Vitamin D3, whether taken as a supplement or made in the skin from sunlight, is biologically inactive. Cholecalciferol, vitamin D3, is circulated to the liver through the bloodstream. In the liver cholecalciferol is hydroxylated (hydrogen and oxygen are added) to form calcidiol, the storage form of vitamin D. Calcidiol can remain in the liver for an extended time as the storage form of vitamin D. Calcidiol is also the major form of vitamin D found circulating in blood. Calcidiol is attached to a binding protein for transport through blood.
With decreased sunlight on the skin, depleted liver reserves, and not enough supplementation of vitamin D, calcidiol concentration in the blood decreases. This makes calcidiol a good indicator of vitamin D status
Calcitriol is one of the most potent
steroid hormones in the human body
To become active, this intermediary form of vitamin D, calcidiol, must be changed to its active form, calcitriol. This transformation to the active form normally takes place in the kidneys, but may also occur in some other tissues. It is only as calcitriol that vitamin D performs its physiological effects. Steroid hormones such as calcitriol are molecules in the body that are made from cholesterol. Steroid hormones act to turn certain genes on and off.
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