Dietary Fibre in the Intestine

Small Intestine

Nutrients in food are degraded and absorbed in the small intestine, but fibres are not due to the lack of fibre-degrading enzymes. Soluble fibre, such as β-glucan and soluble arabinoxylan slow down gastric emptying and the absorption of nutrients possibly by increasing the viscosity of the food mass. A slower hydrolysis and absorption of starch result in lower and more stable blood glucose levels. A minor part of non-starch polysaccharides may be degraded by the intestinal flora harbouring the small intestine of humans. The degradation of arabinoxylans and cellulose is much lower than that of β-glucan.

Large Intestine

In the large intestine dietary fibre is extensively degraded and provides substrate for a complex microbial ecosystem of several hundred species of bacteria that are important for human health. In vitro studies have shown that rye arabinoxylans stimulate the growth of Bifidobacterium longum, while xylooligosaccharides have been demonstrated to stimulate bifidobacteria in vivo. However, these results have not been confirmed by faecal concentrations of Bifidobacteria in humans eating high amount of rye fibre.

The rate and extent of fibre degradation is dependent on the chemical structure, the solubility and degree of lignification of the fibre. β-glucans and soluble arabinoxylans are rapidly degraded in the caecum and proximal colon while the more insoluble fibre, e.g. cellulose and insoluble arabinoxylans, is degraded more slowly at distal locations of the colon.

From the microbial degradation short-chain fatty acids, branched-chain fatty acids, various gases, and other organic compounds are formed. The short-chain fatty acids provide energy, but have also specific health implications. For example, butyrate has regulatory functions in cell proliferation and differentiation, which prevents cancer. Propionate is suggested to modify hepatic cholesterol metabolism, while acetate is a fuel for muscle tissues. Studies have shown that the molar proportion of short-chain fatty acids can be modified by the substrate available for fermentation. Arabinoxylans from wheat and rye are shown to increase the production of butyrate in pigs and rats, by in vitro fermentation, and to increase faecal butyrate concentration in middle-aged human males, but not in women.

The bioavailability and bioactivity of cell wall associated compounds is also affected by microbial activity. Phenolic compounds - ferulic acid and enterolignans - are released and made available for absorption. Enterolactone, the main enterolignan deriving from microbial conversion of plant derived lignans, is significantly higher in plasma and urine after consumption of whole grain wheat and rye compared to refined wheat.