Various food components have been recognized to have prebiotic activity, including various fermentable carbohydrates (lactulose, gums, lactilol, soyoligosaccharides, galacto-oligosaccharides Xylo-oligosaccharides. However, the best studied of these have been those classified as dietary fructans. Dietary fructans can either be derived from naturally occurring oligosaccharides or can be artificially synthesized. These carbohydrates contain one or more fructosyl-fructose links that make up the majority of osidic bonds. They are linear or branched fructose (oligo)polymers with either ,3-2-1 linked inulins or ,-2-6 linked levans. These oligo-saccharides exist naturally in many plants including onions, garlic, the roots of Jerusalem artichoke, asparagus root, chicory root, and wheat (Table 1). Inulin is extractable from root plants particularly Jerusalem artichoke and chicory, while fructooligo-saccharide is hydrolyzed from inulin yielding a shorter chain sugar. It is the degree of polymerization (DP) that distinguishes the fructans. Fructo-oligosaccharides are /3-D-fructans with DP between 2 and 10 while inulin has DP 10-60. Essentially, they are sucrose molecules with 1-3 fructose units linked by a /3-(2,1)-glycosidic bond. Most oligo-saccharides are synthesized from sucrose and therefore usually have a terminal glucose end. Inulin, derived from chicory, is broken down using an inulase enzyme making a smaller (2-10) chain with lower DP (4). Oligofructose is a form synthesized from sucrose by /3-fructofuranosidase linking fructose monomers to sucrose.
Both inulin-derived and synthesized fructo-oligosaccharides have been shown to resist digestion in the upper GI tract. Ninety per cent of consumed inulin and fructooligosaccharide was excreted at the terminal ileum of adult ileostomy patients. Furthermore, the undigested oligosaccharides are
Table 1 Fructo-oligosaccharide (FOS) content of common fruits, vegetables, and grains
FOS concentration (mggm 1)
Apples Banana Banana, ripe Blackberry Orange, navel Peach
Raspberry, red Vegetables
Acorn squash Artichoke, globe Artichoke, Jerusalem Chicory root, raw Garlic Onion, red Onion, white Onion powder Peas, snap Peas, snow Shallot
Barley Oats Rye Wheat Wheat bran Wheat germ
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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