2008 年 56 巻 4 号 p. 155-176
Mature human milk and colostrum contain 12 to 13 g/L and 22 to 24 g/L of oligosaccharides, respectively. Oligosaccharides constitute the third largest component, after lactose and lipids, of the dry matter of human milk. The structures of at least 93 human milk oligosaccharides have been determined to date, while mass spectra data have suggested the presence of more than 130 oligosaccharides in human milk or colostrum. The 93 oligosaccharides can be grouped into 12 series based on their core units. These oligosaccharides are resistant to enzymatic hydrolysis by the intestinal lactase of brush border and there is evidence that the major part survives passage through the small intestine and enters the colon where they are fermented by colonic bacteria. A small fraction of human milk oligosaccharides is absorbed intact, perhaps by receptor-mediate endocytosis, some of which is excreted in the urine. At the colon, the unabsorbed oligosaccharides are believed to act as prebiotics, stimulating the growth of bifidobacteria, and as soluble receptor analogues that inhibit the attachment of pathogenic microorganisms to the colonic epithelial cells. There is an evidence that metabolic activity of the bifidobacteria reduces the colonic pH, which has the effect of inhibiting the proliferation of pathogenic organisms such as Shigella flexneri and Escherichia coli. Although a specific oligosaccharide responsible for stimulating the growth of bifidobacteria was not identified, Kitaoka et al. recently, presented a new hypothesis, based on a novel metabolic pathway for galactose in bifidobacteria, which proposed that Gal(β1-3) GlcNAc(lacto-N-biose I) structures, which are found in type I human milk oligosaccharides, act as specific bifidus factors. Pathogenic bacteria and viruses, to begin their infection, need to attach to the colonic mucosa, which they do by adhering to specific carbohydrate structures of glycoconjugates on the surface of the colonic epithelial cells. Because many milk oligosaccharides contain structural units that are homologous to these carbohydrate structures, it has been suggested that they act as soluble receptor analogues, inhibiting the adhesion of the pathogens, these preventing infection. The observation of 2′-fucosyllactose inhibits the binding of Campylobacter jejuni to H(O) antigen (Fuc(α1-2) Gal(β1-4)GlcNAc) in the infant colon is noteworthy, because this trisaccharide is the most abundant oligosaccharide in human milk. The absorption of small part of human milk oligosaccharides follows that they may alter protein-carbohydrate interactions at systemic level. For example, recent studies suggest that human milk oligosaccharides interfere with the adhesion of neutrophils to vascular endothelial cells and to platelets. These effects appear to be based on structural resemblance of some human milk oligosaccharides to the glycoprotein ligands of selectins.