Dose-response effects of raw potato starch on small-intestinal escape, large-bowel fermentation and gut transit time in the rat

This study was designed to quantify starch digestion within the small and large bowels separately when raw potato starch (RPS) was included at 0-240 g/kg in diets fed to growing male Wistar rats. RPS was incorporated in the diets at the expense of maize starch which was expected to be almost completely digested in the small bowel. The digestibility of the maize starch was 0.99 but only 0.28 of the RPS was digested before the terminal ileum so that with increasing intakes of RPS there was a progressive increase in starch supply to the large bowel (LB). Of this starch 0.77, 0.72 and 0.73 was fermented in the large bowel when RPS constituted 80, 160 and 240 g/kg diet respectively. With increasing RPS intake, there was a curvilinear response in molar proportion of butyrate in caecal contents with a maximum value at about 80 g RPS/kg diet. The molar proportion of acetate increased linearly, that of propionate was unchanged, whilst proportions of the minor short-chain fatty acids all declined markedly with increasing RPS intake. The novel marker Bacillus stearothermophilus spores (BSS) was compared with CrEDTA in estimation of whole-gut mean transit time (MTT) when given together in a single test meal. Whilst estimates of MTT for the two markers were strongly correlated within individual rats (r2 0.72), BSS produced estimates that were 13 h longer than those based on CrEDTA. Neither marker detected a change in MTT with increasing RPS intake but, with both, the rate constant (k1) for the 'largest mixing pool' declined significantly (P < 0.001) as dietary RPS concentration was changed from 0-240 g/kg.