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The present work was undertaken to study the gastrointestinal effects of wheat and oat dietary fibre (DF) using 40-50 kg pigs cannulated in the terminal ileum. The variables studied were: chemical characteristics of the DF, ileal and faecal digestibility of nutrients and bulking properties of polysaccharides and other major constituents. The wheat products studied included refined wheat flour and wheat fractions rich in the following botanical components: aleurone, pericarp/testa and bran. The oat products used were rolled oats and oat bran. The products varied considerably in DF content (g/kg dry matter) and composition; non-starch polysaccharides (NSP) and Klason lignin content ranged from 34 and 1 g/kg respectively in wheat flour, to 465 and 92 g/kg in pericarp/testa. The main NSPs in the wheat were arabinoxylans (AX) (64-69%) and cellulose (15-31%) and in oats mixed linked beta(1----3; 1----4-D-glucans (beta-glucans; 46-63%) and AX (28-32%). The lowest content of soluble NSP was found in the lignified wheat fractions (bran and pericarp/testa) and the highest in oat bran. Eight diets were produced using the wheat and oat products and studied in two series of experiments using wheat flour as the DF-depleted control. The diets in Expt 1 were based on wheat flour and three iso-DF enriched diets prepared by adding DF from the fractions rich in wheat aleurone, pericarp/testa or bran. In Expt 2, oat bran was added to wheat flour to achieve the same DF intake level as in Expt 1. This series also included diets based on rolled oats and rolled oats plus oat bran. Starch was almost completely digested in the small intestine (0.97-1.00). However, there was a tendency to a slightly lower digestibility of oat starch compared with wheat starch. The recovery of wheat NSP in ileal digesta was 82-104% compared with 64-66% for oats. The low recovery of NSP in oat diets was primarily due to the low recovery of beta-glucans (25-36%). In the large intestine NSP and starch residues were extensively degraded. For the DF-depleted control diets or diets based on oats, 8-17% NSP survived breakdown while in the diets enriched with aleurone, pericarp/testa or bran fractions, NSP recovery was 33, 50 and 38% respectively. Fermentative breakdown of carbohydrates in the large intestine was estimated to contribute between 10 and 24% of the energy for maintenance. Energy derived from the inflow of organic acids from the ileum contributed an additional 1-4% of maintenance energy.(ABSTRACT TRUNCATED AT 400 WORDS)