Vitamin D is an example of a fat soluble vitamin which can be synthesized by the human body. It is produced when the skin is exposed to sunlight. 7-Dehydrocholesterol is converted by uv light to Previtamin D3, which is then converted to vitamin D3. Vitamin D is related to a group of hormone-like substances, but is actually a mixture of chemicals: D2 is called ergocalciferol, and D3 is called cholecalciferol.
Vitamin D is required for the absorption and uptake of calcium and phosphorus from the intestines, and is therefore essential for the mineralisation of bones. It is also responsible for the regulation of blood coagulation, along with vitamin K, and optimizes the functioning of the muscles and nerves.
Vitamin D is formed in the skin by the action of the sun, but is also available in fish liver oils.
The RDA is comparatively small, being about 10 micro grams. Children however should have a higher daily intake since their bones are growing. It is possible to overdose on vitamin D.
Deficiencies in vitamin lead to rickets disease, where bones becomes softened and deformed. Infants, nursing mothers and elderly people are especially at risk, and particularly in the summer, when the solar intensity is lower. It is also possible for vitamin D poisoning to occur, if daily intake exceeds 25000 International units. This leads to symptoms including unquenchable thirst, itchy eyes and skin, diarrhoea and frequent urination.