Breast Milk & Cognition
What is the importance of breast milk?
At birth 81% of UK mothers exclusively breast feed their offspring. This decreases to 17% at the 3 month stage postpartum this significantly drops as the months roll by. In this modern age as products are released that offer “safe” alternatives to breast milk, the question remains “What is so important about breast milk?”.
Breast milk is a protein and mineral rich containing concoction that is essential for the healthy growth and development of the newborn infant. The actual composition varies day-to-day depending on the mothers own diet and the environment. The generation of breast milk is controlled via the endocrine system and the hormones oxytocin.
What do we already know?
It has been known for a long time that breast milk has benefits to the immune system. First milk is known as “colostrum”, this is high in IgA or immunoglobin A that acts by coating the newborns gut, offering immuno-protection to the child while its own immune system is built up. It also acts as a mild laxative helping clear the babies gut of gunk.
There is some evidence that was first published in the journal Nature that describes for the first time how breast milk may be critical in programming the infant brain. The study carried out by Dr Bingfang Liu and colleagues highlighted the ever increasing links being discovered between the immune system and the neurological system of the mammalian body. Their report focussed on a specific cytokine protein (TNFα) Tumor Necrosis Factor α. They discovered through a series of experimental hypotheses that mothers lacking TNFα express breast milk that has a different composition to those mothers that have this protein. The composition of the milk differs through the immune factors that it contains. These immune system molecules are known as chemokines or cytokines, factors that are involved in initiating an immune response.
The team carrying out these investigations demonstrated, through the use of a mouse model, that the offspring that feed on this cytokine deficient milk develop a larger hippocampus. The hippocampus is a brain region that is essential in memory, learning and emotion.
In order to test this hypothesis and to rule out the in vivo environment of the offspring (as well as the offsprings own genotype) being the major causal effect, several cross fostering experiments were performed. Pups that were from TNFα deficient mothers were switched at birth with wildtype pups from wildtype mothers. The results were that pups raised by TNFα deficient mothers developed larger hippocampi and so outperformed pups with smaller hippocampi on spatial learning tasks such as the water mazes (or similar) regardless of their genotype. These findings were surprising in that they confirmed the proposed theory that suggested that improved spatial memory was not an artefact of the biomolecular environment of the womb (or indeed maternal care, which didn't differ between the two cohorts) but was in fact due to the molecular components found in the mothers milk.
This lead to the identification of what are thought to be the key molecular players whose absence is required to stimulate the generation of new hippocampi neurons in the offspring. These players have been narrowed down to five chemokines. This was achieved by use of a simple supplementation experiment. Pups being fed the deficient milk were given a cocktail of the five reduced chemokines whilst being fed the altered milk. This had the affect of reducing the growth of the hippocampi of these pups, confirming a pivotal role of the identified chemokines. Although the exact mechanism by which the milk works to improve the memory of the offspring is not known, it is thought that the lack of these specific chemokines not only stimulates neurogenesis in the hippocampi but also increases the number of connections per cell and the activity of synaptic genes in the brain.
This paper comes shortly after TNFα was shown to be expressed in low levels in the brain and adds another layer of complexity to our under standing of the inter-relationship between immune and neurological function. TNFα levels dramatically increase as a consequence of inflammation in microglia, astrocytes and neurons. Something that has been linked to neurodegenerative disease, head trauma and strokes. The concept that the chemical signals received by offspring in the early days/months after parturation can lead to the way in which their adult brain is programmed is fascinating and asks many questions such as cliche: why? and how? One potential reason is adaptive. The environment that the mother faces whilst pregnant and after birth can cause changes in her biochemistry. In this case stress and the inability to readily find food causes TNFα levels to fall. This in turn means that the brains of her young are adaptively programmed for a similar environment, where improved spatial memory may help them survive and reproduce.
Although at this time the pathway has yet to be confirmed in humans. It is evolutionary conserved and almost certainly plays a role in some way or other in the human condition. Whether this is directly through the breast milk or other means. It raises questions concerning modern immunosupressent drugs that are allowed to be taken by mothers during and after pregnancy. Many of these drugs work through the inhibition of TNFα binding, so too do some natural compounds, like those found in tumeric, for example. What are the potential knock on effects of such chemicals on the health and cognition of the offspring? Dr Liu and his team recognise further work is needed but for now the idea that the environment into which an organism is born programs the brain directly through a media like breast milk, in order to deal with the challenges it may face later in life is intriguing.