The present chapter summarizes recent findings from large-scale prospective cohort studies investigating the etiology and prevention of inflammatory bowel disease (IBD), with a particular focus on dietary and environmental factors. Evidence from Western cohorts, such as the UK Biobank, Nurses’ Health Study, and the Crohn’s and Colitis Canada Genetics, Environmental, and Microbial Project, suggests that adherence to healthy lifestyle patterns, including plant-based diets, high dietary fiber intake, and avoidance of ultra-processed foods, may reduce the risk for Crohn’s disease. However, the evidence for ulcerative colitis remains limited, and region-specific investigations are necessary. This chapter also highlights the importance of the gut microbiota as a mediator of dietary effects and advocates nationwide, interdisciplinary research to establish effective IBD prevention strategies tailored to the Japanese population.