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Benefits of Vitamin D supplements for health are well advertised. The value of this small essential magic molecule was recognized well before it was actually identified. The compound was first noted as an important component of cod liver oil that could prevent the rickets disease in dogs. Initially, the molecule was mistakenly documented as Vitamin A, but later Professor Daniel Whistler and Francis Glisson acknowledged it as a distinct entity from vitamin A and renamed it as Vitamin D.
The key advances in our understanding of the structure, function, synthesis, mode of action, metabolism and health benefits of Vitamin D came into the light during the 19th century. Basically, vitamin D is a group of fat-soluble steroids, namely secosteroids, that help in improving the absorption of calcium and phosphate in the intestine. Till date, five different forms of vitamin D (D1-D5) have been identified and out of them most important one for humans are vitamins D2 (ergocalciferol) and D3 (cholecalciferol).
Vitamin D is plentiful in the food and also get produced in our body
The major sources of vitamin D are cod liver oil, fish, caviar (black and red), soy products (tofu and soy milk), fortified cereals, oysters, eggs, mushrooms, milk and milk products. However, unlike the other vitamins, Vitamin D can be autonomously produced in our own bodies (in the skin, specifically cholecalciferol) through direct exposure to the sun light. Frequent exposure of the skin to sunlight or ultraviolet irradiation (UV) results in the production of vitamin D from its dietary precursor 7-dehydrocholesterol. It is the skin that fulfills about 90 percent of total body needs of vitamin D in the form of Vitamin D3. In the commercial production, Vitamin D3 can also be synthesized through UV exposure, for instance by exposing the milk directly to the UV light.
Contrary to vitamin D3, Vitamin D2 is derived from a membrane sterol “ergosterol”. Ergosterol is mainly produced by yeasts, mushrooms, phytoplanktons and invertebrates and consequently get transformed into ergocalciferol (D2) in response to UV irradiation. However, it is not present in the vertebrates and green land plants. In the human body, cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) are converted into calcidiol (25-hydroxycholecalciferol, or 25-hydroxyvitamin D3) and 25-hydroxyergocalciferol (25-hydroxyvitamin D2) and this entire event takes place inside the liver.
Ergocalciferol and calcidiol are two specific stable vitamin D metabolites that are often used as serum markers to determine vitamin D status. The calcidiol is subsequently converted into calcitriol in the kidney. Calcitriol circulate in blood stream as a hormone and regulate the concentration of calcium and phosphate. This compound is known as the most biologically active form of vitamin D.