Fermentation of non-starch polysaccharides in mixed diets and single fibre sources: comparative studies in human subjects andin vitro

Abstract

The present study investigated whether the extent of fermentation of NSP in human subjects could be predicted by an in vitro batch system. Fibre sources studied were five mixed diets containing different amounts and types of fibre and three single fibre sources (citrus fibre concentrate, coarse and fine wholemeal rye bread). Fermentation in human subjects was determined in balance experiments in women who were also donors of the faecal inocula. In vitro fermentations were performed with fibre residues prepared from duplicates of the fibrecontaining foods consumed during the balance trials. Fermentation of total NSP in vivo was between 65 . 8 and 88 . 6 % for the mixed diets and 54 . 4, 58 . 0 and 96 . 9 % for the coarse and fine wholemeal rye breads and the citrus fibre concentrate respectively. For the mixed diets and the citrus fibre concentrate, mean differences between the extent of NSP degradation after 24 h in vitro incubation and that in vivo were between −0 . 7 and 5 . 0 %. Differences were significant for one diet (P < 0 . 05). For the wholemeal rye breads, the fermentation in vitro exceeded that in vivo significantly, but the magnitude of the difference in each case was small and without physiological importance. Particle size of breads had no influence on the extent of NSP degradation. These results indicate that the in vitro batch system used could provide quantitative data on the fermentation in vivo of NSP in mixed diets and some single fibre sources. An in vitro incubation time of 24 h was sufficient to mimic the NSP degradation in vivo.Fermentation: Non-starch polysaccharides: In vitro batch system NSP are the predominant components of dietary fibre (DF). Their susceptibility to bacterial fermentation is of major significance for the actions of DF in the large intestine. Fermentation of NSP affects their structural properties and water-holding capacity and thereby their effect on stool weight. Fibres that are extensively degraded have a lower faecal bulking capacity than more resistant types of fibre (Stephen & Cummings, 1980). This is especially the case if the fermentation is rapid and occurs mainly in the proximal colon ). The short-chain fatty acids (SCFA) acetate, propionate and butyrate formed from bacterial NSP breakdown are efficiently absorbed. Besides their contribution to the body's energy supply (McNeil, 1984), they have specific metabolic functions. Butyrate, for example, is the preferred fuel of colonic epithelial cells (Roediger, 1982) and is thought to contribute to the protective role of fibre against large-bowel cancer (Bingham, 1990).Fermentability of NSP and production of SCFA can be studied rather easily with in vitro batch systems utilizing faecal bacteria. These in vitro systems have several advantages over time-consuming and expensive human fermentation studies. In vitro systems are inexpensive, different NSP sources can be fermented at the same time and they can be used for screening of new fibre sources not yet permitted for human consumption. In vitro studies using rat f...

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