The nutritional significance of ''dietary fibre'' analysis

The term “dietary fibre” (DF) is, in most recent animal literature, used for cell wall or storage non-starch polysaccharides (NSP) and lignin. The DF can be measured as soluble, insoluble and total DF by enzymatic–gravimetric methods or as soluble, insoluble and total NSP by enzymatic–chemical methods and lignin by gravimetry. The NSP comprise 700–900 g kg−1 of the plant cell wall, with the remaining being lignin, protein, fatty acids, waxes, etc. Plant cell wall NSP is a diverse group of molecules with varying degrees of water solubility, size and structure, which may influence the rheological properties of the gastrointestinal contents, flow of digesta and the digestion and absorption process to a variable degree. The action of NSP in the stomach and small intestine is essentially a physical one, in which the plant cell either acts as barrier to the release of nutrients or increases the viscosity of the liquid phase and restricts their absorption. However, contradictory effects of DF are reported in the literature concerning their effects on gastric emptying and rate of glucose absorption in small intestine presumably due to differences among studies in the form in which the DF has been included (isolates or integrated part of cell wall structure). In pigs, approximately 0.8 of fed NSP is recovered in digesta leaving the ileum; the remaining being lost through microbial degradation in stomach and small intestine. The NSP are degraded to a variable degree in the large intestine by anaerobic fermentation. The degradation of NSP in the large intestine depends on the degree of lignification, solubility and structure of the polysaccharides. Analytical values concerning degree of lignification and water solubility provide important information about the degradability of NSP in the large intestine, while the effect of NSP on the digestion and absorption processes in the small intestine is more difficult to predict from any of the chemical parameters currently measured.