Human milk oligosaccharide
Human milk oligosaccharides (HMO, also known as human milk glycans) are sugar molecules, that are part of the oligosaccharides group and which can be found in high concentrations exclusively in human breast milk. 
Human milk oligosaccharides form the third most abundant solid component (dissolved or emulsified or suspended in water) of human milk after lactose and fat. HMOs are present in a concentration of 0.35 - 0.88 oz / L. Approximately 200 structurally different human milk oligosaccharides are known. The composition of human milk oligosaccharides in breast milk is individual to each mother and varies over the period of lactation. The dominant oligosaccharide in 80% of all women is 2'-fucosyllactose, which is present in human breast milk at a concentration of approximately 0.088 oz/ L.
In contrast to the other components of breast milk that are absorbed by the infant through breastfeeding, HMOs are indigestible for the newborn 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 ensures a healthy intestinal flora (intestinal microbiome) and a reduced risk of dangerous intestinal infections.
Recent studies also suggest that HMOs significantly lower the risk of viral and bacterial infections and thus diminish the chance to get 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. 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.
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