Effects of vitamin D on disease activity and serum interleukin-6 in rheumatoid arthritis

Vitamin D₃ or 25(OH)D₃ may have a potential role in rheumatoid arthritis (RA) pathogenesis by inhibiting the expression of pro-inflammatory cytokines including interleukin-6 (IL-6). The aim of this study is to determine the clinical factors of vitamin D deficiency in multi-ethnic Malaysian RA patients and its association with disease activity, functional disability and serum IL-6 levels.

One hundred RA patients and 50 healthy controls, sex- and age-matched, were recruited. Disease Activity Score of 28 joints and Health Assessment Questionnaire scores were assessed. Baseline serum 25(OH)D₃ and IL-6 were measured in all subjects. RA patients who were vitamin D deficient were given loading doses of vitamin D₃ and repeated assessments were done.

Vitamin D deficiency (<50 nmol/L) was found in 63% of RA patients and 76% of healthy controls. Chinese RA patients and healthy controls had significantly more sufficient 25(OH)D₃ levels compared to Malays and Indians (P < 0.001). Serum 25(OH)D₃ level was still negatively associated with body mass index in RA patients (P = 0.002) after adjustment for potential confounding variables. No significant association was seen between 25(OH)D₃ levels and disease activity or serum IL-6 levels in both pre- and post-treatment groups. A negative association was observed between serum 25(OH)D₃ and functional disability, including a 33% improvement post-treatment (mean ± SD: 0.30 ± 0.46 to 0.20 ± 0.18).

Vitamin D deficiency is prevalent in Malaysian RA patients. This study suggests that vitamin D is not associated with disease activity or serum IL-6 levels but it may have a role in functional disability in RA patients.