Breast milk is the result of thousands of years of mammalian evolution. It is so exquisitely designed by nature that it is specifically tailored for each mother-infant pair. Moreover, it is a dynamic biological fluid: its composition changes over time; within each feed and over the duration of lactation, adapting to the baby’s needs.
The composition of human milk is highly complex. It can be divided into two categories:
- Nutritional components: fats, proteins, carbohydrates
- Bioactive components: they have purposes other than feeding the infant, like the development of the immune system or the maturation of the gut microbiota
Nutritional components
Lipids
These are different types of fats and the main source of energy for the baby. On top of their nutritional value, breast milk lipids are believed to protect the gut mucosa from infections.
Some lipids are:
- Triacylglycerides – which constitute the highest proportion of lipids in human milk
- Fatty acids – There are over 200, the most abundant being oleic, palmitic and linoleic acid. The rest are found in very low concentrations.
- Short-chain fatty acids (SCFAs) – are important as a source of energy and for normal maturation of the gastrointestinal tract.
Proteins
There are more than 400 proteins in breast milk. They can be divided into three groups: caseins, whey and mucin proteins.
Apart from their nutritional value, they are also antimicrobial and modulate the immune system.
Carbohydrates
There is a large variety of them. Two examples are:
Lactose – is the most abundant, which is explained by the high energy the human brain demands.
Human milk oligosaccharides (HMO) – there are more than 200 types of and their production is genetically determined by the mother. They are indigestible for the infant but digestible for some beneficial gut bacteria, like Bifidobacterium infantis. In addition, the different HMO affects the types of microbes colonising infants and the timing of the establishment of the microbiota.
Bioactive components
Nucleotides – they have many beneficial roles in the infant’s development and they are also beneficial for the development of the microbiota.
Antibodies – there are different types of these and the most predominant in human milk is one called secretory IgA (SIgA). Colostrum has the highest concentration and it decreases as the milk matures with the baby’s age. This reflects their importance in protecting the newborn whose immune system is very immature. The antibodies protect the baby from infections and there are even specific SIgA for gut and respiratory pathogens.