PREBIOTIC MECHANISMS OF ACTION11

Food ingredients and novel compounds are increasingly being examined for their ability to do more than provide nutrition, according to George Fahey. Most of this expanding research activity is focused on health promotion or disease reduction. At the top of the list of food ingredients being studied for nonnutrition activity are the nondigestible oligosaccharides (NDOs). Nutritionally, NDOs are known mostly for their low caloric value and ability to enhance mineral absorption, but they are also becoming known for their potential to lower the risk of infections and diarrhea, modulate the immune system, and modulate the microbiota. At the beginning of the prebiotic era, in the mid-1990s (Gibson and Roberfroid, 1995), scientists spent a great deal of time in particular thinking about how NDOs might be used to increase the presence of beneficial bifidobacteria (members of the genus Bifidobacterium) and lactobacilli (members of the genus Lactobacillus) in the microbiota while decreasing the presence of pathogenic bacteria. Many NDOs have the ability to alter the composition of the colonic microbiota in a positive manner, thus satisfying a key criterion for what defines a prebiotic—the selective stimulation of growth and/or activity of those bacteria that contribute to colonic and host health.

What Are the Major Dietary Sources of Prebiotics?

There are several well-established major dietary sources of prebiotics, primarily fructins (including chicory root extract, inulin, oligofructose, and short-chain fructooligosaccharides). Two other major dietary sources of prebiotics are the galactooligosaccharides and the stool softener lactulose.

There is a long list of potential prebiotic candidates, including soybean oligosaccharides, glucooligosaccharides, cyclodextrins, gentiooligosaccharides, oligodextrans, glucorinic acid, pectic oligosaccharides, isomaltooligo-saccharides, lactosucrose, xylooligosaccharides, human milk oligosaccharides, mannanoligosaccharides (yeast cell wall), lactose, resistant starch and derivatives, oligosaccharides from melobiose, N-acetylchitooligosaccharides, poly-dextrose, sugar alcohols, and konjac glucomannan. These are widely variable types of compounds, Fahey noted. Several are natural ingredients (e.g., soybean oligosaccharides), and several are widely used in both human and animal diets (e.g., yeast cell wall, which is very rich in mannanoligosaccharides). Some are very simple from the point of view of chemical composition (e.g., glucooligosaccharides); others are “really strange,” according to Fahey (e.g., N-acetylchitooligosaccharide). These and other candidates are considered “potential” because research on their prebiotic characteristics is incomplete,

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11 This section summarizes the presentation of George Fahey.



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