Adapted from Campbell J, Bauer L, Fahey G, Hogarth AJCL, Wolf B, and Hunter D (1997) Journal of Agricultural and Food Chemistry 45: 3076-3082.

not recovered in the fecal mass indicating they are completely fermented in the colon. In many ways, prebiotics behave as a form of dietary fiber that has specific effects on colonic flora.

The mother's milk is a key factor in the early establishment of the infant's colonic environment. Up to 10% of the carbohydrates in human milk are not lactose, and human milk contains high concentrations of other carbohydrates and glycoconju-gates that fall under the general category of prebiotic food substances. The monosaccharides of breast milk include D-glucose, D-glactose, sialic acid, L-fructose, and N-acetylglucosamine. Chain lengths range from three to ten with the majority having a lactose end. Combinations of these monosaccharides result in more than 130 varieties of oligosaccharides in human milk. These galacto-oligosaccharides in breast milk have lactose as their reducing end. Many human milk oligosaccharides elongate by enzymatic attachment of N-acetylglucosamine linked to a galactose residue. Several of these carbohydrates, including N-acetylglucosamine, are considered 'bifidus factors' or 'bifidogenic,' increasing the growth and establishment of bifidobacteria in the intestine of the breast fed infant. Human milk oli-gosaccharides also appear to prevent attachment of pathogenic microorganisms by competing with epithelial ligands for bacterial binding sites. Several types of human milk oligosaccharides appear to be bacteria specific. For example, sialyated oligo-saccharides inhibit attachment of Pneumococci and influenza viruses, while galacto-oligosaccharides and fructosylated oligosaccharides can inhibit E.coli attachment. The bifidogenic effects, as well as those of direct interaction with the intestinal mucosa, are considered to be some of the mechanisms by which these agents confer a protective effect on the lactating infant.

Oligosaccharide content of the breast milk varies among individuals and within an individual. Levels are highest in the newborn period peaking after 5 days and slowly declining through the first 3 months. The levels of oligosaccharide in the breast milk also are dependent on time of feeding and generally are higher at the beginning of the feed.

Other, less well-studied oligosaccharides including maltose, soya, and xylose-oligosaccharides have some effect on increasing microbe colonization; however, they are weak prebiotics because of the lack of specificity of their fermentation.

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