It occurs naturally in food and human body can also produce it when human skin is exposed to sunlight. Skin synthesis is limited by various determinants, including pigmentation, age, zenith angle of the sun, poor air quality and % of the skin surface area available for exposure.
The two major physiologically relevant forms of vitamin D are D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 is photosynthesized in the skin of vertebrates by the action of solar ultraviolet (UV) B radiation on 7-dehydrocholesterol. Vitamin D2 is produced by UV irradiation of ergosterol, which occurs in molds, yeast, and higher-order plants.
Almost all of the U.S. milk supply is fortified with about 3 mcg (120 IU) vitamin D per cup. Many plant-based alternatives such as soy milk, almond milk, and oat milk are similarly fortified.
Vitamin D is added to many breakfast cereals and to some brands of orange juice, yogurt, margarine, and other food products.
There are some dietary sources of vitamin D. Fatty fish such as salmon, tuna, and mackerel are among the best sources. Beef liver, egg yolks, and cheese have small amounts of vitamin D.
It has long been known to help the body absorb and retain calcium and phosphorus; both are critical for building bone. Vitamin D sustains the normal mineralization of bone, muscle contraction, nerve conduction and general cellular function in all cells of the body.
Vitamin D in general
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Vitamin D2 (ergocalciferol) |
