were identified from the Child Benefit Register and randomly selected to participate. Parent and teacher reports and standardized assessments were used to collect information on children’s problem behaviors, expressive vocabulary, and cognitive abilities at age 3 and 5 years. Breastfeeding information was collected via maternal report. Propensity score matching was used to compare the average treatment effects on those who were breastfed.
Before matching, breastfeeding was associated with better development on almost every outcome. After matching and adjustment for multiple testing, only 1 of the 13 outcomes remained statistically significant: children’s hyperactivity (difference score, –0.84; 95% confidence interval, –1.33 to –0.35) at age 3 years for children who were breastfed for at least 6 months. No statistically significant differences were observed postmatching on any outcome at age 5 years.
Although 1 positive benefit of breastfeeding was found by using propensity score matching, the effect size was modest in practical terms. No support was found for statistically significant gains at age 5 years, suggesting that the earlier observed benefit from breastfeeding may not be maintained once children enter school.
Comments
RE: Formula Feeding Exposure not Homogenous
All breastfeeding research faces the age-old challenge of innate selection-bias. Girard et al, recently utilized advanced quasi-experimental techniques to mimic randomization, in combination with a large contemporary cohort, making a novel contribution to the field (1).
This study has received significant press attention, mainly because the findings contradict previous studies that report positive relationships between breastfeeding and offspring cognitive development/performance. However, there is a crucial lack in discussion about a potentially critical contributing factor to these conflicting results – the exposure (formula-fed) group is not homogeneous. Infant formula feeding options are incredibly vast and differ between cultures. Combining all formula-fed infants into one single exposure diminishes such studies’ ability to make clean comparisons. Furthermore, formula options and formulations are constantly changing, making it very difficult to draw conclusive insights from compiled historical breastfeeding research that spans decades.
The famous PROBIT trial utilized cluster-randomization to study the impact of breastfeeding-support interventions in Belarus between 1996-1997. At 6.5 years the intervention group exhibited significantly higher Wechsler Abbreviated Scales of Intelligence ranging between 2.9-7.5 points (2). The current Girard manuscript studied infants born between 2007-2008 in Ireland. The difference in exposure between breast vs formula-fed infants in the Girard study is likely vastly different than that of infants born over a decade earlier in Belarus. A similar principle can be applied to the well-publicized 2015 study that reported breastfeeding associated with higher adult IQ (averaging 3.8 points), education and income in a 1982 Brazilian birth cohort (3). While these older studies remain impactful, it is noteworthy (and often ignored) that infant formula options have since evolved and advanced, making comparisons with the Girard study difficult.
We can further observe results form a recent, well-conducted meta-analyses of 18 cohort studies that reported significantly higher performance on adult intelligence tests among breastfed offspring, averaging 2.6 higher IQ points (4). This meta-analysis included birth cohorts that spanned almost 4 decades (1968-2006). The authors provided no discussion of the inevitable heterogeneity in the formula fed group(s), both within cohorts and across decades. Neither did they attempt to control for either of these factors.
While breast milk is undeniably the ideal feeding choice, advances in infant formula nutrition continue to shrink the gap in many infant outcomes. Continuing with our example of cognitive development - infants randomized to receive an experimental formula supplemented with bovine milk fat globule membrane from 0-6 months exhibited 6 point higher Bayley scale scores at 12 months, similar to breastfed controls (5). This difference in Bayley scores is noteworthy because it is similar in nature to the differences in IQ between breast vs formula fed infants from the historical studies noted above.
Moving forward, the field needs to acknowledge that “formula-feeding” is not a homogeneous exposure. Researchers need to begin characterizing differences in formula exposure and incorporating these differences into studies of infant outcomes. We also need to acknowledge that insights from historical comparisons lose relevance as time passes and infant formula continues to evolve at a faster pace than research is conducted.
1. Girard L, Doyle O, Tremblay R, Breastfeeding, Cognitive and Noncognitive Development in Early Childhood: A Population Study. Pediatrics. 2017; 139 (4): e20161848
2. Kramer MS, Aboud F, Mironova E, Vanilovich I, Platt RW, Matush L, et al. Breastfeeding and child cognitive development: new evidence from a large randomized trial. Arch Gen Psychiatry. 2008;65(5):578-84.
3. Victora CG, Horta BL, Loret de Mola C, Quevedo L, Pinheiro RT, Gigante DP, et al. Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from Brazil. The Lancet Global health. 2015;3(4):e199-205.
4. Horta BL, Loret de Mola C, Victora CG. Breastfeeding and intelligence: a systematic review and meta-analysis. Acta Paediatr. 2015;104(467):14-9.
5. Timby N, Domellof E, Hernell O, Lonnerdal B, Domellof M. Neurodevelopment, nutrition, and growth until 12 mo of age in infants fed a low-energy, low-protein formula supplemented with bovine milk fat globule membranes: a randomized controlled trial. Am J Clin Nutr. 2014; 99: 860-8.
RE: Response to "Confused about Table 1"
Figure 1 is a report on the frequency distributions (cumulative percentages) of two separate questions (i.e., ‘how old was <baby> when he/she completely stopped being breastfed’ and ‘how old was <baby> when he/she completely stopped being exclusively breastfed’) that were graphed together for simplicity and space. Thus, the sum of the two distributions would not equal to 100%. Breastfeeding rates in Ireland are in fact among the lowest across Europe although have steadily been increasing as mentioned. Due to inclusion criterion for our study (full term babies only and complete data on all matching variables), our statistics are on a subsample of the GUI Infant cohort (N = 9,854) and not the entire sample of 11, 134. In our sample, 2.5% (n= 249) of babies were still being fully breastfed at interview and 6.1% (n= 603) were being partially breastfed at interview. The graph should be interpreted as 1 representing the percentage of infants breastfed 1 day or more, 2 as 32 days or more, and 3 as 181 days or more.
RE: Confused about Table 1
According to Table 1 in this paper, 5490 infants were "ever breastfed." Of those infants, 60% were exclusively breastfed from 0 to 31 days of life, and nearly 50% were partially breastfed from 0 to 31 days of life. Shouldn't the percentage of exclusive and partially breastfed infants add up to 100%, not nearly 110%?
According to Table 1, over 40% of Irish infants in the sample were exclusively breastfed at point 2 (32 to 180 days) and 30% of that same sample were partially breastfed.
According to Brick and Noland, ("Maternal Country of Birth Differences in
Breastfeeding at Hospital Discharge in Ireland" ) breastfeeding initiation in Ireland in 2007 was about 81% and in 2008, about 82%. These totals are the sum of exclusively and partially breastfed infants during their hospital stay after birth.
Tarrant and Kearney (2008) lamented, as do my Irish lactation consultant colleagues, that Irish breastfeeding rates are so low, and call for governmental support and education of healthcare professional and the the public to increase the number of Irish babies being breastfed.
In Growing Up in Ireland: a Longitudinal Study Infants and Their Families Report 1, (2010), it says, " Among the Growing Up in Ireland infants, 82% of mothers who had ever breastfed had completely ceased by the time of the interview at age nine months. The mean duration of breastfeeding, for those who had ever breastfed but had since stopped, was 12 weeks and ranged between just one day and nine months. A total of 9% of infants were still being partially breastfed at the time of interview, while 2% were still being exclusively breastfed." ("In line with Perinatal Statistics 2007")
Comparing the graph in Table 1 with what Irish statistics report and with the notoriously low breastfeeding rates reported in mainstream media ("Ireland has the lowest breastfeeding rates in the world" Independent.ie News January 16, 2016) leaves me confused.