The main source of carbon in the human large intestine comes from carbohydrates like starches and oligosaccharides which remain unchanged by gastric digestion. These polysaccharides are metabolised in the colon by saccharolytic bacteria whose composition is dependent upon the substrate availability. Among the metabolites produced, the short-chain fatty acids (SCFA) are important for colon function and to prevent diseases. In particular, butyrate affects several cellular functions (proliferation, membrane synthesis, sodium absorption), and it has been shown to be protective against colorectal cancer. In addition, faecal bacteria are responsible for the conversion of primary bile acids (BA) to secondary BA, which are considered tumor promoters. In this study we investigated the in vitro effect of different substrates (CrystaLean starch, xylo-oligosaccharides, corn starch) supplied to human faecal micro-flora, on the SCFA production, on the bowel micro-flora composition and on the primary BA conversion rate. In addition, with corn starch as substrate, we considered the effect of enriching normal human faecal micro-flora with lactobacilli and bifidobacteria, on the above reported parameters.