Rural–urban differences in disease activity and vitamin D status in ankylosing spondylitis

Background: Vitamin D has important immunomodulatory effects and may influence disease activity in ankylosing spondylitis (AS). Rural–urban differences in sunlight exposure, diet, and healthcare access may affect vitamin D status. This study evaluated the impact of residential background on serum vitamin D levels and disease activity among patients with AS in Northeast India. Methods: This cross-sectional observational study included 40 consecutive patients fulfilling the ASAS 2009 classification criteria for AS at Assam Medical College and Hospital, Dibrugarh (2024–2025). Serum 25-hydroxyvitamin D [25(OH)D] was measured by enzyme-linked fluorescent assay (ELFA). Disease activity was assessed using BASDAI, ASDAS-ESR, and ASDAS-CRP. Rural and urban groups were compared using the Mann–Whitney U test, and correlations were assessed using Spearman's rank correlation. Results: The mean age was 41.1±14.1 years, with 55% males; 62.5% were from rural areas. Vitamin D deficiency, insufficiency, and normal levels were observed in 45%, 32.5%, and 22.5% of patients, respectively. Mean serum 25(OH)D levels were similar in rural and urban groups (15.3±5.8 vs. 14.7±6.1 ng/mL; p=0.715). High disease activity (BASDAI ≥4) was present in 76.0% and 80.0% of rural and urban patients, respectively, with no significant differences in BASDAI or ASDAS-ESR scores. Serum vitamin D levels showed significant negative correlations with BASDAI, ASDAS-ESR, ASDAS-CRP, ESR, and CRP (all p<0.001). Conclusions: Vitamin D deficiency was common and strongly associated with higher disease activity, although no significant rural–urban differences were observed. Routine assessment of vitamin D may be beneficial in patients with AS irrespective of residential background.