Higher incidence of paediatric inflammatory bowel disease by increasing latitude in Norway, but stable incidence by age.

Aim: To examine possible geographical and temporal differences in the incidence of childhood-onset inflammatory bowel disease (IBD) in Norway, motivated by previous research indicating relevant environmental factors explaining changing epidemiology.

Methods: We analysed data from children born in Norway from 2004 to 2012 (n = 541 036) in a registry-based nationwide study. After validating registry diagnoses against medical records, we defined IBD as ≥2 entries of International Classification of Diseases, 10th revision (ICD-10) codes K50, K51 and K52.3 in the Norwegian Patient registry. We estimated hazard ratios (HR) for IBD across four geographical regions with a south-to-north gradient and the incidence by period of birth.

Results: By the end of follow-up on 31 December 2020, 799 IBD diagnoses were identified (Crohn's disease: n = 465; ulcerative colitis, n = 293, IBD: unclassified, n = 41). Compared to children in the southernmost region, there was almost a two-fold HR for IBD in children in the most Northern region (HR = 1.94, 95% Cl = 1.47-2.57; Mid region: HR = 1.68, 95% CI = 1.29-2.19, p <0.001). These estimates remained largely unchanged after adjustment for potential confounding factors. The cohorts born in 2004-2006 and 2010-2012 had comparable cumulative incidences, with a slightly higher incidence for those born in 2007-2009.

Conclusion: We observed an increase in the risk of IBD by increasing latitude which may suggest that environmental factors influence the development of IBD, although non-causal explanations cannot be ruled out.