In mice given eight azoxymethane injections over 28 weeks, a 7.5% refined inulin diet produced colon tumors in roughly 70% of animals, versus 10-20% on low-fiber, cellulose, agar or pectin diets. Inulin shifted the gut microbiome away from short-chain fatty-acid producers and cross-feeding taxa, with succinate, fumarate and lactate accumulating while propionate and amino acids fell. These changes appeared only after carcinogen exposure. Cecal extracts from inulin-fed mice also stimulated HT29 cancer-cell proliferation.
This is mouse evidence from a preprint that has not been peer-reviewed, so it should not change what anyone eats. It does add weight to a growing caution: inulin, the chicory-root prebiotic in many bars, yogurts and powders, is not uniformly protective. Earlier work from the same group and others found that inulin can worsen tumors or liver cancer in certain mouse models, especially when the microbiome is already altered. Human data remain far more reassuring, since high intake of mixed whole-food fiber consistently tracks with lower colorectal cancer risk, and nothing here tests that. Key limits: male mice only, a chemical carcinogen rather than natural tumor development, a single dose, and a single refined inulin source. Because the intervention is isolated fiber, the result does not speak against fiber from vegetables, legumes or whole grains. The practical takeaway is modest: fiber type may matter, and trials of inulin supplements in people at elevated colorectal cancer risk deserve scrutiny. Confirmatory of an emerging concern rather than paradigm-shifting.