Vitamin D levels in ankylosing spondylitis: Does deficiency correspond to disease activity?

Ankylosing spondylitis (AS) is an inflammatory disorder that presents with arthritis of the axial skeleton, including sacroiliac joints. Vitamin D is a secosteroid hormone with a long-established role in calcium and phosphate homeostasis, and in the regulation of bone formation and resorption. It is now known that vitamin D plays an immunosuppressive role in the body, and there is interest of late in the role of vitamin D in autoimmune diseases. Inflammation may be responsible for some of the loss of bone mineral density seen in AS. We reviewed the literature for studies assessing vitamin D level as a marker of AS disease activity and those examining vitamin D levels in AS in comparison to healthy controls. Four of 7 studies found a significant negative correlation between vitamin D levels and Bath Ankylosing Spondylitis Index (BASDAI), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). In a review of 8 case-control studies, the mean level of 25-hydroxyvitamin D₃ was 22.8 ± 14.1 ng/mL in 555 AS patients versus 26.6 ± 12.5 ng/mL in 557 healthy controls. When compared with a 2-sample t test, vitamin D levels were significantly higher in healthy controls (p < 0.01). We conclude that patients with AS appear to have lower vitamin D levels versus healthy controls; however, the cause is unclear. Existing studies do not demonstrate a consistent link between vitamin D levels and disease activity in AS. Further studies are in need to determine if a causative link exists between vitamin D deficiency and AS.