Breastfeeding and adult intelligence – Authors' reply
Authors: Bernardo Lessa Horta, Cesar G Victora
Journal: The Lancet Global Health (2015)

Colin Cooper argues that the positive association between breastfeeding and adult intelligence might be due to residual confounding by maternal intelligence quotient (IQ).1Victora CG Lessa Horta B Loret de Mola C et al.Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from Brazil.Lancet Glob Health. 2015; 3: e199-e205Summary Full Text Full Text PDF PubMed Scopus (389) Google Scholar This is indeed an important issue in high-income countries, but not in low-income or middle-income settings. Although we did not measure maternal IQ in 1982, we found no associations between breastfeeding duration and strong correlates for IQ, including schooling and family wealth;2Strenze T Intelligence and socioeconomic success: a meta-analytic review of longitudinal research.Intelligence. 2007; 35: 401-426Crossref Scopus (580) Google Scholar in our cohort participants, the correlation coefficient between IQ and years of schooling was 0·64. Had maternal IQ been a confounder in our study, our adjustment for proxy variables (maternal and paternal education, family income, and household assets) would weaken the association between breastfeeding and IQ, rather than strengthen it, as described in our paper. Furthermore, findings from a meta-analysis3Horta BL Victora CG Long-term effects of breastfeeding: a systematic review. World Health Organization, Geneva2013: 74Google Scholar showed that, even in studies that controlled for maternal IQ, breastfeeding was associated with increased IQ (mean pooled difference 2·19, 95% CI 0·89–3·50).3Horta BL Victora CG Long-term effects of breastfeeding: a systematic review. World Health Organization, Geneva2013: 74Google Scholar Finally, the only randomised study on this issue (the PROBIT trial)4Kramer MS Aboud F Mironova E et al.Breastfeeding and child cognitive development: new evidence from a large randomized trial.Arch Gen Psychiatry. 2008; 65: 578-584Crossref PubMed Scopus (742) Google Scholar reported a 7·5-point increase in IQ among Belarusian children born in maternity hospitals in which breastfeeding was promoted.4Kramer MS Aboud F Mironova E et al.Breastfeeding and child cognitive development: new evidence from a large randomized trial.Arch Gen Psychiatry. 2008; 65: 578-584Crossref PubMed Scopus (742) Google Scholar Zeljka Buturovic and Suzana Ignatovic argue that factors related to the child can also confound the association. Adjustment for the child's sex, birthweight, and gestational age did not affect our results. We did not measure the “baby's ability and willingness to nurse”, which they suggest would lead to reverse causation—that is, infants who learn to suckle more quickly would breastfeed for longer. This is a novel hypothesis in our view, but it would not explain the results from the PROBIT trial,4Kramer MS Aboud F Mironova E et al.Breastfeeding and child cognitive development: new evidence from a large randomized trial.Arch Gen Psychiatry. 2008; 65: 578-584Crossref PubMed Scopus (742) Google Scholar nor would it make much sense in evolutionary terms since extended breastfeeding was universal in early mankind and lack of breastfeeding would greatly increase the risk of death. We hope that this hypothesis can be tested in future studies by obtaining detailed information on suckling intensity, although it might be necessary to wait for another 30 years to verify whether this variable can explain findings such as ours. Buturovic and Ignatovic rightly point out that present-day formula is different from the type of milk received by Brazilian infants born in the early 1980s, particularly because long-chain polyunsaturated fatty acids have been added to modern products. Nevertheless, modern biology is constantly uncovering new properties of breastmilk. In addition to the effect on myelinisation, these properties include the presence of stem cells, exosomes, epigenetic programming, and changes in the microbiome.5Azad MB Konya T Maughan H et al.Gut microbiota of healthy Canadian infants: profiles by mode of delivery and infant diet at 4 months.CMAJ. 2013; 185: 385-394Crossref PubMed Scopus (650) Google Scholar, 6Yatsunenko T Rey FE Manary MJ et al.Human gut microbiome viewed across age and geography.Nature. 2012; 486: 222-227PubMed Google Scholar, 7Verduci E Banderali G Barberi S et al.Epigenetic effects of human breast milk.Nutrients. 2014; 6: 1711-1724Crossref PubMed Scopus (123) Google Scholar, 8Torregrosa Paredes P Gutzeit C Johansson S et al.Differences in exosome populations in human breast milk in relation to allergic sensitization and lifestyle.Allergy. 2014; 69: 463-471Crossref PubMed Scopus (94) Google Scholar, 9Hassiotou F Hartmann PE At the dawn of a new discovery: the potential of breast milk stem cells.Adv Nutr. 2014; 5: 770-778Crossref PubMed Scopus (82) Google Scholar These mechanisms might lead to further insights on how breastmilk leads to improved health and development, and make it difficult, if not impossible, for formula to mimic the effects of a live substance. Ossie Ferdinand Uzoigwe suggests that we should also have considered parity and birth intervals as confounding variables. In our cohort, mothers who breastfed for longer had larger families, rather than smaller families, as in many low-income settings. Additionally, higher parity was associated with lower IQ, and therefore parity was a negative confounder. Indeed, the IQ difference between the extreme breastfeeding categories increased slightly to 3·87 points when adjusted for parity. We also restricted the analyses to singletons and the effect size was maintained (3·45 IQ points). Regarding the birth interval following the index participant, very similar effect sizes were noted among those who had a younger sibling born within 3 years (effect 3·61 points) and those who did not (3·92 points). We declare no competing interests. Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from BrazilBreastfeeding is associated with improved performance in intelligence tests 30 years later, and might have an important effect in real life, by increasing educational attainment and income in adulthood. Full-Text PDF Open AccessBreastfeeding and adult intelligenceCesar Victora and colleagues (April, 2015)1 report that breastfeeding is associated with children developing higher intelligence quotients (IQ) in later life. Victora and colleagues viewed this association as being causal. However, their experimental design ignored two important facts: first, that the children were biologically related to their mothers; and second, that adult IQ is strongly heritable (narrow heritability of at least 0·5, meaning that additive genetic effects account for half the variation in intelligence from person to person in Western populations). Full-Text PDF Open AccessBreastfeeding and adult intelligenceCesar Victora and colleagues1 used a large Brazilian sample to investigate the effect of breastfeeding on children's intelligence quotient (IQ), education, and earnings at age 30 years. They noted that, after adjustment for confounders, infants who were breastfed for 12 months or longer had, on average, about 4 points higher IQs, about 1 year more of schooling, and a monthly income that was roughly 350 Brazilian reals higher than did children who were breastfed for less than 1 month. Importantly, they identified a dose-response effect for breastfeeding—with almost every increase in the duration of breastfeeding, there was a slight rise in IQ. Full-Text PDF Open AccessBreastfeeding and adult intelligenceCesar Victora and colleagues1 use a robust method to show an association between the duration of breastfeeding and adult intelligence, educational attainment, and professional success. However, some potentially important confounders are overlooked. These confounders include the birth interval between children and family size. Full-Text PDF Open Access