Vitamin D

Vitamin D is a group of structurally related, <a href="/facts/Fat-soluble/AWpsR8Xe">fat-soluble</a> compounds responsible for increasing intestinal absorption of <a href="/facts/Calcium/hf252CC0">calcium</a>, <a href="/facts/Magnesium/XyIhd2FG">magnesium</a>, and <a href="/facts/Phosphate/FDGVCxUG">phosphate</a>, along with numerous other biological functions. In humans, the most important compounds within this group are vitamin D3 (<a href="/facts/Cholecalciferol/ILqn7syS">cholecalciferol</a>) and vitamin D2 (<a href="/facts/Ergocalciferol/cpRFLyNz">ergocalciferol</a>).
Unlike the other twelve vitamins, vitamin D is only conditionally essential, as with adequate skin exposure to the <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet B (UVB) radiation</a> component of <a href="/facts/Sunlight/WPkJRC7s">sunlight</a> there is <a href="/facts/Chemical_synthesis/3KXsVVwC">synthesis</a> of cholecalciferol in the <a href="/facts/Epidermis/JvRABubP">lower layers of the skin's epidermis</a>. For most people, skin synthesis contributes more than diet sources. Vitamin D can also be obtained through diet, <a href="/facts/Food_fortification/Yr96v3Jj">food fortification</a> and <a href="/facts/Dietary_supplement/SHXNNxaS">dietary supplements</a>. In the U.S., cow's milk and plant-based milk substitutes are fortified with vitamin D3, as are many breakfast cereals. Government dietary recommendations typically assume that all of a person's vitamin D is taken by mouth, given the potential for insufficient sunlight exposure due to urban living, cultural choices for the amount of clothing worn when outdoors, and use of <a href="/facts/Sunscreen/W6J13MRz">sunscreen</a> because of concerns about <a href="/facts/Health_effects_of_sunlight_exposure/pcUI9WMr">safe levels of sunlight exposure</a>, including the risk of <a href="/facts/Skin_cancer/r9BWEX7W">skin cancer</a>.: 362–394  
Cholecalciferol is converted in the liver to <a href="/facts/Calcifediol/H8jsbUpR">calcifediol</a> (also known as calcidiol or 25-hydroxycholecalciferol), while ergocalciferol is converted to ercalcidiol (25-hydroxyergocalciferol). These two vitamin D metabolites, collectively referred to as 25-hydroxyvitamin D or 25(OH)D, are measured in serum to assess a person's vitamin D status. Calcifediol is further hydroxylated by the kidneys and certain immune cells to form <a href="/facts/Calcitriol/3SXesdFb">calcitriol</a> (1,25-dihydroxycholecalciferol; 1,25(OH)2D), the biologically active form of vitamin D. Calcitriol attaches to <a href="/facts/Vitamin_D_receptor/G6UAEBhn">vitamin D receptors</a>, which are <a href="/facts/Nuclear_receptor/9aYqKxnM">nuclear receptors</a> found in various tissues throughout the body.
The discovery of the vitamin in 1922 was due to an effort to identify the dietary deficiency in children with <a href="/facts/Rickets/Fd1r1k78">rickets</a>. <a href="/facts/Adolf_Windaus/NKDps602">Adolf Windaus</a> received the <a href="/facts/Nobel_Prize_in_Chemistry/8Nb8Ajjy">Nobel Prize in Chemistry</a> in 1928 for his work on the constitution of <a href="/facts/Sterols/QrVlPnWn">sterols</a> and their connection with vitamins. Present day, government food fortification programs in some countries and recommendations to consume vitamin D supplements are intended to prevent or treat vitamin D deficiency rickets and osteomalacia. There are many other health conditions linked to vitamin D deficiency. However, the evidence for the health benefits of vitamin D supplementation in individuals who are already vitamin D sufficient is unproven.

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Vitamin D