1. Male Sprague-Dawley rats were fed on either a purified, fibre-free diet or a diet in which half the maize starch was replaced with uncooked amylomaize or potato starch (equivalent to 100 or 200 g amylase-resistant starch (ARS)/kg diet respectively). Changes in short-chain fatty acids (SCFA), pH, ammonia and a number of bacterial variables in caecal contents were then assessed.
2. Both ARS supplements decreased caecal content pH by approximately 1–2 units, with an associated reduction in ammonia concentration. Potato starch significantly decreased the concentration of SCFA in the hind-gut, while amylomaize supplementation increased propionic and butyric acids but decreased the occurrence of minor, branched-chain fatty acids.
3. Caecal bacterial biotransformation activities (β-glucosidase (EC 126.96.36.199), β-glucuronidase (EC 188.8.131.52), reduction of p-nitrobenzoic acid, apparent ammonia formation) were consistently decreased by both ARS sources.
4. The results demonstrate that amylase-resistant carbohydrate altered toxicologically important functions in the large-intestinal flora of the rat.