Replacing animal-sourced with plant-sourced proteins alters gut microbiota and biomarkers associated with colorectal cancer risk in healthy adults: A randomized feeding study

Epidemiological studies associate replacing animal-sourced with plant-sourced protein with lower colorectal cancer (CRC) risk, but controlled feeding studies have rarely tested how this substitution alters the gut microbiota and CRC-related biomarkers. In a 12-week randomized controlled feeding study, 136 healthy adults consumed diets providing 30 energy percent (E%), 50 E%, or 70 E% of protein from plant sources. Change in gut microbiota composition was the primary outcome; stool characteristics, fecal N-nitroso compounds (NOCs), and fecal and plasma bile acids (BAs) were secondary outcomes. The 70% plant-protein diet significantly restructured the gut microbiota, increasing fiber-associated Lachnospiraceae and Roseburia while reducing Methanobrevibacter and several asaccharolytic taxa. Microbial functional potential shifted toward plant-carbohydrate degradation, with lower amino acid degradation and methanogenesis. Fecal pH decreased, stool characteristics suggested faster colonic transit, and potentially carcinogenic total and haem-derived NOCs declined progressively with plant-protein replacement. The 70% plant-protein diet also reduced fecal secondary and taurine-conjugated BAs, including the CRC-associated BAs DCA and LCA. Plasma glucuronidated BAs increased, expanding the circulating pool of readily excreted BA conjugates. Higher insoluble fiber intake was associated with greater microbiota restructuring and lower fecal NOCs. Methanogen carriers had higher baseline fecal NOCs and features of slower colonic transit, and showed weaker microbiota and less favorable BA responses to the 70% plant-protein diet. Baseline methanogen carriage thus could be exploited for participant stratification in future dietary interventions.