Vitamin D3 is formed in the skin, converted to a pre-hormone in the liver and becomes a fully active hormone in the kidney

Vitamin D3 (cholecalciferol) is formed in the skin by a photoreaction between ultraviolet (UV) rays of sunlight and a pre-cholesterol compound (7-dehydrocholesterol) in the skin.  Vitamin D3 is inert, that means it is not bioactive.  Vitamin D3 is also unstable, especially in the presence of air or oxygen.

As shown in the figure above, Vitamin D3 is transported in the blood stream to the liver where it is converted into the pre-hormone form, 25-hydroxy Vitamin D3 by the enzymatic addition of a single hydroxyl [OH-] group.  This pre-hormone form is also largely inert and unstable in the presence of air or oxygen.  This pre-hormone form is also called calcidiol.

The pre-hormone form of Vitamin D3 is then transported from the liver to the kidneys where it is converted to the active hormone form, 1, 25-dihydroxy Vitamin D3 by the enzymatic addition of a second hydroxyl [OH-] group.  The hormone form of Vitamin D3 is highly bioactive and also unstable in the presence of air or oxygen.  This hormone form is also called calcitriol.

• Cellular and Immune Functions – The active hormone bind into the Vitamin D receptor (VDR) that is an essential component to most cellular and immune functions.
• Vitamin D Associated Diseases – Vitamin D deficiencies can cause or contribute to diseases such as colorectal and prostate cancers, high blood pressure, and kidney and heart diseases, which affect black Americans at higher rates than whites.

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