Sources and properties of nondigestible oligosaccharides

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7.3.1 Definition

Following worldwide authorities on chemical nomenclature and terminology (i.e. the International Union of Biochemistry (IUB) and the International Union of Pure and Applied Chemistry (IUPAC), an oligosaccharide is a molecule containing a small number of monosaccharide residues (degree of polymerisation, DP from 2 to 10). NDOs resist hydrolysis and digestion in the human digestive system and are partially or completely fermented by the colonic microbiota in the large intestine.

7.3.2 Chemical structure and origin

Several NDOs are considered DFs, itself a broad category. As illustrated in Table 7.1, NDOs may occur naturally in many plants - mainly vegetables, whole grains and fruits (Meyer 2004). Another natural source of NDOs is milk (cow milk galacto-oligosaccharides and human milk oligosaccharides). Moreover, several NDOs - often added in food for their technological properties - may be synthesised from simple or complex carbohydrates. NDOs present in the diet differ from one another in their chemical structure, in other words the number (DP) or the type of hexose moeties (glucosyl-, fructosyl-, galactosyl-, xylosyl-), the position of links between the hexose moeties and their conformation (a- versus P-) (Delzenne 2003). All these characteristics have consequences on the physical properties of NDOs - and therefore on their putative usefulness as food ingredients and their effects and metabolism in the gastro-intestinal tract. Owing to interest in their nutritional properties, biotechnology (enzymatic or thermal processes) has been applied to obtain new types of NDOs either through enzymatic synthesis from simple sugars, or enzymatic hydrolysis from more complex carbohydrates (Murphy 2001). Short-chain fructo-oligosaccharides (FOS), for example, may be obtained by synthesis from saccharose, or through controlled and partial hydrolysis from chicory root inulin (Roberfroid & Slavin 2000).

FOS are often cited as the most important dietary oligosaccharides. They may be of plant origin (garlic, onions, banana, artichoke, chicory root and cereals) (Delzenne 2003). FOS are a linear polydisperse carbohydrate material consisting mainly, if not exclusively, of fructose residues linked by P-2,1 fructosyl-fructose linkages. The generic term for FOS, namely fructans, includes the oligofructose (OFS) (DP 4), high-molecular-weight inulin (Inu) (DP 25) and a Synergy 1 (Syn) consisting of a mix of OFS and Inu (DP 2-60).

Although specific attention is often given to FOS, one must not forget that human milk, remains another 'natural' source that is extremely rich in diverse oligosaccharides. Their presence could explain some of the many health-promoting effects of breast feeding.

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Table 7.1 Dietary NDOs available in food products

Type of NDO

Natural occurrence

Industrial production process

Fructo-oligosaccharides

Onions, banana, garlic, Jerusalem artichokes

Synthesis from saccharose

Hydrolysis from chicory-root inulin

Galacto-oligosaccharides

Cow milk, soybean, legumes

Enzymatic process from lactose

Isomalto-oligosaccharides

Miso, soy sauce, sake, honey

Hydrolysis or glycosyl transfer from starch

Palatinose or isomaltulose

Honey, cane juice

Produced from sucrose by a-glucosidase reaction of

Protaminobacter rubrum

Theanderose

Sugar, honey

Produced using fungus Mucor javanicus

Lactulose

Synthesis from lactose

Lactosucrose, glycosylsucrose

Synthesis from saccharose and/or lactose

Xylo-oligosaccharides

Wheat bran

Hydrolysis from polyxylans

Stacchyose, raffinose

Soybean

Fibersol-2

Produced by pyrolysis of corn starch

Glucomannan

Konjac root

Cyclodextrin

Synthesis from starch

7.3.3 Technological and nutritional properties (including estimated intake)

NDOs are readily water soluble and exhibit some sweetness, which decreases with longer chain length, whereas water binding and gelling properties increase with the number of hexose molecules and reticulation. These properties, together with some interesting physiological effects (low caloric value - close to 1.5 kcal per gram of oligosaccharides, low cariogenicity, prebiotic effect, improvement of mineral absorption, etc.) are used to promote the addition of some NDOs to foodstuffs that normally contain low or no significant amount of such nutrients. Since they are added to foods, quantifying the dietary intake of NDOs in adult humans is difficult. One study, based on the intake of FOS in fruits and vegetable, has estimated the daily intake of inulin-type fructans for a 75-kg person to be 3.2-11.3 g in Western Europe, and 1-4 g in the United States, (van Loo et al. 1995). The usual intake of most other NDOs is difficult to assess due to the lack of adequate published data.

7.3.4 Effect of non-digestible oligosaccharides in the gastro-intestinal tract

NDOs, once ingested, resist digestion by hydrolytic enzymes secreted or active in the intestine, such as a-glucosidase or maltase/isomaltase. NDOs, which mostly escape the upper digestion, are important sources of energy for bacteria in the caeco-colon, which express enzymes such as P-fructosidase, P-galactosidase, xylanase or any other hydrolases (Bernalier et al. 1999). The ingestion of some NDOs may thus lead to the advantageous proliferation of certain types of bacteria, which are generally considered beneficial (e.g. bifidobacteria, lactobacilli) and suppression of more harmful bacteria. Dietary FOS have been demonstrated in many studies in animals and humans to (re)equilibriate the colonic biotope, first defined as the 'prebiotic effect' in 1995 by Gibson and Roberfroid, 1995. In addition, more recently, promising results have been obtained with other NDOs such as galacto-oligosaccharides (GOS) (Delzenne & Williams 2002). The dose and the duration of NDO intake, the place where fermentation mainly occurs (proximal or distal colon) as well as the initial composition of faecal flora, are important factors influencing the extent of the prebiotic effect -namely the increase in bifidobacteria (Rao 2001; Tuohy et al. 2001). On the basis of the results of well-designed human studies that have shown significant changes in the composition of human faecal flora, it can be concluded that FOS are prebiotic at a dose of between 5 and 15 g per day. For the GOS, an increase in bifidobacteria and lactobacilli in response to doses ranging from 3 to 10 g per day has been reported. A dose of isomalto-oligosaccharides of 13.5 g per day for 2 weeks significantly increased the number of bifidobacteria in adult and elderly volunteers (Roberfroid 2003).

For soybean oligosaccharides, a dose of 10 g given twice daily for 3 weeks significantly increases the number of bifidobacteria, while slightly decreasing clostridia counts; a dose of 3 g per day increases bifidobacteria, bacter-oides and eubacteria. A bifidogenic effect has also been demonstrated in humans after ingestion of lactulose, lactilol and xylo-oligosaccharides (Meyer 2004). A low baseline bifidobacteria count was significantly associated with the bifidogenic response to NDO ingestion (Bouhnik et al. 1999, 2004). Recentlys a new quantitative approach for determining in vitro pre-biotic potential of dietary oligosaccharides has been developed (Vulevic et al. 2004). The measure of the prebiotic effect (MPE) includes quantitative changes in the number of bacterial groups, fermentation end products, such as short-chain fatty acids (SCFAs), and substrate assimilation. Although the approach is not meant to define health values, it is formulated to better inform the choice of prebiotic NDOs.

The major products of NDO metabolism are SCFA and gases (hydrogen and carbon dioxide), produced through fermentation by colonic flora. The pattern of fermentation (the proportion of the different short-chain acids (acetate, propionate, butyrate, lactate) produced in the caecum and/or the colon) varies with the nature of the NDO, at least in animals. More butyrate is produced from FOS and lactitol than from raffinose (Nyman 2002). The proportion of SCFA also changes with the duration of the treatment, as was well illustrated by the study of Le Blay et al. (1999) performed with FOS. The SCFA are absorbed into the bloodstream of the host. Butyrate is absorbed and used by colonocytes as the preferred fuel, whereas the other SCFA are transferred to various organs - primarily the liver - where they enter different metabolic pathways. Interesting methods have been developed to determine SCFA turn-over in animals and human subjects, using stable isotopes (Pouteau et al. 2003).

SCFA have an important effect in the intestinal tract. It is largely accepted that butyrate exerts an essential role in maintaining the metabolism, and the proliferation/differentiation of different epithelial cell types. SCFA, and mostly butyrate, are often evoked to explain the role of NDO in gut immunity, or cell turn-over.

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