These have been defined as “non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon that can improve the host health”. They therefore encourage the growth of lactic-acid producing organisms such as lactobacilli and bifidobacteria that are already present in the gut. The majority of prebiotics are oligosaccharides (short-chain carbohydrates) of the monosaccharides fructose or galactose. Lactulose, an indigestible disaccharide sometimes prescribed as a laxative, may also act as a prebiotic. Other potential prebiotics include oligosaccharides from soy beans and a variety of man-made oligosaccharides. Insulin, found in Jerusalem artichokes, is a prebiotic that has been used to study the effects of these materials, and you will also sometimes see or hear the term “FOS” (fructo-oligosaccharides). Most of the substances now presented as prebiotics certainly provide a substrate for gut bacteria. The presence of these and similar materials has often been given as the explanation for the flatulence experienced after consumption of such foods as onions, beans, or Jerusalem artichokes; this occurs when the bacteria ferment the carbohydrate that our own enzymes cannot break down.
The most common use for prebiotics, especially insulin, at the moment is as a dietary fiber, bulking agent, and fat replacer in foods, and in future the food uses are likely to increase. There have been few studies of the potential benefits, and although a mixture of oligosaccharides, including insulin, has been shown in one study to lower plasma cholesterol, further research will be necessary to investigate other beneficial effects.
In some countries “synbiotics” (a mixture of prebiotic material and probiotic bacterial culture) are being added to foods. The assumption here is that the prebiotic will encourage the growth of the accompanying bacteria in the gut. So you might have prebiotic FOS in a food together with a Bifidobacter species. However, research is still needed to find appropriate mixtures, as different bacteria may need different prebiotics to be effective.