Konjac glucomannan (KGM) has been shown to increase human colon microbial ecology and reduce faecal toxicity in mice. The main goal of the present study was to assess the effects of a KGM supplement into a low-fibre diet on precancerous markers of colon cancer in a double-blind, placebo- and diet-controlled study. Adult volunteers consumed defined diets supplemented with konjac (4·5 g/d) or placebo (maize starch) for 4 weeks. Stools collected before and at the end of the supplementation were analysed for β-glucosidase, β-galactosidase and β-glucuronidase activities, microflora and bile acids. Faecal water was co-incubated with Caco-2 cells, a model of human colonocytes, to determine the cytotoxicity and DNA-damaging effect as assessed by the comet assay. The results indicated that the KGM supplement significantly decreased faecal β-glucuronidase activity by 25·6 (se 7·8) % and faecal secondary bile acid level by 42·4 (se 11·8) %. In contrast, consuming the defined diet supplemented with placebo for 4 weeks did not improve these determinants. The KGM-supplemented diet, but not the placebo diet, significantly increased the survival rate (%) of Caco-2 cells co-incubated with faecal water for 1 and 3 h, respectively. In addition, KGM significantly reduced the DNA damage induced by the faecal water alone or in combination with H2O2. The faecal bifidobacteria and lactobacilli levels increased only with the KGM-supplemented diet. Therefore, we conclude that supplementation of KGM into a low-fibre diet improved the faecal microbial ecology and metabolites, which may contribute to the reduced toxicity of faecal water and precancerous risk factors of human colon cancer.