Vitamin D (D3 and D2)

Vitamin D is required for calcium and phosphorus absorption, normal mineralization of bone, and mobilization of calcium from bone. This vitamin has been called the sunshine vitamin since ultraviolet light is involved in the conversion of provitamin substances to vitamins D2 and D3.

Vitamin D is the general name given to a group of fat-soluble compounds that are essential for maintaining the mineral balance in the body.

The main forms are vitamin D2 and vitamin D3. In nature only a few foods contain vitamin D, such as fatty fish, liver and egg yolks.

Vitamin D is a generic term of all steroids including vitamin D3 and vitamin D2. Vitamin D3, also known as cholecalciferol, and D2, known as ergocalciferol.

Vitamin D3, is the primary form of the vitamin encountered in zoologic species; it is stored in a number of tissues, including the liver and skin. Fish liver oils are a rich natural source of this material.

Vitamin D3, produced in the skin of humans and animals by the action of sunlight (ultraviolet light 290 to 320 nm) on its precursor molecule, the cholesterol derivative 7-dehydrocholesterol (a normal metabolite of cholesterol found in the skin). Absorption of light energy induces breakage of the 9, 10 carbon bond; a spontaneous isomerization (shifting of double bonds) then occurs. It is said to be more bioactive.

Ergocalciferol, or vitamin D2, is derived from ergosterol, a plant steroid and only enters the body via the diet, from consumption of foods such as oily fish, egg yolk and liver. It is the form of this vitamin normally used to fortify such foods as milk, bread, and cereals.

Vitamin D is absorbed from the diet in the intestinal tract in association with liquids and the presence of the bile slats. Once in the liver, one metabolite 25 hydroxy-vitamin D3 is formed, which is a about four times as active as vitamin D.

Vitamin D (D3 and D2) in physiologic replacement and pharmacologic doses has been used to correct vitamin D depletion in the elderly and to prevent vitamin D deficiency at all ages.
Vitamin D (D3 and D2)

Synthesis of vitamin D in the skin

The human body also is able to synthesize this vitamin from components of the skin through exposure to ultraviolet or sunlight. Most of the world’s population relies on natural exposure to sunlight to maintain adequate vitamin D nutrition.

To get all vitamins D is to go outside. That’s because vitamin D is made when the sun shines on the human skin.

In the skin, 7-dehydrocholesterol in the epidermis and dermis changes conformation ultraviolet B (UVB) radiation in the wavelength range of 280-315 nm passes through these skin layers to form pre-vitamin D3.

Subsequent enzymic conversion occurs outside the skin predominantly, although both the necessary enzymes, 25-hydroxylase and 1 α-hydroxylase are present in the skin.  Thus the active form of vitamin D,1,25-dihydroxyvitamin D3 may be present in the skin.

While the reaction to form pre-vitamin D3 takes minutes, the reaction converting it to vitamin D3 takes hours to occur and is a rate-limiting step.

The vitamin D3 gets carried to liver, where it gets changes into a more active form; from there, it goes to kidneys, where it becomes even more active.

Some of the vitamin D3 stays in liver and kidneys, where it helps reabsorb calcium from blood.
Synthesis of vitamin D in the skin