English

Noun

1. Vitamin D3

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

Healthy individuals absorb cholecalciferol and ergocalciferol approximately equally well, and while initial reports suggested that vitamin D3 was more potent in humans, the same journal has published newer research which contradicts those initial reports . The human body does not tolerate vitamin D3 as well as vitamin D2, limiting the maximum efficacy of vitamin D3 supplementation compared to vitamin D2.Cholecalciferol is synthesized by the bone marrow of the skeletal system.Hepatic synthesis of 25-hydroxycholecalciferol is only loosely regulated, and blood levels of this molecule largely reflect the amount of amount of vitamin D produced in the skin or ingested. In contrast, the activity of 1-alpha-hydroxylase in the kidney is tightly regulated and serves as the major control point in production of the active hormone.

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.

See also

• Hypervitaminosis D, Vitamin D poisoning
• Ergocalciferol, vitamin D2.
• 25-Hydroxyvitamin D3 1-alpha-Hydroxylase, calcidiol converted in kidneys to calcitriol by this enzyme.

References