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BACKGROUND: The increasing incidence of inflammatory bowel disease (IBD) in western countries has led to the hypothesis that obesity-related inflammation could play a role in the etiology of IBD. However, this hypothesis lacks confirmation in studies of individuals prior to the typical onset of IBD in young adulthood.METHODS: In a cohort of 316,799 individuals from the Copenhagen School Health Records Register (CSHRR), we examined whether BMI at ages 7 through 13 years was associated with later IBD. Linking the CSHRR to the Danish National Patient Register, we identified cases of Crohn’s disease (CD) and ulcerative colitis (UC) diagnosed during follow-up. Cox regression was used to estimate the hazard ratios (HR) with 95% confidence intervals.RESULTS: During 10 million person-years of follow-up, 1500 individuals were diagnosed with CD and 2732 with UC. At all examined ages, a 1 unit increase in BMI z-score was associated with a significantly decreased risk of UC (HRs = 0.9) and with a significantly increased risk of CD when diagnosed before age 30 (HRs = 1.2). We observed no associations between changes in BMI z-score between 7 and 13 years and later risk of CD or UC.CONCLUSION: We found a direct association between childhood BMI and CD diagnosed before 30 years of age, and an inverse association between childhood BMI and UC irrespective of age. Our results support the previous hypotheses of obesity being a risk factor for CD, and suggest that childhood underweight might be a risk factor for UC. [ABSTRACT FROM AUTHOR]