Seasonal variation in vitamin D levels in psoriatic arthritis patients from different latitudes and its association with clinical outcomes.

Objective. Vitamin D insufficiency appears to be a pandemic problem and is more common in inhabitants of high latitude compared to low latitude areas. We aimed to determine the prevalence of vitamin D deficiency/insufficiency in patients with psoriatic arthritis (PsA), its seasonal and geographic variation, and the possible association with demographics and disease activity.Methods. This study was conducted in a northern geographic area and in a subtropical region from March 2009 to August 2009. Most subjects were assessed in both winter and summer. Demographics, clinical data, skin photo type, and serum 25-hydroxyvitamin D (25[OH]D) levels were determined. Multivariate linear and logistic mixed models were used to assess the relationship with serum 25(OH)D levels.Results. In total, 302 PsA patients were enrolled. Two hundred fifty-eight patients were evaluated during the winter,while 214 patients were evaluated during the summer. 25(OH)D levels in winter and summer were adequate (north: 41.3%winter and 41.4% summer, south: 42.1% winter and 35.1% summer), insufficient (north: 55.7% winter and 58.6% summer,south: 50.9% winter and 62.2% summer), and deficient (north: 3% winter and 0% summer, south: 7% winter and 2.7%summer) among patients. There was no association between 25(OH)D levels, geographic and seasonal interaction, race,employment status, and skin photo type or disease activity in both seasons. No association between disease activity in summer and vitamin D levels in winter could be found.Conclusion. A high prevalence of vitamin D insufficiency among PsA patients was found. There was no seasonal variation in 25(OH)D levels among PsA patients in the southern and northern sites. No association could be established between disease activity and vitamin D level.