The Degradation of Dietary Fibre in the Intestine

Small 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. 2000).

Large Intestine

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 al. 2000).

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 al. 2000)

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 acids.

Mechanism of Action of Dietary Fibre and Unabsorbed Carbohydrates in Increasing Colonic and Faecal Weight and Bulk

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