Prebiotics are non-digestible carbohydrates, that are able to act as a food source for colonic microorganisms and favourably influence populations of beneficial bacteria. Fructooligsaccharides (FOS) have been extensively studied for their prebiotic effects and beneficial properties have been reported. However, other non-digestible oligosaccharides may also have health benefits. For example, arabino-xylooligosaccharides (AXOS) are a non-digestible carbohydrate which are fragmentation products of arabino-xylans (AX) from the outer wall of certain cereal grains. Arabino-xylans are constructed of a main chain of β(1-4) linked D-xylopyranosyl units with O-2 and O-3 L-arabino-furanosyl linked units, with the average degree of polymerisation being 3000 to 6000. Fragmentation of AX using endo-β-1,4-D-xylanase produces AXOS, which are more readily fermented by colonic bacteria due to their lower molecular weight. Graded ethanol precipitation and ultracentrifugation can further isolate the AXOS, with the arabinose to xylose ratio (degree of substitution) depending on the exact fragmentation process.
The effects of AXOS on gastrointestinal motility and colonic bacterial metabolism has been investigated in healthy humans. One study used AXOS with an average degree of polymerisation of 15, and average degree of arabinose substitution of 0.26 (AXOS-15-0.26). Five test meals containing different quantities of AXOS-15-0.26 (0.0, 0.2, 0.7, 2.2 and 4.9g) were administered to 12 healthy volunteers with 1 week wash-out periods between tests. The results showed that the gastric emptying and oro-caecal transit time (OCTT) of the subjects (as measured by breath tests) were not influenced by the quantity of AXOS in the test meal. However, the 2.2 and 4.9g AXOS containing test meals increased urinary 15N-excretion and significantly decreased faecal 15N-excretion. The shift from urinary to faecal 15N-excretion suggested a higher uptake and incorporation of ammonia by bacteria due to an increase in growth or metabolism. A significant increase in hydrogen excretion after 2,2 and 4.8g AXOS may have resulted from bacterial growth.
These results suggest that ~2.2g of AXOS is required to have a significant effect on the growth and metabolism of colonic bacteria. The lack of effects of the AXOS on gastric emptying and gut motility suggests that the effects of increased colonic fermentation were not sue to an increased rate of supply of the oligosaccharides. Therefore AXOS may provide benefits as a prebiotic in humans. A healthy colon is important for health because gut bacteria can improve immunity, provide vitamins to the host, produce short chain fatty aids for energy and can inhibit the growth of potential pathogenic bacteria through competition for substrates. Overgrowth of pathogens such as yeasts and pathogenic bacterial species in the colon is increasingly being associated with disease and poor health. The use of prebiotic substances is one method to ensure a healthy colon, and might be particularly useful following a course of antibiotics.
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