Cholecalciferol

Overview
Cholecalciferol is a form of Vitamin D, also called vitamin D3. It is structurally similar to steroids such as testosterone, cholesterol, and cortisol (though vitamin D3 itself is a secosteroid).

1g of pure vitamin D3 is 40 000 000 (40x106) IU, or in other words, one IU is 0.025 μg.

Forms
Cholecalciferol has several forms:
 * Calciol, is an inactive, unhydroxylated form of vitamin D3)
 * calcidiol (also called 25-hydroxyvitamin D3), is the blood calcium form
 * calcitriol (also called 1,25-dihydroxyvitamin D3), is the active form of D3.

Metabolism
7-Dehydrocholesterol is the precursor of vitamin D3 and only forms the vitamin after being exposed to UV radiation. This creates calciol.

After exposure to the sun, calciol is sent to the liver to be hydroxylated where it becomes Calcidiol.

Next, it is sent to the kidney and once again hydroxylated becoming Calcitriol. Calcitriol is the active hormone form of vitamin D3, for this reason vitamin D is often referred to as a prohormone.

As food fortification
Cholecalciferol is the form of vitamin D normally added during fortification of foods. Cholecalciferol is produced industrially by the irradiation of 7-dehydrocholesterol extracted from lanolin found in sheep's wool. In products where animal products are not desired, the alternative is to use ergocalciferol (also known as vitamin D2) derived from the fungal sterol ergosterol.

Dose
While the absorption of cholecalciferol and ergocalciferol is approximately equivalent in healthy individuals, vitamin D3 is three to ten times more potent than vitamin D2, and has more prolonged effects. However, the human body cannot tolerate intake of as much vitamin D3 per day compared to the body's upper-limit intake of vitamin D2, limiting the overall efficacy of taking vitamin D3 supplements as opposed to vitamin D2 supplements despite vitamin D3's higher potency. The current standard is to recommend equivalent doses of vitamin D2 as vitamin D3, but with regard to the mentioned mechanisms it has been suggested that instead, the Recommended Daily Allowances (RDA) or International Units (IU) of vitamin D2 and D3 should be changed to recommend approximately 10 times more daily intake for vitamin D2 than vitamin D3.

For a current and illustrated discussion on this topic see:

Tavera-Mendoz, Luz E. & White, John H. (2007, November). Cell Defenses and the Sunshine Vitamin. Scientific American, pp.62-72.

Stability
Cholecalciferol is very sensitive to UV radiation and will rapidly, but reversibly break down to form supra-sterols, which can further irreversibly convert to tachysterol.