Digestion of polysaccharides and other major components in the small and large intestine of pigs fed on diets consisting of oat fractions rich in β-D-glucan

The digestibility of polysaccharides and other major components and the metabolic response of the microflora in the small and large intestines to oat diets varying in mixed linked beta(1-->3; 1-->4)-D-glucan (beta-glucan) were studied in experiments with ileum-cannulated pigs. The oat fractions for diets were prepared in a dry milling process in which oat groats were milled into two endosperm fractions (oat flour 1 and oat flour 2) and oat bran. The digestibility of polysaccharides and the metabolic response of the microflora were followed for the two contrasting diets, oat flour 1 and oat bran, from ingestion to excretion while the digestibility of oat groats and oat flour 2 were estimated only at the ileum and in faeces. There was no degradation of beta-glucan from either oat flour 1 or bran in the stomach and the first, middle and distal thirds of the small intestine (average digestibility approximately 0), while in the terminal ileum digestibility increased to 0.30 to 0.17 respectively. The digestion of starch in the first third of the small intestine was lower for the high-beta-glucan oat-bran diet (0.49) than for the low-beta-glucan flour diet (0.64). However, digestibility differences between the two diets levelled out as the digesta moved aborally in the small intestine and the digestibility at the terminal ileum was almost complete (0.970-0.995) for all diets. Oat non-starch polysaccharides (NSP) were an easily digestible energy source for the microflora in the large intestine less than 13% of dietary NSP being recovered in faeces. The bulk of beta-glucan which survived the small intestine was degraded in the caecum and proximal colon while arabinoxylan was more slowly degraded. The amount of residues passing the ileo-caecal junction has little impact on the density of micro-organisms in the large intestine, which on the flour and bran diets were in the range of 10(10)-10(11) viable counts/g digesta, but a high impact on the activity of the flora in colon. Oat bran resulted in a higher proportion of butyric acid in large intestinal content compared with the flour diet. The faecal bulking effect of oat bran was mainly caused by an increased excretion of protein and fat, presumably of bacterial origin. Of all the diets tested the oat-bran diets had the lowest digestibilities of protein and fat at the terminal ileum and in the faeces.