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.
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.
is the precursor of vitamin D3
and only forms the vitamin after being exposed to solar UV radiation
. This creates calciol.
Calciol is then hydroxylated in the liver to become calcidiol.
Next, calcidiol is once again hydroxylated, this time in the kidney, and becomes 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
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 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.
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
A 2008 study published in Cancer Research has shown the addition of vitamin D3
(along with calcium) to the diet of mice fed a regimen nutritionally similar to a new Western diet prevented colon cancer development.
There's a minority view, often associated with Trevor Marshall
, which asserts that low levels of 25D are often due to overconversion into calcitriol because of chronic infection rather than 25D deficiency.