Diet composition shapes tissue function and disease risk by modulating nutrient availability, metabolic state and cellular dynamics1. In the gastrointestinal tract, obesogenic high-fat diets enhance small-intestinal stem cell activity and tumorigenesis2. However, the impact of ketogenic diets (KDs), which contain even higher lipid content but reduce circulating insulin and induce ketogenesis, remains poorly understood3. This is particularly relevant for patients with familial adenomatous polyposis who face a high risk of small-intestinal tumours4. Here we combine dietary, genetic and metabolic manipulations in mouse models of spontaneous intestinal adenoma formation to dissect the role of systemic and epithelial ketogenesis in intestinal cancer. We show that KD accelerates tumour burden and shortens survival, independent of ketone metabolites. Through genetic manipulation of the ketogenic pathway, we modulate the production of local and systemic ketone metabolites; however, neither inhibition nor augmentation of the ketogenic enzyme 3-hydroxy-3-methylglutaryl-coenzyme A synthase 2 nor disruption of ketolysis altered tumorigenesis. Combined intestinal loss of PPARα/δ/γ attenuates KD-driven intestinal stem cell expansion, proliferation and clonogenicity, whereas inhibition of downstream fatty acid oxidation through CPT1A loss limits adenoma formation specifically under KD, linking tumour initiation to fatty acid oxidation of dietary lipids rather than lipid accumulation. These findings reveal that dietary lipid content, through fatty acid oxidation rather than ketone metabolism, influences intestinal tumorigenesis and highlight the need for nuanced consideration of dietary strategies for cancer prevention in genetically susceptible populations.