Figure 1.

Metabolic pathways by which vitamin D exerts its many effects in the body. (A) The prevailing scheme before recognition of the role of peripheral 1-α-hydroxylation. In this scheme, essentially all conversion of 25-hydroxyvitamin D [25(OH)D] to calcitriol occurs in the kidney, and the synthesized calcitriol appears in the serum, where it can be measured. Calcium-binding protein (CaBP) is a stand-in for the complex calcium absorptive apparatus induced in the enterocyte by calcitriol. (B) The current scheme, explicitly incorporating extrarenal 1-α-hydroxylation, with the resulting calcitriol appearing mainly intracellularly, where it is clinically unmeasureable. (Copyright Robert P. Heaney, 2008. Used with permission.)