Two experiments were carried out to examine the short (1–3 d) and medium (14 d) term adaptations of rat large-bowel (LB) fermentation to alterations in substrate supply brought about by including cooked haricot beans (Phaseolus vulgaris) in diets based on white bread. Changes in organic matter (OM) flow from the ileum occurred within 1 d and were stable for 14 d but the pattern of caecal short-chain fatty acids took much longer to stabilize with considerable increases in butyrate between 1 and 3 d and up to 14 d. This suggests that the metabolic activity of the LB microflora may take a considerable time to stabilize after an abrupt change in substrate supply. Despite an almost fourfold change in OM supply to the LB, the proportion of this OM apparently fermented in that organ (0·46) remained fairly constant. None of the apparent resistant starch measured in the diets could be detected in faeces. Dietary non-starch polysaccharides (NSP) were extensively fermented with similar values (0·82) for both bread and bean NSP and there was little indication of any interaction between the two diet components on NSP fermentation. An attempt was made to fractionate ileal and faecal OM to provide a basis for a quantitative model of LB stoichiometry.

To investigate diurnal changes in large-bowel metabolism rats were fed once daily on a cooked maize-based diet without or with 200 g wLeat bran/kg diet and containing Cr2O3 as an indigestible marker. After 17 d four rats on each diet were killed at 4, 10, 16 and 22 h postfeeding. Emptying of dry matter (DM) from the stomach occurred by an apparently zero-order process at about 0·5 g/h in both diet groups. Feeding wheat bran had little effect on caecal pH or total short-chain fatty acid (SCFA) concentration but was associated with a marked increase in molar proportion of butyrate and a fall in propionate. There were substantial changes in caecal total SCFA concentration and in the molar proportions of individual SCFA throughout the day but no evidence of an interaction between diet and time interval after feeding. Caecal transit time (TT) was significantly reduced by feeding wheat bran whilst colonic TT was unaffected. Although when averaged across both diets there were no significant time effects on caecal or colonic TT, there was a significant diet × time interaction for caecal TT. The extent of coprophagy was measured. The proportion of stomach DM derived from ingested faeces increased with time interval after feeding but it was always a minor contribution so that in these circumstances coprophagy is unlikely to result in significant bias in estimates of digesta flow-rates or TT.