Background: Patients with inflammatory bowel disease (IBD) often develop alterations in body composition in terms of their proportions of lean mass and fat mass, as well as reduced bone mineral density (BMD). However, there are limited data on the skeletal muscle index (SMI) and percentage fat (fat %) for young adults with childhood-onset IBD. Our aim was to investigate the body compositions of these patients, with the focus on SMI and fat %.Methods: Body composition was estimated by dual x-ray absorptiometry for 94 young adults with childhood-onset IBD aged 18-27 years, 65 of whom had ulcerative colitis. The Z-scores for SMI, fat %, and BMD were calculated using the normative data from 1,289 individuals with corresponding age. Based on the SMI and fat % Z-scores, each patient was classified as having a body composition profile that was: (i) normal; (ii) obese (fat % Z-score >1); (iii) myopenic (SMI Z-score < -1); or (iv) myopenic-obese.Results: A higher proportion of young adults with childhood-onset IBD had a body composition profile classified as myopenic (24%) or myopenic-obese (9%), as compared to the controls (myopenic [16%, p = .016]; myopenic-obese [2%, p = .002]). Patients with the myopenic or myopenic-obese profile had significantly lower total body BMD Z-scores (-1.3 ± 0.7 and -1.4 ± 0.9, respectively) than patients with the normal profile (-0.2 ± 1.1; p < .001 and p = .004, respectively). Diagnosis of IBD in childhood represented an additional risk for low BMD, regardless of SMI Z-score.Conclusion: Young adults with childhood-onset IBD have a high risk for having altered body composition traits.SummaryYoung adults with childhood-onset IBD carry a high risk for altered body composition traits. The myopenic and myopenic-obese body composition profiles were more frequently observed in patients with IBD than controls, and these profiles were strongly associated with low BMD.