There are ten compounds of vitamin D, called vitamin D1 through vitamin D10. The most important of these compounds are D2 (ergocalciferol) and D3 (cholecalciferol).
Ergocalciferol is found exclusively on plant foods. It is formed from plant sterol ergosterol by the action of UV light.
Cholecalciferol is found in animal foods such as eggs and fish oils, most is synthesized in the skin. Cholecalciferol is formed directly from the steroid known as 7-dehydrocholesterol by the action of sunlight, or other source of ultraviolet light.
Cholecalciferol and ergocalciferol in the diet are absorbed and carried to the liver in chylomicrons.
Cholecalciferol and ergocalciferol circulated for only 1 or 2 days. This quick turnover is due to rapid hepatic conversion and uptake by fat and muscle cells.
In the liver, cholecalciferol and ergocalciferol are converted in to calcidiol and then sent to the kidneys. Calcidiol is the main circulating form of the vitamin.
The kidneys perform the final step – the formation of 1,25-dihydroxyvitamin D3 which is also called calcitriol.
1,25-dihydroxyvitamin D3 is the active form of vitamin D, and the body derives about 90 percent of its 1,25-dihydroxyvitamin D3 from the cholecalciferol synthesized in the skin.
Vitamin D2 and D3: cholecalciferol and ergocalciferol