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Definitions and background
Prebiotics are 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, and thus improve host health (Gibson and Roberfroid, 1995).
Most potential prebiotics are carbohydrates, but the definition does not exclude non-carbohydrates to be used as a prebiotic. In theory, any antibiotic that would reduce the number of potentially harmful bacteria and favour health promoting bacteria or activities, can be considered a prebiotic.
The definition does not emphasize a specific bacterial group. Often, however, it is assumed that a prebiotic should increase the number and/or activity of bifidobacteria and lactic acid bacteria, as these groups of bacteria are claimed to have several beneficial effects on the host. A product that stimulates (or claims to stimulate) bifidobacteria is considered a bifidogenic factor. Some prebiotics may thus also act as a bifidogenic factor and vice versa, but the two concepts are not identical, see intermezzo below.
Prebiotics, related terminology and misunderstandings
Any component stimulating the growth of bifidobacteria.
Non-digestible oligosaccharides (NDOs):
Short-chain carbohydrates that are not digested in the small intestine and enter the large intestine unaltered.
Prebiotics are bifidogenic:
To have a beneficial effect on the host, it is not necessary to stimulate bifidobacteria. Stimulation of other species may also have beneficial effects. Prebiotics thus are not necessarily bifidogenic.
Bifidogenic factors are prebiotics:
A bifidogenic factor increases bifidobacteria either in the intestine, or in other conditions (fermented dairy products for example). When bifidobacteria in the intestine are stimulated, a bifidogenic factor may be considered prebiotic, but only if this stimulation has a beneficial effect on the host. In other conditions, bifidogenic factors are not considered prebiotics.
NDOs are bifidogenic or prebiotics:
NDOs may stimulate beneficial bacteria, which may result in beneficial effects. If so, they can be considered prebiotic.
NDOs may also stimulate unwanted bacteria and thus have negative effects (for example in lactose intolerance).
NDOs may stimulate bifidobacteria and thus be considered bifidogenic, which may or may not be considered prebiotic.
NDOs may also have no effect at all on the intestinal flora.
Thus depending on the type, NDOs may be prebiotic, bifidogenic, harmful or inert.
The definition also states that a change in metabolic activity may result in an improvement of the host health. This means that no specific group of bacteria has to be stimulated, but that the metabolic activity of the intestinal flora as a whole has to be modified. This generally means an increase in carbohydrate fermentation and a decrease in protein degradation and fermentation.
Carbohydrate fermentation generally results in harmless or even beneficial end products, whereas protein fermentation results in the production of potential harmful products, see figure below.
Figure 1. Schematic fermentation of protein and carbohydrates by the intestinal microflora. Protein and fermentation products mainly derived from protein fermentation are underlined. SCFA = Short Chain Fatty Acids, BCFA = Branched Chain Fatty Acids
The gas H2S is very reactive and can have negative effects on the intestine. The other gases, hydrogen (H2), CO2 and methane (CH4) have no negative effects, except flatulence and bloating. SCFA and lactate are beneficial for both the intestinal microflora (lowering of the pH, making the intestine more acidic) and the intestinal cells (which need SCFA for energy). Ethanol is rapidly metabolised by other intestinal bacteria and has no effect on the host. BCFA, ammonia (NH3), amines, phenols and indoles are irritants to the intestinal cells, possibly mutagenic or may have a negative effect on the immune system in high concentrations
It is thus beneficial to increase the carbohydrate fermentation in the intestine and to suppress the protein fermentation. This change in metabolic activity, however, does not necessarily correlate with an increase in the numbers of the possible beneficial bacterial groups.
The concept of prebiotics thus is clear: increase beneficial bacteria and/or increase carbohydrate metabolism.
Effects of prebiotcs
Prebiotics are specifically targeted to act on the flora in the large intestine.
The following effects of prebiotics have been claimed:
- relief of constipation
- reduce intestinal pH
- restore intestinal bacterial balance
- effect blood cholesterol level
- reduce risk on colorectal cancer
- effects on the immune system
- better intestinal flora in infants
Relief of constipation.
This is a well-documented effect, which is similar as to the effect of dietary fibre. Many prebiotics are carbohydrates, which are fermented in the large intestine. With this fermentation gasses are produced, which increase the volume and reduce the transit time of the digesta in the intestine. Constipation is the result of a very slow intestinal passage (slow transit time). Reducing the transit time thus helps against constipation.
In addition to this, many carbohydrates also increase the water content of the intestine and the acids produce increase intestinal motility. These two effects also reduce the transit time.
The best effects are obtained by carbohydrate mixtures, which include dietary fibres. It is based on a change in metabolism (more carbohydrates) and not on the stimulation of specific bacteria.
Reduce of intestinal pH
This effect is also due to a change in metabolism from protein fermentation (resulting in ammonia and high pH) to more carbohydrate fermentation (resulting in acid). Several intestinal diseases, such as Crohn's disease and IBS (irritated bowel syndrome), are characterized by a high pH. Reducing the pH thus reduces the symptoms of this disease (not curing!), which is beneficial to the patient.
A low intestinal pH also increases bowel movement and may protect against pathogenic bacteria.
The best effects are obtained by rapidly fermenting carbohydrate mixtures. It is based on a change in metabolism (more carbohydrates) and not on the stimulation of specific bacteria.
Restore intestinal bacterial balance
Prebiotics may restore intestinal balance after a disturbance due to antibiotics, diarrhoea, stress or other drugs (non antibiotics). By either selectively stimulating a certain group of bacteria the balance can be restored. This may be possible for many different bacterial groups. This can be either by direct stimulation (the selected bacteria grow on the prebiotic) or by indirect stimulation (the bacteria create a favourable environment for other bacteria).
In this case both selective stimulation and changes in metabolism play a role.
Effect on blood cholesterol level
A beneficial effect has been claimed for some oligosaccharides, but this effect has not been proven and, when described, is not very high.
Reduce risk on colorectal cancer
This effect has not been proven. However, there are many indications that the products formed by protein fermentation may increase the risk of colorectal cancer. Reducing the protein fermentation thus reduces the risk. In addition, the products from carbohydrate fermentation may reduce the risk on colorectal cancer.
Again, the best risk-lowering effect may be due to changes in metabolism, not by changes in specific groups of bacteria.
Effects on immune system
Prebiotics itself will have no effect on the immune system. However, by changing the intestinal flora, the immune system may be influenced. This stimulation may be beneficial or non beneficial. Beneficial would mean that the immune system is activated against possible pathogens. Non-beneficial may be that allergic reactions are increased due to the stimulation.
There have been many studies on the effect of the intestinal microflora on the immune system. However, at present no prebiotic substances are known that beneficially affect the immune system.
Better intestinal microflora in infants
The intestinal microflora of children under 4 years is very instable. Many oral pathogens may disturb the microflora. Substances that stabilize the flora can thus be considered prebiotics. There have been several studies that commercial oligosaccharides have a stabilizing effect, which is a combination of changing the flora and reducing the pH.
Most commercial prebiotics are oligosaccharides and dietary fibres. Selected non-digestible oligosaccharides will increase the carbohydrate fermentation and thus be effective for most of the claims described above. However, it should be kept in mind that not all oligosaccharides will have a beneficial effect. To have an effect a (mixture of) oligosaccharide(s) should be thoroughly tested in animals and humans.
At present, there are no oligosaccharides that selectively stimulate a certain group of bacteria. It has been claimed that several oligosaccharides selectively stimulate bifidobacteria and lactic acid bacteria, but experiments in vivo (in humans) and in vitro (laboratory) have shown that all commercial oligosaccharides are fermented by a large number of bacterial species in the intestine.
All commercial oligosaccharides, when effective, thus act by changing the metabolism, not by selectively stimulating a certain bacterial group.
As the commercial oligosaccharides increase carbohydrate fermentation, they also increase gas formation (see figure above). This means that the main side effects are flatulence and bloating. These effects may be present at 5 grams in sensitive persons, but absent at 40 grams in tolerant persons. The side effects are thus due to the type of oligosaccharide and the tolerance of the host.
A normal diet provides between 5 and 10 grams of non-digestible carbohydrates per day. This includes oligosaccharides from vegetable origin (mainly fructo-oligosaccharides). Effective doses of oligosaccharides are in the range of 5-10 grams/day for healthy adults. Doses under 5 grams are generally considered ineffective.
Some commercial oligosaccharides:
Beneficial effects described
Fructo-oligosaccharides (FOS) and inulin
Oligofructose, raftilose, actilight, frutafit, frutalose
yes (in animals)
Prebiotics are specifically targeted to act on the flora in the large intestine. Probiotics (link to probiotics) are designed and most effective in the small intestine.
Synbiotics are a combination of a prebiotic and a probiotic in a single product. The idea is that the product contains a beneficial agent in the small intestine (the probiotic) and one for the large intestine (the prebiotic). The two thus act synergistically, hence syn-biotics.
A common misunderstanding is that the prebiotic should stimulate the probiotic (often referred to as sym-biotics, from symbiosis). As by stimulating the probiotic the prebiotic is used, it will have no effect in the large intestine, and thus this concept is not useful.
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