Vitamin D -Do our patients need not just a room with a view, but one with sunshine?

摘要

Vitamin D comes in two forms, D_2 (ergocalciferol) and D_3 (cholecalciferol), that have the same chemical structure except that D_2 has a methyl group on Carbon 24 of the parent molecule. D_2 is ingested in foods whereas D_3 can be synthesized in the skin from the action of ultraviolet light on 7-dehydrocholesterol. Both compounds are metabolically inert. The active compound is the product of two chemical reactions-the first occurs in the liver mediated by 25-hydrox-ylase which forms 25-hydroxyvitamin D or 25OHD, and the second in the kidney where 1alpha-hydroxylase converts 25OHD to the biologically active hormone 1,25(OH)_2 vitamin D (calcitriol), active in the regulation of serum calcium and phosphate levels. 25OHD is carried in the bloodstream by vitamin D binding protein (DBP). Following its synthesis in the kidney, 1,25(OH)_2, calcitriol, is transported to its sites of action by the same DBP. Even though calcitriol is the biologically active mol- ecule, most studies of the effects of vitamin D have measured its precursor, 25OHD. Other than an additional methyl group on Carbon 24 there is no known difference between 25OHD derived from ergocalciferol (25OH vitamin D_2) or from cholecalciferol (25OH vitamin D_3), and therefore, these observational studies focus on total 25OHD levels. 25OHD bound to DBP circulates in the blood and when calcium levels decrease, or for other unknown reasons, the 25OHD is taken up by the kidney and hydroxylated to the biologically active calcitriol. The process is tightly controlled and calcitriol is not produced in the kidney unless there is a need.