Background Recently, vitamin D has been shown to play an important role in the immune responses, increasing evidence that it can contribute to the pathogenesis of systemic lupus erythematosus (SLE) as well as affect disease course and activity. Therefore, this study aims to determine whether there is a correlation between seasonal vitamin D levels and clinical manifestations or disease activity.

Methods Seasonal measurements of serum 25(OH)D3 were performed in December to February in winter time and in July to September in summer time. We included patients with SLE who measured serum 25(OH)D3 from 2013 to 2016, with 407 patients measured during winter and 375 patients measured during summer. Vitamin D-deficient groups were classified based on 20ng/ml, 25ng/ml, and 30ng/ml for each summer and winter. The relationship between vitamin D concentrations and clinical manifestations or disease activity was analyzed using logistic regression analysis.

Results There were seasonal differences in the reference concentration of vitamin D, which affects disease activity or clinical manifestations, and the values were 20ng/ml and 30ng/ml in winter and summer, respectively. In the winter vitamin D-deficient group (less than 20ng/ml), the erythrocyte sedimentation rate was 23.9mm/hr, which was marginally higher than that in the vitamin D-sufficient group (22.4mm/hr, p = 0.08). In addition, oral ulcer was significantly less frequent in winter vitamin D-sufficient group (Odds ratio [OR] 0.530, p = 0.047). In summer, oral ulcer (OR 0.278, p = 0.019) and skin rash (OR 0.221, p= 0.015) were significantly less common in vitamin D-sufficient group with cut off of 30ng/ml.

Conclusions In conclusion, this study suggests that seasonal variations in serum vitamin D may affect the clinical manifestations of SLE, and that vitamin D-deficiency leads to increased oral ulcer, skin rash, and inflammatory marker.