UVB and Vitamin D in Psoriasis

Phototherapy (broadband UVB, narrowband UVB (NBUVB) and heliotherapy – treatment with natural sunlight) is a commonly used treatment modality for widespread psoriasis. A similar wavelength spectrum of UVB (280-315 nm) is responsible for vitamin D synthesis in the skin. Vitamin D3, or cholecalciferol, is produced from 7-dehydrocholesterol in the basal epidermis when exposed to UVB, and is then hydroxylated in the liver into 25hydroxyvitamin D [25(OH)D], which is the major circulating metabolite. Further hydroxylation into 1,25-dihydroxyvitamin D [1,25(OH)2D] occurs primarily in the kidneys. Hydroxylation in the kidneys is stimulated by parathyroid hormone (PTH) and suppressed by phosphate. Homeostatic mechanisms include parathyroid activity, serum calcium and serum 1,25(OH)2D3 itself. Vitamin D is an essential steroid not only for calcium homeostasis and skeletal health, but also for regulation of cellular growth, cell proliferation and cell differentiation. Vitamin D is obtained by skin production in response to UVB or by intake of vitamin D rich food or supplements. Vitamin D status is measured by serum/blood concentration of its metabolite 25(OH)D.