High levels of serum vitamin D-binding protein in patients with psoriasis: A case-control study and effects of ultraviolet B phototherapy

The role of vitamin D in psoriasis remains contradictory despite the fact that vitamin D analogues constitute an established treatment for psoriasis. It has been proposed that the ability of vitamin D to exert anti-inflammatory effects might not depend solely on the concentration of serum 25(OH)D but also on the concentration of vitamin D-binding protein (DBP). High concentrations of DBP might diminish vitamin D's biologic action. The aims of this study were (i) to analyze the serum levels of DBP, total and calculated free 25(OH)D in patients with psoriasis and compare the results with healthy controls and (ii) to study the effect of ultraviolet B (UVB) phototherapy on DBP levels. Caucasian subjects (n = 68) with active plaque psoriasis were compared with a population-based sample of men and women (n = 105), matched for age and sex. Season of enrollment was taken into consideration. The patients were also studied before and after UVB phototherapy. The severity of the disease was calculated as Psoriasis Area Severity Index (PASI). DBP, free 25(OH)D index and total 25(OH)D were higher in patients with psoriasis compared with controls (P= 0.004, P = 0.045 and P < 0.0001, respectively). DBP did not change after phototherapy, whereas 25(OH)D increased and intact parathyroid hormone (iPTH) decreased (P < 0.001 for both). Psoriasis improved and PASI decreased after phototherapy (P < 0.001). There was no correlation between DBP and 25(OH)D or between DBP and PASI. Measurement of DBP is recommended when evaluating vitamin D status in patients with psoriasis. High DBP levels in psoriasis imply a disturbed vitamin D pathway that warrants further investigation. Direct measurement of free 25(OH)D, instead of total 25 (OH)D that circumvents abnormally high levels of DBP, could be considered.