Breastfeeding and cognitive development

Since the report by Hoefer and Hardy in 1929 (1) several studies have reported that children who are breastfed score higher on tests of cognitive development than children who are formula fed (2) and that this beneficial effect becomes more pronounced with increasing duration of breastfeeding (3). Although many investigators reported that differences in cognitive development persist after adjustment for important covariates, other investigators showed that these differences were no more significant after appropriate covariate adjustment, including maternal intellectual quotient (IQ) and parental skills (4, 5). Whether these effects reflect residual confounding factors due to educational and parenting differences between groups, therefore, remains uncertain (6). In preterm infants, the evidence that human milk may promote neurodevelopment and higher IQ is stronger (6, 7), but clear attribution of such finding, even if causal, to some components of breast milk (long-chain polyunsaturated fatty acids) is precluded since human milk contains many other substances not present in formulae (e.g. hormones) that may theoretically influence neurodevelopment (6, 8, 9). The cognitive development of an infant is a complex process influenced by multiple genetic and environmental factors that interact with one another (2). Because randomized controlled trials are not feasible in this area, except in preterm babies where “management trials” seem ethically justified (10–12), all of the cohort surveys and case control studies on full-term, healthy children analysed in the recently published reviews (2, 13–16) were observational in design. Observational studies, no matter how well controlled, restrict the validity of comparisons by potential inherent biases (5, 9, 16). Indeed, it is important to realize that the comparison of breast versus formula-feeding is complex because there are differences in what is fed, who feeds and how is it fed. The fact is that formulae differ from human milk in several nutritional components that may affect the biological basis of mental function, including hormones and growth factors; mothers who succeed at breastfeeding are different from those who feed formulae, and the process of breastfeeding itself has a profound effects on the behaviour and physiology of the mother and of the mother–infant dyad (17). Nonrandomized comparisons may be significantly flawed by the major sociobiological differences between the groups, including smoking, parental educational achievement, socioeconomic status (SES), family size and population density (9). In general, breastfeeding mothers have higher SES, better educational level, higher intelligence, fewer symptoms indicative of depression and greater preoccupation with infant development; they are less authoritarian, provide an nhanced home environment (5) and have more positive health behaviours (12). Therefore, adjustment is essential but often difficult, and failure to adjust generally favours an apparent breastfeeding benefit (9). Whether these are biological, nutritional or environmental effects, or some combination of these factors, is unclear (16). Is the association between breastfeeding and intelligence attributable largely to differences in intellectual ability and quality of parenting (genetic or socioenvironmental factors) between breastfeeding and non-breastfeeding mothers rather than to the nutritional benefit of breast milk on neurodevelopment?(4).