Vitamin D from natural sources

Very few naturally occurring foods are rich in vitamin D. Typically animal foods contain cholecalciferol, while plant foods contain ergocalciferol. Some food items that naturally contain small amounts of vitamin D include oil fish such as salmon, mackerel and blue fish. Cod liver oil is the best source of vitamin D.

Liver meat is another source of abundant vitamin D. For vegetarians or vegans, all edible mushrooms have some content of vitamin D2 and ergosterol, which becomes activated with UVB exposure.

The two basic substances with vitamin D activity D2 and D3, occur only in yeast and fish liver oils. Vitamin D is also presents in small quantities in vegetables, meat and egg yolk.

The main food sources are those to which crystalline vitamin D has been added. Milk, because it is commonly used, has proved to be the most practical carrier. In United States fluid milk is voluntary fortified with 400 IU per quart of vitamin D.

The most efficient source of the vitamin D is not a food at all, but exposure to sunlight, which transforms a related pro-vitamin substance in the skin into a substance which the kidney can change into active vitamin D.

Sunlight provides 90 to 100% of the requirement for most people who are exposed to adequate sunlight. Vitamin D3 is synthesized in human skin from 7-dehydrocholesterol following exposure to ultraviolet B radiation with wavelength 290 to 320 nm.

Like other fat soluble vitamins, the sunlight activated pro-vitamin D can be stored away in the liver.
Vitamin D from natural sources

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