Bone mineral density, body composition, and muscle strength in young adults with juvenile idiopathic arthritis: a case–control study

Juvenile idiopathic arthritis (JIA) frequently leads to musculoskeletal complications that persist into adulthood. This study aimed to comprehensively assess bone mineral density (BMD), body composition, and muscle strength in young adults with JIA compared with healthy controls. In this cross-sectional case-control study, 147 participants (81 JIA patients, 66 healthy controls) were evaluated. BMD and body composition were assessed via dual-energy X-ray absorptiometry; muscle strength was measured by handgrip strength (HGS) dynamometry. Sarcopenia was defined according to the European Working Group on Sarcopenia in Older People (EWGSOP2) criteria. Data were analyzed using analysis of covariance (ANCOVA) adjusted for age, sex, and height. Adjusted odds ratios (OR) for sarcopenia and low BMD (Z-score ≤ − 2.0) were calculated using Firth’s penalized logistic regression to account for age and sex. Patients with JIA had significantly lower BMD at all sites ( p < 0.01), with the largest deficit in total forearm BMD (0.524 ± 0.094 vs. 0.691 ± 0.100 g/cm², p < 0.001). JIA patients also showed a significantly lower skeletal muscle index (SMI) (6.05 ± 1.52 vs. 6.89 ± 1.19 kg/m², p < 0.001) and markedly reduced HGS (18.0 [14.0; 25.0] vs. 25.5 [22.0; 37.5] kg, p < 0.001). Sarcopenia was present in 54% of JIA patients and 0% of controls ( p < 0.001). A significant mass–strength dissociation was identified: in patients with low disease activity (LDA) (defined by the Juvenile Arthritis Disease Activity Score-27 [JADAS27]), SMI was comparable to controls (6.99 ± 1.47 vs. 6.89 ± 1.19 kg/m², p = 1.00), yet their HGS remained significantly lower (22.0 [18.0; 29.0] vs. 25.5 [22.0; 37.5] kg, p < 0.001). Overall, JIA was associated with much higher odds of sarcopenia (aOR 107.8) and low BMD (aOR 13.6). Young adults with JIA demonstrated lower BMD, reduced lean mass and muscle strength compared with healthy controls, with persistent muscle strength impairment even in patients with relatively preserved muscle mass and low disease activity. These comparisons should be interpreted in light of the age difference between the JIA and control groups.