Correlations Between Serum Vitamin D, Synovial Succinic Acid Levels and Disease Activity in Patients with Rheumatoid...

Rheumatoid arthritis (RA) is a chronic autoimmune synovitis characterized by progressive joint destruction. Vitamin D exerts immunomodulatory and anti-inflammatory effects, and its insufficiency is highly prevalent in RA and closely related to exacerbated inflammatory status. Synovial succinate acts as a pro-inflammatory metabolic mediator that aggravates intra-articular inflammation. However, the interactive associations among vitamin D, synovial succinate, and RA disease activity remain poorly defined. This cross-sectional study aimed to elucidate their correlations in RA patients.

A total of 120 RA patients were enrolled and stratified according to serum 25-hydroxyvitamin D [25(OH)D] levels (deficiency: < 20 µg/L). Demographic parameters, 28-joint Disease Activity Score (DAS-28), serum C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and synovial succinate levels were compared between groups. Pearson correlation and multiple linear regression analyses were performed to evaluate correlations and independent determinants of disease activity.

RA patients with vitamin D deficiency presented a significantly lower BMI, higher serum CRP and elevated synovial succinic acid levels compared with those without vitamin D deficiency, whereas no inter-group differences in DAS-28 scores and ESR were observed. Correlation analysis revealed that serum vitamin D levels were negatively correlated with CRP (P = 0.012), ESR (P = 0.049) and synovial succinic acid levels (P = 0.030), while no significant correlation was found between vitamin D and DAS-28 scores. Additionally, synovial succinic acid levels were positively correlated with DAS-28 scores (P = 0.002), CRP (P = 0.035) and ESR (P = 0.031), respectively. Multiple linear regression analysis validated that age, ESR and synovial succinic acid were independent positive influencing factors for increased DAS-28 scores in RA patients.

This study demonstrates a high prevalence of vitamin D deficiency in RA patients, which is associated with elevated synovial succinate. Synovial succinate is significantly correlated with disease activity and serves as an independent influencing factor for DAS-28, together with age and ESR. Given the cross-sectional design, these findings confirm associative rather than causal relationships.