In contrast to the other components of breast milk that are absorbed by the infant through breastfeeding, HMOs are indigestible for the nursing child. However, they have a prebiotic effect and serve as food for intestinal bacteria, especially bifidobacteria. The dominance of these intestinal bacteria in the gut reduces the colonization with pathogenic bacteria (probiosis) and thereby promotes a healthy intestinal microbiota and reduces the risk of dangerous intestinal infections. Recent studies suggest that HMOs significantly lower the risk of viral and bacterial infections and thus diminish the chance of diarrhoea and respiratory diseases.
This protective function of the HMOs is activated when in contact with specific pathogens, such as certain bacteria or viruses. These have the ability to bind themselves to the glycan receptors (receptors for long chains of connected sugar molecules on the surface of human cells) located on the surface of the intestinal cells and can thereby infect the cells of the intestinal mucosa. Researchers have discovered that HMOs mimic these glycan receptors so the pathogens bind themselves to the HMOs rather than the intestinal cells. This reduces the risk of an infection with a pathogen. It has also been demonstrated that HMOs can bind to several intestinal viruses, such as norovirus and Norwalk virus, moreover they can reduce the virus load from influenza and RSV.
In addition to this, HMOs seem to influence the reaction of specific cells of the immune system in a way that reduces inflammatory responses. It is also suspected that HMOs reduce the risk of premature infants becoming infected with the potentially life-threatening disease necrotizing enterocolitis (NEC).
Some of the metabolites directly affect the nervous system or the brain and can sometimes influence the development and behavior of children in the long term. There are studies that indicate certain HMOs supply the child with sialic acid residues. Sialic acid is an essential nutrient for the development of the child’s brain and mental abilities.
In experiments designed to test the suitability of HMOs as a prebiotic source of carbon for intestinal bacteria it was discovered that they are highly selective for a commensal bacteria known as Bifidobacteria longum biovar infantis. The presence of genes unique to B. infantis, including co-regulated glycosidases, and its efficiency at using HMOs as a carbon source may imply a co-evolution of HMOs and the genetic capability of select bacteria to utilize them.