AbstractSecondary bile acids (BA) may be involved in the pathogenesis of colorectal cancer.In vivo, starch malabsorption has been shown to reduce fecal excretion of secondary BA. The presentin vitrostudy was performed to investigate the effect of starch fermentation on BA metabolism by colonic bacteria. Fecal samples of healthy volunteers were incubated in anaerobic batch cultures for 48 hours with the primary bile acids cholic (0.6 g/l) and chenodeoxycholic acid (0.4 g/l). Media were starch free or enriched with starch (10 gll). The pH was controlled and held at 6 or 7.In the starch‐free incubations, secondary BA were rapidly formed, and degradation of primary to secondary BA was complete within 24 hours. The formation of secondary BA was partially inhibited by the addition of starch to the media. This effect was stronger at pH 6 than at pH 7. Starch was rapidly and completely fermented. In conclusion, this study showed that formation of secondary BA by fecal bacteria is inhibited when starch is simultaneously fermented, an effect that is mainly, but not completely, explained by reduction of pH.