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In Gut, a study by Osswald et al1 links diet and microbially generated bile acids (BA) to colorectal cancer (CRC), a leading cause of cancer-related mortality worldwide. Dietary habits are a predominant modifiable risk factor, where Western diets, characterised by high saturated fat, low fibre and highly refined carbohydrates, are associated with increased CRC incidence.2 A long-standing hypothesis posits that Western diet-induced alterations in gut microbiota and, consequently, BA metabolism, particularly increased hydrophobic secondary BAs in the distal intestine, promote colorectal carcinogenesis.3 However, despite a large body of correlative human data,4 definitive causal evidence linking gut microbiota-derived secondary BAs to CRC development has remained elusive. The study by Osswald et al provides a critical advance by integrating large-animal models, gnotobiotic studies and multiomics analyses to establish a direct causal role for deoxycholic acid (DCA) generating gut bacteria in CRC progression.1