Abstract
Vitamin D
3
and its plant‐derived counterpart vitamin D
2
(collectively referred to as vitamin D) are by themselves biologically inert for most of their actions. After cutaneous synthesis and transport into the general circulation, vitamin D disappears from the serum within a week. The vitamin D receptor (VDR) has an extraordinarily broad distribution among human tissues. The mechanism of action of the active form of vitamin D, 1,25‐(OH)
2
D, is similar to other steroid hormones. The classical, endocrine actions of 1,25‐(OH)
2
D involve its regulation of calcium and phosphate flux across bone, gut, and kidney. The parathyroid glands are the target of 1,25‐(OH)
2
D that inhibits the synthesis and secretion of PTH and prevents the proliferation of PTH‐producing cells in the glands. Adequate sunlight exposure is the most cost effective means of obtaining vitamin D. Whole body exposure to a 0.5 minimal erythema dose of sunlight has been calculated to provide the equivalent of 10,000 IU vitamin D
3
.