The Role of Vitamin D in Disease Activity in Axial Spondyloarthritis

Axial spondyloarthritis (axSpA) is a chronic inflammatory disease which predominantly involves the axial skeleton and sacroiliac joints. The aetiology remains unknown but is thought to be immune driven. Vitamin D is a focus for research in numerous autoimmune conditions, especially because it is now thought to have an important role in immunoregulation. It has been hypothesised that low levels of vitamin D increase the risk of autoimmune disease. Considering that vitamin D is implicated in immune regulation and autoimmunity, a question that arises is whether vitamin D deficiency can lead to increased disease activity in axSpA. Through this narrative review of the literature the authors explore potential links between vitamin D and axSpA. This review highlights that larger and more methodologically robust prospective longitudinal studies are required to answer this key question. There was considerable heterogeneity between studies, including in the definition of vitamin D deficiency, latitude where the study took place, and seasonal variation. Another clinically relevant aspect to address is whether correcting vitamin D deficiency leads to improved makers of disease activity in patients with ankylosing spondylitis. This may justify mandatory food fortification and specific supplementation programmes in countries at risk. For example, in Finland there is a low prevalence of vitamin D deficiency in the general population because of food fortification.