FIGURE 2.

Overview of the vitamin D metabolic pathway. Vitamin D₃ is obtained from the diet or by photoconversion in the skin. It is converted to its active form, 1,25-dihydoxyvitamin D₃ or calcitriol, by two consecutive hydroxylation steps, first in the liver and then in the kidneys. Calcitriol travels to its target organs (bones, intestines, kidneys) bound to vitamin D binding protein (DBP). It mediates its biological effects by activating the vitamin D receptor (VDR), which in turn modulates the expression of several genes involved in calcium homeostasis. Genes identified in the whole-genome scan are highlighted in red.