Early-Life Body Mass Index and Inflammatory Bowel Disease Risk: A Scandinavian Birth Cohort Study.

Introduction: Childhood overweight and obesity are emerging global health issues with potential implications for immune function. We aimed to investigate childhood body mass index (BMI) as a risk factor for later inflammatory bowel disease (IBD).

Methods: ABIS (Sweden) and MoBa (Norway) are population-based cohorts following participants prospectively from birth (1997-2009) until 2023. We retrieved anthropometric data at birth, 1, 3, and 7-8 years to examine the association of age-specific standardized BMI percentiles and categories (underweight, normal [reference), overweight, obesity) with later IBD. We also analyzed IBD risk according to BMI trajectories across ages. IBD diagnosis was identified in national health registries. Cohort-specific hazard ratios (aHRs) were adjusted for sociodemographics, parental BMI, IBD, and smoking and pooled using a random-effects model.

Results: Overall, among 54 890 children with 803 444 person-years of follow-up, we identified 246 IBD events. Eight-year-olds living with obesity had a 5-fold increased risk of ulcerative colitis (pooled aHR = 5.10; 95% CI , 1.51-17.27), but not a significantly increased risk of Crohn's disease (pooled aHR = 1.38; 95% CI , 0.24-7.98) and IBD overall (pooled aHR = 1.89; 95% CI , 0.71-5.04). Children with overweight or obesity at age 3 had no increased risk of IBD compared to normal-weight children (pooled aHR = 1.15, 95% CI , 0.74-1.77; and 1.05, 95% CI , 0.43-2.58, respectively). Early-life BMI trajectories were not consistently associated with IBD.

Conclusion: In this Scandinavian birth cohort, 7-8 year-old children with obesity had an increased risk of developing ulcerative colitis later in life. Given the high prevalence of childhood obesity, this observation should be corroborated and possible mechanisms behind the association clarified.