A very promising alternative increasing in application in food products in the U.S. and Europe is the use of prebiotics. Prebiotics are non-digestible carbohydrates, usually oligosaccharides, which survive digestion in the stomach and small intestine and enter the colon. The important feature of prebiotics, however, is that they are then selectively fermented by the indigenous “probiotic” bacteria, increasing their population and activity. The ingestion of prebiotics has repeatedly been shown to alter the composition of fecal bacteria. Generally, a positive effect would be to see increases in the populations of lactobacilli and bifidobacteria and decreases in less desirable organisms such as clostridia and protein-degrading bacteroides. These organisms produce toxins and tumor-promoters from metabolism of proteins that have escaped digestion in the upper gut. On the contrary, bifidobacteria and lactobacilli produce a complex range of compounds which inhibit the growth of invading pathogens. They also are capable of modulating the activity of the immune system.
In the U.S. and Europe, there are a limited range of prebiotics available for use in food processing. These include fructans and galacto-oligosaccharides. The fructans are either inulin, derived from chicory, or fructo-oligosaccharides (FOS). FOS can be made from chicory inulin by partial hydrolysis or from sucrose by enzyme transfer reactions. Galacto-oligosaccharides (GOS) are manufactured from lactose by enzyme transfer reactions and thus represent a potential economic upgrading of a by-product from the dairy industry. The disaccharide lactulose is also a prebiotic at sub-laxative doses. It is not, however, used in foods at the present time, presumably as it commands a higher price as a medicine for treatment of constipation and hepatic encephalopathy.
Of these prebiotics, it is the fructans that currently have the greater weight of evidence of effect from human studies. There is now extensive literature on the prebiotic properties of FOS and inulin and the health consequences of consumption. In studies in the laboratory, in small animals and in humans, both inulin and FOS consistently display a prebiotic effect. Surprisingly, GOS have received much less attention than the fructans. In laboratory studies using fecal culture, they consistently display prebiotic effects, but these have not always been reproduced in human volunteer trials. Undoubtedly, there is a need for more research on GOS as functional food ingredients.
There is a much wider range of prebiotic ingredients available in Japan, as well as development of prebiotic food products in Japan. These include isomalto-oligosaccharides (IMO), soya oligosaccharides (SOS), xylo-oligosaccharides (XOS), lactosucrose (LS) and gentio-oligosaccharides (GeOS). For some of these, there is only scant experimental data in humans. In many cases, human studies have been poorly controlled and have involved a small number of subjects. However, it is possible some of these ingredients may have very desirable health or functional properties and receive renewed attention by food scientists. IMO, for instance, have been given approval in the E.U., although they have not yet been taken up commercially.
There are many health effects ascribed to prebiotics. Most of these are thought to be mediated by the indigenous microbiota, particularly bifidobacteria and lactobacilli. Some of the most extensively researched include:
* Antipathogen Activities. Many bifidobacteria and lactobacilli are known to produce antimicrobial compounds such as organic acids and antimicrobial peptides. Such activity can be seen in pure culture and in fecal cultures. In laboratory studies, treatment of fecal cultures with prebiotics frequently decreases the survivability of added pathogens such as E. coli, Salmonella or Campylobacter, and similar effects have been seen in animal models. It is very difficult to substantiate this effect in humans, and we have little data to prove this effect works on human consumption.
* Increased Mineral Absorption. Ingestion of prebiotics increases uptake of minerals such as calcium and magnesium from the colon. Feeding studies have shown that consumption by adolescent girls of inulin together with calcium supplements can significantly increase calcium uptake by around 18%. A long-term feeding study has confirmed these findings and shown that bone mineral density also is increased. Given the potential impact on osteoporosis in later life, this is an area that requires further study.
* Effects on Colon Cancer. Many of the non-probiotic organisms, resident in the human colon, produce genotoxins and tumor promoters, largely from the metabolism of proteins. Such metabolites are not produced from bifidobacteria and lactobacilli. Consumption of prebiotics might, therefore, be expected to reduce the levels of these undesirable metabolites and, perhaps, have an impact on the chances of development of colon cancer. Laboratory studies with fecal cultures have shown that prebiotics do, indeed, reduce the levels of these compounds, and studies in animals and humans have shown reductions in biomarkers of cancer.
What the Future HoldsA very exciting area of development is the rational combination of prebiotics and probiotics. Such a combination is known as a synbiotic. With a synbiotic we have the possibility of combining a well-developed probiotic strain (with known health benefits) with a prebiotic (which should provide a selective advantage to the probiotic when it is consumed). Commercial application of synbiotic products is in its infancy, but this approach shows much promise for developing advanced products with defined health benefits. The potential is to develop prebiotics targeted at specific probiotic strains to optimize the health benefits. This approach is being investigated for application in elderly individuals, formula-fed infants and companion animals.
So, what does the future hold for prebiotics? There undoubtedly will be significant development in this area in the next few years, as manufacturers catch onto the benefits of prebiotics. There already is much research at the laboratory scale on the development of novel, enhanced prebiotics. These include ones with more precisely defined health benefits, multiple functionality, some targeted at specific food products and others targeted at specific probiotic strains. Microbiologists who study the human colonic microbiota now have a range of sophisticated DNA-based techniques with which to characterize the microbial diversity present. Similarly these can provide a much more accurate picture of the results of dietary interventions. These molecular techniques are particularly relevant to the human gut, where many of the organisms present are not cultivable using traditional microbiology techniques. Novel species are being discovered all the time, at a rate much faster than their functional significance in the gut can be defined. It is clear the targets of prebiotic intervention will not be restricted to bifidobacteria and lactobacilli in the future. An example of this is the desirable microbial metabolite, butyric acid. Butyrate is a fuel for colonocytes and induces apoptosis, or programmed cell death; therefore, it may play a role in inhibiting the development of colon cancer. Bifidobacteria and lactobacilli do not produce butyrate; however, other members of the colonic microbiota do, and there is interest in stimulating butyrate production in the colon.
Another trend is to look at food processing by-products as sources of novel prebiotics. For instance, wheat bran, citrus waste and sugar beet pulp are being evaluated for their potential to be sources of prebiotic oligosaccharides. Wheat bran can be processed to produce arabinoxylans and sugar beet pulp to give arabino-oligosaccharides. Both of these are undergoing evaluation for their prebiotic functionality. In some instances, we may be able to generate prebiotics with multiple functionalities. For example, pectic oligosaccharides derived from citrus processing prevent the adhesion of certain pathogens and toxins to cells. They also seem to mimic butyrate and stimulate apoptosis in colon cancer cells. Multiple activities will expand the definition of prebiotics in the future.
With increasing commercial application of prebiotics in the market, some of these “next generation” prebiotics will no doubt make the transition from the laboratory to the supermarket.
Website Resources:http://jn.nutrition.org/cgi/content/full/129/7/1478S — Pro- and prebiotics and colon cancer inhibition
http://content.karger.com/ProdukteDB/produkte.asp?Aktion=Citation&ArtikelNr=90345&ProduktNr=228391&Ausgabe=231585 — Article: “Probiotics and Prebiotics in Human Health”
Sidebar: Area to WatchAn area of growing interest is the use of prebiotics in pet foods. This application is not yet very well developed, partly due to economic considerations and partly due to deficiencies in our understanding of the gut microbiology of companion animals. There is currently much research being carried out in this area, however, and the market certainly will see developments in the future.
Editor's note: Chicory root has now been successfully launched in numerous Nestlé Purina pet foods around the world. Brands include Winalot, Vital Balance, Lucky Dog and ALPO. It has also been incorporated into the Pro Plan Indoor Cat Formula.