The Degradation of Dietary Fibre in the Intestine
The majority of nutrients in food are degraded and
absorbed in the small intestine. Soluble fibre, such as b-glucan and soluble arabinoxylan, may slow down the gastric emptying rate
and the absorption of nutrients from the lumen of the small intestine, possibly by
increasing the viscosity of the food mass. This leads to the delayed hydrolysis of starch
and absorption of nutrients, which results in lower and more stable blood glucose levels.
This is beneficial e.g. with respect to prevention of development of type 2 diabetes.
Some minor degradation of non-starch polysaccharides may also occur in the small intestine
of humans. The degradation depends on the type of polysaccharide. Rye has a higher
arabinoxylan content than other cereals. The degradation of arabinoxylans and cellulose is
much lower than that of the mostly soluble b-glucan (Bach Knudsen et al. 1997, Karppinen et al.
Most microbial degradation of dietary fibre occurs
in the large intestine. Dietary fibre provides substrates for the complex ecosystem there,
which consists of several hundred species of bacteria that are important for human health.
The long term effects of the level of dietary fibre intake on the composition of human
intestinal microflora have not been thoroughly investigated. It seems, however, that the
microflora can be altered as early as after 2 weeks of increased intake of dietary fibre
(Rao 1995). The intermediate and end products of fermentation also are partly determined
by the composition of the polysaccharide substrate.
The fact that the gut microflora can be altered for the benefit of human health has
motivated the development of new functional food ingredients. A probiotic is a live microbial food
supplement, which beneficially affects the host by improving its intestinal microbial
balance (Gibson et al. 2004). A prebiotic is
defined as a nondigestible food ingredient that beneficially affects the host by
selectively stimulating the growth and/or activity of one or a limited number of bacteria
in the colon, and thus improves host's health (Gibson et al. 2005).
The most studied prebiotic food ingredients are oligosaccharides, which change the gut
microflora in the favour of Bifidobacteria. The bifidogenic nature of
fructo-oligosaccharides originating from chicory inulin has been well documented (Van Loo
et al. 1999). There is also evidence that xylo-oligosaccharides increase the amount of
Bifidobacteria in humans (Okazaki et al. 1990, Crittenden et al. 2002). The potential of
rye arabinoxylan to selectively stimulate some groups of bacteria in the colon remains to
be studied. Recently it was shown in pig experiments that whole grain rye is a much better
stimulator of fermentation and enterolactone production than white wheat bread (Glitsø et
It can be estimated that the amount of carbohydrates potentially available for
fermentation in the large intestine is about 12 g/100 g of whole grain rye bread, of which
about 80% is in the form of non-starch polysaccharides. This level is about three times
higher than in white wheat bread. Moreover, whole grain rye bread provides 2 g of lignin
per 100 g of dry matter, compared with 0.3 g in white wheat bread. The large amount of
carbohydrates available for bacterial fermentation is beneficial for bowel physiology
(Bach Knudsen et al. 1997, Gråsten et al. 2000).
An increased intake of dietary fibre will inevitably influence bowel function because it
stimulates microbial growth and short-chain fatty acid production, and lowers the pH in
the gut, and also because of the mechanical action and the water holding properties of
fibre (MacFarlane et al. 1991). All of these processes lead to increased bulk in the colon
and a shorter feacal transit time, as also shown for high intake of rye bread (Gråsten et
The significance of the colonic fermentation lies mainly in the types of products that are
formed and their fate in the body. Carbohydrates and proteins are broken down through a
variety of intermediate products to short-chain fatty acids, various gases, branched-chain
fatty acids and other organic compounds. The short-chain fatty acids
are generated to supply the
host with energy but have also specific metabolic roles with health implications. For
example, butyrate has regulatory functions in cell proliferation and differentiation
properties, which prevents cancer. Propionate has been proposed as a modifier of hepatic
metabolism, and acetate is used as fuel for muscle tissues (Bergman 1990).
Studies have shown that the substrate available for fermentation influences the molar
proportion of short-chain fatty acids. In vitro faecal fermentation studies indicated that
rye bran and its fractions were good producers of butyrate and propionate (Karppinen et
al. 2001). Results from both animal and human experiments point to rye dietary fibre as a good source of
butyrate generation (Bach Knudsen et al. 2003;
McIntosh et al. 2003).
Whole grain rye bread lowers total bile acid concentration in faeces and reduces the
concentration of free secondary bile acids, primarily because of a much higher
concentration of saponifiable bile acids in faeces. The concentration of faecal litocholic
acid, the most toxic of the bile acids, was significantly lower in a rye bread diet than
in a wheat bread diet (Korpela et al. 1992, Gråsten et al. 2000). It is believed that
saponifiable bile acids are less co-carcinogenic and co-mutagenic than free secondary bile
Mechanism of Action of Dietary Fibre and Unabsorbed Carbohydrates in Increasing
Colonic and Faecal Weight and Bulk
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