Key questions in Vitamin D research

Figures & data

Figure 1. Vitamin D metabolism, transport and body pools. Endogenously produced vitamin D diffuses into the blood, binds to the vitamin D binding protein (DBP) and transports to the liver. In contrast, dietary vitamin D is first packaged into chylomicrons after intestinal absorption, transported to the lymph into the peripheral blood where the chylomicrons are metabolized by lipoprotein lipase (LPL) and then transported to the liver in the rsulting remnant particles. Regardless of source, vitamin D is hydroxylated in the liver to 25-hydroxyvitamin D [25(OH)D], which is transported in the blood bound to DBP. In the kidney and possibly a wide-variety of extra-renal tissues, 25(OH)D is again hydroxylated to the active 1,25 dihydroxyvitamin D [1,25(OH)₂D]. Both 1,25(OH)₂D and 25(OH)D can be catabolized to the inactive calcitroic acid or 24,25(OH)₂D, respectively, by the 24-hydroxylase (CYP 24A1) that is also widely expressed. Circulating 25(OH)D can be taken up by one of two mechanisms, the ubiquitous diffusion of free 25(OH)D or the DBP-receptor mediated endocytosis by megalin/cubulin, which is selectively expressed in a tissue, developmental and pathophysiologic specific manner (adapted from Jones [44]).

Jones G. Pharmacokinetics of vitamin D toxicity. Am J Clin Nutr 2008;88(suppl):582S–586S.

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