**Background:** The prevalence of childhood obesity has risen dramatically worldwide, and a parallel increase in inflammatory bowel disease (IBD) incidence has been observed. While obesity is known to promote a proinflammatory state, its impact on pediatric IBD outcomes remains poorly understood and controversial. Existing adult data are conflicting, with some studies suggesting increased disease flares and complications in obese patients, while others show no impact or even reduced need for surgery. Data in pediatric populations are particularly scarce. This study aimed to determine the prevalence of overweight and obesity in a large Swiss pediatric IBD cohort and assess their impact on disease phenotypes and clinical outcomes.
**Methods:** This was a prospective analysis of pediatric IBD patients enrolled in the Swiss Inflammatory Bowel Disease Cohort Study (SIBDCS) and its pediatric arm (SPIBDCS) between May 2006 and May 2020. Patients underwent structured assessment at enrollment and annual follow-up visits. Using BMI at enrollment and latest follow-up, patients were categorized according to WHO child growth standards: underweight (<3rd percentile), normal weight (3rd–90th percentile), overweight (90th–97th percentile), and obese (>97th percentile). Data collected included demographics, disease location (Paris classification), disease behavior, extraintestinal manifestations (EIM), medication history, disease severity (anti-TNF use, IBD-related surgery), clinical disease activity (PCDAI for CD, PUCAI for UC), biological activity (CRP, fecal calprotectin), and quality of life (KIDSCREEN, IMPACT III). Complicated disease course was defined as development of fistulizing or stricturing phenotype, intestinal surgery, or IBD-related hospitalizations. Statistical analyses used Wilcoxon/Mann-Whitney tests, Kruskal-Wallis tests, Fisher's exact test, Kaplan-Meier curves, and Cox proportional hazard models. A P value <0.05 was considered significant.
**Key Results:** A total of 327 pediatric patients were included (146 CD, 181 UC). At baseline, 13 (4.0%) were underweight, 272 (83.2%) normal weight, 22 (6.7%) overweight, and 20 (6.1%) obese. The combined prevalence of overweight/obesity was 12.8%. Median follow-up was 2.0 years (IQR 0.9–3.9). Obese patients were more likely to be male (80% vs 49.3%, P=0.041) and more likely to have been treated with steroids (80% vs 38.5%, P=0.017). In CD patients, overweight/obese patients had significantly higher rates of perianal abscess (41.2% vs 5.8%, P<0.001) and surgery for fistula or abscess (35.3% vs 4.2%, P<0.001) compared with normal-weight patients. Arthritis was more prevalent in obese CD patients (47.1% vs 21.7%, P=0.034). However, there were no significant differences in clinical disease activity (PCDAI median 2.5 in both groups, P=0.983), CRP (median 3.1 vs 3 mg/L, P=0.561), or fecal calprotectin (median 385 vs 147 µg/g, P=0.792). In UC patients, obese patients had a higher PUCAI score (median 5 vs 0, P=0.034), though both medians corresponded to clinical remission. No significant differences were found in biological activity, disease location, or EIMs in UC. IBD-related hospitalization rates over the last 12 months were similar between overweight/obese and normal-weight patients for both CD (11.8% vs 10.8%, P=1.000) and UC (16.0% vs 10.6%, P=0.493). No overweight/obese UC patient required intestinal surgery. Quality of life measures showed no significant differences between BMI groups in CD patients. In UC, overweight/obese patients had a significantly lower Kidcope escape-oriented strategy score (median 7 vs 10, P=0.002), suggesting better adaptive coping strategies.
**Clinical Implications:** This study provides important evidence that in pediatric IBD, overweight and obesity are not associated with broadly worse disease activity, severity, or hospitalization rates. However, the significant association between obesity and perianal abscesses and related surgery in CD patients, as well as increased arthritis, identifies specific high-risk subgroups that may benefit from closer monitoring and targeted management. The prevalence of overweight/obesity (12.8%) mirrors general pediatric population trends in Switzerland, and the low rate of underweight (4%) confirms that IBD diagnosis should not be excluded based on elevated BMI. The finding of better coping strategies in overweight/obese UC patients warrants further investigation. Limitations include the small sample size of overweight/obese patients (n=42), limited median follow-up (2.0 years), and inability to assess anti-TNF trough levels. Longer-term studies are needed to determine whether obesity's proinflammatory effects manifest over extended disease duration.