Epidemiological studies have identified increased colorectal cancer (CRC) risk with high red meat (HRM) intakes, whereas dietary fibre intake appears to be protective. In the present study, we examined whether a HRM diet increased rectal O⁶-methyl-2-deoxyguanosine (O⁶MeG) adduct levels in healthy human subjects, and whether butyrylated high-amylose maize starch (HAMSB) was protective. A group of twenty-three individuals consumed 300 g/d of cooked red meat without (HRM diet) or with 40 g/d of HAMSB (HRM+HAMSB diet) over 4-week periods separated by a 4-week washout in a randomised cross-over design. Stool and rectal biopsy samples were collected for biochemical, microbial and immunohistochemical analyses at baseline and at the end of each 4-week intervention period. The HRM diet increased rectal O⁶MeG adducts relative to its baseline by 21 % (P< 0·01), whereas the addition of HAMSB to the HRM diet prevented this increase. Epithelial proliferation increased with both the HRM (P< 0·001) and HRM+HAMSB (P< 0·05) diets when compared with their respective baseline levels, but was lower following the HRM+HAMSB diet compared with the HRM diet (P< 0·05). Relative to its baseline, the HRM+HAMSB diet increased the excretion of SCFA by over 20 % (P< 0·05) and increased the absolute abundances of theClostridium coccoidesgroup (P< 0·05), theClostridiumleptumgroup (P< 0·05),Lactobacillusspp. (P< 0·01),Parabacteroides distasonis(P< 0·001) andRuminococcus bromii(P< 0·05), but loweredRuminococcus torques(P< 0·05) and the proportions ofRuminococcus gnavus,Ruminococcus torquesandEscherichia coli(P< 0·01). HRM consumption could increase the risk of CRC through increased formation of colorectal epithelial O⁶MeG adducts. HAMSB consumption prevented red meat-induced adduct formation, which may be associated with increased stool SCFA levels and/or changes in the microbiota composition.