Vitamin D deficiency is a worldwide health problem. Over a billion people worldwide are vitamin D deficient or insufficient. Vitamin D, is described as the "Sun Vitamin". It is a steroid with hormone like activity. It regulates the functions of over 200 genes and is essential for growth and development. Two major forms of vitamin D [are] vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is made naturally in the body when skin is exposed to ultraviolet radiation in sunlight. It is converted to 25-hydroxyvitamin D in the liver. 25-Hydroxyvitamin D then travels through the blood to the kidneys, where it is further modified to 1,25-dihydroxyvitamin D, or calcitrol, the active form of vitamin D in the body. The most accurate way of evaluating a person's vitamin D status is to measure the level of 25-hydroxyvitamin D in the blood. Usually 50 to 90% of vitamin D is produced by sunshine exposure of skin and the remainder comes from the diet.[The] natural diet, most people consume, contain[s] little vitamin D. Traditionally the human vitamin D system begins in the skin, not in the mouth. However, important sources of vitamin D are egg yolk, fatty fish, fortified dairy products, and beef liver.
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