The cohort study by Julvez et al. extends the body of research that shows a link between breastfeeding and neurodevelopment. Although it is well established that breastfed children have better neurodevelopment, it has been contended that this could be due to confounding factors such as socio-economic status and maternal IQ, which are fairly easy to control for, and less easily controlled factors such as mother-child bonding. Randomized controlled trials in this area are not feasible or ethical. However, one cleverly designed study randomized mothers to a breastfeeding promotion condition and showed superior neurodevelopmental outcomes with the higher numbers of mothers in the promotion arm who breastfed their children, although this latter study has been criticized for unblinded assessments of the children. This latest cohort study addressed the potential confounding of maternal bonding by measuring and controlling for maternal attachment and psychopathology, along with a comprehensive array of other variables including maternal education, IQ, socio-economic status, and environmental contaminants. The outcomes still showed a significant, independent link between exclusive, long-term breastfeeding and improved neurocognitive development in children at 4 years of age.
The developing brain relies on a supply of essential macro- and micronutrients, and will not develop optimally (or at all) without them. It is becoming increasingly accepted that absence of nutritional deficiency and observable outcomes (e.g. cretinism as a result of iodine deficiency) does not necessarily equate to optimal brain development. There is a continuum of nutritional sufficiency with a range between deficiency and optimal levels, and suboptimal levels can be observed in psychological performance before physical symptoms are apparent. This provides a plausible platform for observed links between breastfeeding and neurodevelopmental outcomes.
The cohort study published here found improved outcomes with more than 6 months’ exclusive breastfeeding. This is interesting given that a review for the World Health Organization identified that very little iron and zinc (essential for neurodevelopment) is provided by breast milk and the infant can rely on prenatally supplied stores for up to 6 months. However, the definition of exclusive breastfeeding in this study did not exclude non-milk liquids and allowed for ‘syrup forms of vitamins, minerals, and medicines’. In either case, this nutritional component does not fully account for nutrients supplied by infant formula, which is increasingly including nutrients of previously unknown importance such as omega-3 fatty acids. Are there other compositional factors that contribute to associations between extended breastfeeding and neurodevelopment?
Recent years have seen an explosion of interest in the tens of trillions of microbiota that inhabit the human body (outnumbering our own cells), and a growing understanding of the numerous important roles they play in human health – including micronutrient synthesis. It is suggested that the adult microbiota profile is largely determined in infancy. The strongest determinants of beneficial infant gut microbiota are method of delivery (vaginal home birth) and breastfeeding. In terms of neurodevelopment, the gut-brain axis is of interest, given the rich and complex concentration of microbiota in our gut and the enteric nervous system, which has been described as our ‘second brain’. Animal studies have shown altered emotional responses and brain biochemistry with modification of gut flora. A recent study was the first to show this in humans: brain regions controlling emotional and sensory processing were altered in a group of healthy females provided with a probiotic yoghurt formula compared with controls, showing a link between gut microbiota and brain function. It is possible, therefore, that the beneficial microbial environment provided by breast milk may play a role in influencing children's neural development.
Although there is still much to understand about mechanisms of benefit, this latest cohort study combined with growing understanding of the beneficial properties of human breast milk provide further support for clinicians to recommend exclusive, long-term breastfeeding to mothers of newborn infants. It is possible that supplementary liquid nutrients are needed after 6 months; as this does not appear to have been factored into the present study, this is yet to be elucidated.