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 phototype, 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 phototype 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.