Vitamin D

 History of vitamin D:
American researchers Elmer McCollum and Marguerite Davis in 1913 discovered a substance in cod liver oil which later was called "vitamin A". British doctor Edward Mellanby noticed dogs that were fed cod liver oil did not develop rickets and concluded vitamin A or a closely associated factor, could prevent the disease. In 1921, Elmer McCollum tested modified cod liver oil in which the vitamin A had been destroyed. The modified oil cured the sick dogs, so McCollum concluded the factor in cod liver oil which cured rickets was distinct from vitamin A. He called it vitamin D because it was the fourth vitamin to be named. It was not initially realized that unlike other vitamins, vitamin D can be synthesised by humans through exposure to UV light.
In 1923, it was established that when 7-dehydrocholesterol is irradiated with light, a form of a fat-soluble vitamin is produced (now known as D3). Alfred Fabian Hess showed "light equals vitamin D." Adolf Windaus, at the University of Göttingen in Germany, received the Nobel Prize in Chemistry in 1928, for his work on the constitution of sterols and their connection with vitamins. In 1929 a group at NIMR in Hampstead, London, were working on the structure of vitamin D, which was still unknown, as well as the structure of steroids. A meeting took place with J.B.S. Haldane, J.D. Bernal and Dorothy Crowfoot to discuss possible structures, which contributed to bringing a team together. X-ray crystallography demonstrated that sterol molecules were flat, not as proposed by Windaus. In 1932 Otto Rosenheim and Harold King published a paper putting forward structures for sterols and bile acids which found immediate acceptance. The loose association between the team members Bourdillon, Rosenheim, King and Callow was very productive and led to the isolation and characterization of vitamin D. At this time the policy of the MRC was not to patent discoveries, believing that results of medical research should be open to everybody. In the 1930s Windaus clarified further the chemical structure of vitamin D.

In 1923, American biochemist Harry Steenbock at the University of Wisconsin demonstrated that irradiation by ultraviolet light increased the vitamin D content of foods and other organic materials. After irradiating rodent food, Steenbock discovered the rodents were cured of rickets. A vitamin D deficiency is a known cause of rickets. Using $300 of his own money, Steenbock patented his invention. His irradiation technique was used for foodstuffs, most memorably for milk. By the expiration of his patent in 1945, rickets had been all but eliminated in the US.

Vitamin D

Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique because it can be ingested as cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2) and because the body can also synthesize it (from cholesterol) when sun exposure is adequate. Although vitamin D is commonly called a vitamin, it is not actually an essential dietary vitamin in the strict sense, as it can be synthesized in adequate amounts by all mammals from sunlight. An organic chemical compound is only scientifically called a vitamin when it cannot be synthesized in sufficient quantities by an organism and must be obtained from their diet. However, as with other compounds commonly called vitamins, vitamin D was discovered in an effort to find the dietary substance that was lacking in a disease, namely, rickets, the childhood form of osteomalacia. Additionally, like other compounds called vitamins, in the developed world vitamin D is added to staple foods, such as milk, to avoid disease due to deficiency.

Upper intake levels:

The Tolerable Upper Intake Level is defined as "the highest average daily intake of a nutrient that is likely to pose no risk of adverse health effects for nearly all persons in the general population”. Although tolerable upper intake levels are believed to be safe information on the long-term effects is incomplete and these levels of intake are not recommended:

0–6 months of age: 1,000 IU

6–12 months of age: 1,500 IU

1–3 years of age: 2,500 IU

4–8 years of age: 3,000 IU

9–71+ years of age: 4,000 IU

Pregnant/lactating: 4,000 IU

Dietary sources:

Vitamin D2
Plant: Alfalfa (Medicago sativa subsp. sativa),
Fungus: Mushrooms.
Vitamin D2, or ergocalciferol found in fungi, is synthesized from viosterol which in turn is activated when ultraviolet light stimulates ergosterol. Human bioavailability of vitamin D2 from vitamin D2-enhanced button mushrooms via UV-B irradiation is effective in improving vitamin D status and not different to a vitamin D2 supplement. From UV-irradiated yeast baked into bread is bioavailable. By visual assessment or using a chromometer, no significant discoloration of irradiated mushrooms, as measured by the degree of "whiteness", was observed. Claims have been made that a normal serving (approx. 3 oz or 1/2 cup, or 60 grams) of fresh mushrooms treated with ultraviolet light have increased vitamin D content to levels up to 80 micrograms, or 2700 IU if exposed to just 5 minutes of UV light after being harvested.
Vitamin D3
In some countries, staple foods are artificially fortified with vitamin D. Dietary sources of vitamin D include:

Fatty fish species, such as: Catfish, Salmon, Mackerel, Sardines, Tuna, Eel,
A whole egg, Beef liver, Fish liver oils, such as cod liver oil also contain a lot of vitamin D.