Prevalence of Vitamin D Deficiency and its Association with Disease Activity in Patients with Systemic Lupus Erythematosus: A Cross-Sectional Study from Southeast Iran

Background: Vitamin D plays a critical immunomodulatory role, and its deficiency has been consistently associated with the pathogenesis and activity of various autoimmune diseases. Despite high ambient sunlight in Southeast Iran, sociocultural factors and disease-related sun avoidance may increase hypovitaminosis D risk in patients with systemic lupus erythematosus (SLE). Objective: To determine the prevalence of vitamin D deficiency among patients with SLE in Zahedan, Iran, and to evaluate its association with disease activity. Methods: We conducted a cross-sectional study of 100 adult SLE patients at a major referral center. We measured disease activity using the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K). Blood samples were tested for 25-hydroxyvitamin D [25(OH)D] levels using a chemiluminescence immunoassay. We grouped patients into three categories: deficient (below 20 ng/mL), insufficient (20-29.9 ng/mL), or sufficient (30 ng/mL or above). Data were analyzed using Spearman's correlation and one-way ANOVA. Results: The mean serum 25(OH)D level was 22.9 ± 12.2 ng/mL. Only 23% of patients exhibited sufficient vitamin D levels, whereas the remaining 77% demonstrated suboptimal levels (33% deficient, 44% insufficient). A significant inverse correlation was observed between serum 25(OH)D levels and SLEDAI-2K scores (r = -0.375, p<0.001). Furthermore, vitamin D levels demonstrated a graded decline with increasing disease severity; patients with high disease activity had significantly lower mean 25(OH)D concentrations compared to those with mild activity (11.95 ± 7.96 ng/mL vs. 25.18 ± 13.03 ng/mL; p=0.048). Conclusion: Vitamin D deficiency is highly prevalent in SLE patients in Southeast Iran and is significantly associated with greater disease activity. Routine screening and further research into vitamin D supplementation are warranted.