The Impact of Body Mass Index at the Diagnosis on Disease Activity and Lower Extremity Involvement in Juvenile Idiopathic Arthritis

Juvenile idiopathic arthritis (JIA) is the most common chronic inflammatory disease of childhood, and body mass index (BMI) may influence its course. This study aimed to assess the impact of BMI on disease activity and lower-extremity joint involvement by analyzing demographic, laboratory, and clinical characteristics of children with JIA. A cross-sectional study was conducted in 153 patients who were diagnosed with JIA according to the International League for Rheumatology classification criteria and who attended routine outpatient clinic visits between March 2025 and June 2025. Demographic, clinical, and laboratory characteristics were compared among groups (underweight, normal weight, overweight, and obese). The study included 153 patients (54.9% female, n=84). The prevalence of overweight or obesity among patients with JIA was 30.7%. Underweight patients had an earlier disease onset (p=0.007) and a higher frequency of oligoarticular JIA (p=0.028), whereas overweight patients exhibited a higher frequency of enthesitis-related arthritis (ERA) (p=0.025) and ankle involvement (p=0.030). While underweight patients developed oligoarticular disease at a younger age, overweight or obese patients were more frequently classified as having ERA and exhibited a higher incidence of ankle involvement. Although BMI was not associated with overall disease activity, it may represent an important factor contributing to lower extremity joint stress and inflammation.