Vitamin D is not a vitamin in the strict definition because it can be produced by exposure of the skin to sunlight.
Vitamin D is synthesized in the skin from 7-dehydrocholesterol (provitamin D3 or pro D3) which is derived from cholesterol.
While a majority of cholesterols are synthesized in the liver, some cholesterol such 7-dehydrocholesterol are made in the skin.
Vitamin D is created when adequate exposure to sunlight (ultraviolet light-B) is available to promote the synthesis of vitamin D in the skin from 7-dehydrocholesterol.
It is a fat soluble steroid hormone precursor that contributes to the maintenance of normal levels of calcium in the blood stream by increasing absorption of calcium from food and reducing urinary calcium loss (reabsorption by the kidneys).
Both effects keep calcium in the body and therefore spare the calcium that is stored in bones. When necessary, vitamin D transfers calcium from the bone into the bloodstream, which does not benefit bones.
The bones grow denser and stronger as they absorb and deposit the calcium.
Although the overall effect of vitamin D on the bones is complicated, some vitamin D is necessary for healthy bones and teeth.
Other major physiological function is vitamin D is to maintain extracellular fluid concentrations of calcium and phosphorus within a normal range.
Nutritional vitamin D becomes essential when sunlight is insufficient to meet daily needs. This has become particularly acute as more people reside in urban centers where they are exposed to suboptimal level of sunlight.
What is vitamin D?
