Serum Vitamin D Levels and Disease Activity in Systemic Lupus Erythematosus: Association with Anti-dsDNA Antibodies and Selected Lifestyle Factors

Background: Vitamin D is involved not only in calcium–phosphate homeostasis but also in immune and endothelial regulation. Vitamin D deficiency has been suggested to worsen disease activity in systemic lupus erythematosus (SLE). Environmental and lifestyle factors, including seasonal sun exposure, smoking, diet, and supplementation, may influence vitamin D status and disease manifestations. This study aimed to evaluate the association between serum 25-hydroxyvitamin D [25(OH)D] levels, disease activity, and anti-double-stranded DNA (anti-dsDNA) antibody titers in patients with SLE, taking selected lifestyle and environmental factors into account. Methods: Serum 25(OH)D concentrations, SLE disease activity assessed by the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) score, and anti-dsDNA antibody titers were measured in patients with SLE and healthy controls. Blood samples were collected during sunny (April–September) and non-sunny (October–March) months. Information on vitamin D supplementation, smoking status, and dietary habits was obtained using a structured questionnaire. Associations between vitamin D status, disease activity, anti-dsDNA seropositivity, season of blood collection, supplementation, smoking, and diet were analyzed statistically. Results: Patients with SLE had significantly higher mean serum 25(OH)D levels than controls, mainly due to frequent vitamin D supplementation. No significant associations were observed between serum 25(OH)D levels and SLEDAI-2K scores or anti-dsDNA antibody positivity. Seasonality, smoking status, and adherence to special diets were not significantly related to disease activity or anti-dsDNA seropositivity. Vitamin D supplementation was strongly associated with sufficient 25(OH)D levels but did not translate into reduced disease activity or lower anti-dsDNA prevalence. Conclusions: Serum 25(OH)D concentration was not associated with clinical or immunological activity of SLE in this cross-sectional study, despite effective correction of deficiency through supplementation. These findings likely reflect the heterogeneity of SLE and the limitations of single time-point assessments, although regular monitoring and individualized vitamin D supplementation may still be considered in SLE care, particularly in the context of recommended photoprotection.