Vitamin D Deficiency and Psoriatic Arthritis

A large new study has found further links between vitamin D deficiency and chronic inflammatory rheumatic diseases (CIRDs) such as psoriatic arthritis.

Researchers in Spain compared 677 controls to 721 patients with psoriatic arthritis, 775 patients with rheumatoid arthritis and 738 with ankylosing spondylitis. They found vitamin D deficiencies — defined as less than 20 ng/ml — in blood samples from 26.7% of the control patients, 40.9% of the psoriatic arthritis patients, 40.5% of the rheumatoid arthritis patients and 39.7% of the ankylosing spondylitis patients.

Median levels of 25-hydroxyvitamin D were 24.8 ng/ml (25^th percentile to 75^th percentile, 18.4-32.6 ng/ml) in controls, 20.0 ng/ml (14.0-28.8 ng/ml) in psoriatic arthritis patients, 20.4 ng/ml (14.4-29.2) ng/ml in rheumatoid arthritis patients and 20.9 ng/ml (13.1-29.0 ng/ml) in ankylosing spondylitis patients.

Further analysis of these figures found a significant relationship between rheumatoid arthritis and vitamin D deficiency (adjusted odds ratio [OR], 1.46; 95 % confidence interval [CI], 1.09-1.96; p = 0.012). The positive association did not reach statistical significance for patients with psoriatic arthritis (OR, 1.32; 95 % CI, 0.94-1.84) or patients with ankylosing spondylitis (OR, 1.23; 95 % CI, 0.85-1.80).

When the research team compared common metrics of disease severity and vitamin D levels, it found no significant associations in psoriatic arthritis patients. Team members did, however, find a marginally significant association between vitamin deficiency and ACPA positivity in rheumatoid arthritis patients (OR, 1.45; 95 % CI, 0.99-2.12; p = 0.056) and a similar association between vitamin deficiency and BASFI in patients with ankylosing spondylitis patients (OR, 1.08; 95 % CI, 0.99-1.17; p = 0.07).

“Vitamin D plays an important role in the immune regulation. Vitamin D deficiency has been observed in some autoimmune diseases, in particular in systemic lupus erythematosus and rheumatoid arthritis. However, information related to undifferentiated spondyloarthropathies and ankylosing spondylitis is limited. It is also the case for psoriatic arthritis,” the study authors wrote in Arthritis Research & Therapy.

“We feel that our results may be considered of potential interest in daily clinical practice… Although a recent umbrella review of systematic reviews and meta-analyses of observational studies and randomized trials did not demonstrate that supplementation of vitamin D improves the health of the general population, we believe it is important to monitor and supplement vitamin D to patients with CIRD and vitamin D deficiency, regardless of whether the deficiency of vitamin D may or may not have a pathogenic role, or whether it is merely an epiphenomenon associated with inflammatory disease.”

The study authors noted several limitations to their work. Most notable among them was the fact that the control subjects were merely individuals without any diagnosed CIRD rather than fully healthy subjects. A high percentage of control group subjects had osteoarthritis, osteoporosis and/or other musculoskeletal diseases that are themselves known to be associated with vitamin D deficiencies. Another control-related limitation was the fact that control group members and study group members were recruited at different times of year, which may have affected their relative levels of sun exposure.

“Patients with CIRD have less mobility and life outdoors, which would also contribute negatively to maintain adequate levels of vitamin D,” the study authors wrote. “Therefore, vitamin D deficiency in these patients may be explained by a dual mechanism. On the one hand, chronic diseases can predispose to vitamin D deficiency directly by decreasing synthesis or increasing vitamin D catabolism, and on the other hand, indirectly lowering sunlight exposure in phases of reduced mobility and ability to spend time outdoors in patients with worse functional status.”