Body composition, bone mineral density, and functional impairment in axial spondyloarthritis: a 36-month longitudinal study

Background: Axial spondyloarthritis (ax-SpA) is a chronic inflammatory disease affecting the axial skeleton, peripheral joints, And entheses, often leading to pain, stiffness, And extra-articular complications. Its impact on muscle and bone health has gained attention, as chronic inflammation, reduced physical activity, sedentary behaviour, and glucocorticoid therapy, may alter body composition, particularly lean mass and adipose tissue distribution. This study explores the association between disease activity, body composition, and bone parameters in ax-SpA patients over a 36-month period. Methods: This longitudinal study was conducted at Padua University Hospital (Italy). The following data was collected at baseline And 36 months: medical history, phospho-calcium metabolism, anthropometric measurements, handgrip strength (using hand dynamometer), sit-to-stand test, and Dual-Energy X-ray Absorptiometry assessments. Patients completed Bath Ankylosing Spondylitis Functional Index (BASFI), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), and Health Assessment Questionnaire (HAQ). Correlations between questionnaire scores and clinical variables were analysed, and changes from baseline to follow-up were assessed using paired comparisons. Results: Twenty participants (10 ax-SpA, 10 matched controls) were enrolled. At baseline, no significant differences in bone and body composition were found between groups. In ax-SpA patients, BASFI correlated with BMI (r = 0.800, p < 0.01), fat percentage (r = 0.808, p < 0.01), and fat mass index (r = 0.903, p < 0.01), while BASDAI correlated with sit-to-stand performance (r = 0.677, p < 0.05) and fat percentage (r = 0.700, p < 0.05). After 36 months, significant improvements were observed in sit-to-stand scores [from 17.37 (7.47) to 11.98 (3.81), p = 0.02] and femoral neck BMD [from 0.89 (0.13) to 1.02 (0.14), p = 0.01]. Sit-to-stand improvements correlated with BASFI (r = 0.78, p < 0.01), and ASMMI changes correlated with HAQ (r = 0.92, p < 0.001). Conclusion: Our findings suggest that greater muscle mass and physical performance are associated with lower disease activity and improved quality of life in ax-SpA patients. These associations support integrating pharmacologic treatment with structured exercise, although causal inferences cannot be drawn from this observational design. Further studies are needed to clarify the directionality and underlying mechanisms. Trial registration: Not applicable.