2.4.1 Prebiotic definitions, characteristics and classes

Prebiotics are non-digestible, fermentable food components that result in ‘the selective stimulation of growth and/or activity of one or a limited number of microbial genera/species in the GI microbiota that confer health benefits to the host’ [5]. This is the most recent definition and updates earlier versions to account for the fact that prebiotics can influence the activity, as well as the numbers, of specific bacteria.

There are three essential characteristics of a prebiotic: (i) its resistance to digestion in the upper GI tract; (ii) its ability to be fermented by the host microbiota; and (iii) for this to impact on the growth or activity of specific bacteria only [5]. Resistance to small intestinal digestion is the result of humans lacking enzymes that hydrolyse the various polymer bonds. This allows the prebiotic to reach the colon intact and undergo fermentation, but only by a limited number of genera/species.

The most common classes of prebiotics are inulin-type fructans (e.g. inulin, oligofructose, fructo-oligosaccharides) and galactans (galacto-oligosaccharides, e.g. stachyose, raffinose) (Table 2.4.1). Lactulose, the synthetic non-digestible carbohydrate used as an osmotic laxative for constipation, is also a prebiotic but is rarely used in functional food preparations and therefore is not reviewed here. The potential prebiotic properties of other novel oligosaccharides (e.g. isomalto-oligosaccharides, soybean oligosaccharides) are also under investigation.

Inulin-type fructans (ITF) consist of linear polymers of fructose monomers joined by beta(2 → 1) linkages, some of which also have a terminal glucose monomer. The major ITFs are inulin, oligofructose (OF) and fructo-oligosaccharides (FOS), which are defined based upon their source and the number of monomers in the polymer chain (degree of polymerisation, DP) (see Table 2.4.1). In general, inulin has a wide-ranging polymer length (DP 2–60), whereas OF and FOS have a shorter length (DP 2–10) [5]. Inulin-type fructans are found in very large amounts in chicory root (35.7–47.6 g/100 g), which is the most common source for commercial preparations of inulin, but is also present in Jerusalem artichoke (16–20 g/100 g) and garlic (9–16 g/100 g) [6]. ITFs are found in small amounts in cereals such as wheat (1–4 g/100 g), which is actually the most common dietary source in the United Kingdom [7] and the United States due to its widespread consumption.

Galactans include the prebiotics galacto-oligosaccharides (GOS), which are short polymers (DP usually <10) of galactose monomers joined by beta(1 → 6), beta(1 → 3) and/or beta(1 → 4) linkages to a terminal glucose monomer [9]. Galacto-oligosaccharides are widely contained and consumed within pulses, where the type of linkages may differ.

2.4.2 Evidence of selective stimulation of gastrointestinal microbiota

The first key human study to demonstrate the prebiotic effect compared the impact of 15 g/day of oligofructose or inulin, in a randomized, cross-over trial of eight healthy people consuming a controlled diet. Both compounds resulted in almost a 1 log₁₀ increase in luminal bifidobacteria [10].

Many in vitro, animal, human and even some clinical studies have now demonstrated the ability of ITFs and GOS to stimulate the growth of bifidobacteria (the prebiotic effect), and these have been extensively reviewed and tabulated elsewhere [5]. Indeed, some studies in healthy humans have also shown increases in other bacteria including lactobacilli [10] and Faecalibacterium prausnitzii [11]. Although not frequently investigated in vivo, the increase in bifidobacteria following prebiotic supplementation (sometimes referred to as bifidogenesis) has been shown to be at the expense of a range of other bacteria including reductions in clostridia [10]and bacteroides [12]. Interestingly, the majority of prebiotic studies use supplements and the effect of prebiotics naturally occurring in foods has received little attention in the literature. Therefore, the effects of a diet high in ITFs (e.g. chicory, onion, wheat) and GOS (e.g. beans, pulses) are as yet unclear.

The mechanism of how prebiotics are able to selectively stimulate the growth of specific bacteria has until recently received little attention in the literature. In one study it was shown that some bacteria have ‘fructan utilisation locus’ genes that enable them to acquire and ferment ITFs [13]. Studies in mice have shown that the functional expression of the ‘fructan utilisation locus’ in different bacteria was highly predictive of the ability of that strain to be selectively stimulated when the mice were fed inulin [13]. Therefore it is thought that during prebiotic supplementation, bacteria with the ability to express genes coding for fructan utilisation are better able to access the carbon source in the ITF, use it for bacterial metabolism and therefore are better able to proliferate, a process of competitive selection.

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