Vitamin D is a fat-soluble vitamin that can be obtained in a few different ways. It can be obtained from sun exposure, foods, and in supplement forms. It is traditionally found in two types, D2 and D3. D2 comes from plant sources, such as mushrooms and fortifies foods. D3 comes from animal sources like fish oil, liver, and egg yolks. It is also produced when the body is exposed to sunlight. Vitamin D is essential for the regulation of phosphorus and calcium and for ensuring the immune system is functioning optimally.

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Vitamin D, in its various forms, is not an essential nutrient – as it is produced endogenously within the body, as a reaction to the skin’s exposure to the sun – and thus not commonly considered a vitamin. Despite its technical classification, vitamin D (a hormone), is nevertheless an important biologically-active compound, necessary for the calcium homeostasis and metabolism, along with increasing the absorption of magnesium and phosphate.

Vitamin D also works to promote adequate calcium absorption in the intestinal tract, which is needed to enable normal bone mineralization and to prevent involuntary muscle contractions that lead to cramps and spasms. Vitamin D is also needed for bone growth and bone remodeling performed by specific cells call osteoblasts and osteoclasts.

Other roles that this fat-soluble vitamin play in the body include immune function, glucose metabolism, as well as modulation of cell processes, such as cell growth.

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Vitamin D3 Label

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References:

Holick MF (December 2004). "Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease." The American Journal of Clinical Nutrition. 80 (6 Suppl): 1678S–88S. PMID 15585788.
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Norman AW (August 2008). "From vitamin D to hormone D: fundamentals of the vitamin D endocrine system essential for good health." The American Journal of Clinical Nutrition. 88 (2): 491S–499S. PMID 18689389.
"Vitamin D." NIH Office of Dietary Supplements. February 11, 2016. "Vitamin D Tests." Lab Tests Online (USA). American Association for Clinical Chemistry. Retrieved June 23, 2013.
Hollis BW (January 1996). "Assessment of vitamin D nutritional and hormonal status: what to measure and how to do it." Calcified Tissue International. 58 (1): 4–5. doi:10.1007/BF02509538. PMID 8825231.
Holick MF, Schnoes HK, DeLuca HF, Suda T, Cousins RJ (July 1971). "Isolation and identification of 1,25-dihydroxycholecalciferol. A metabolite of vitamin D active in intestine." Biochemistry. 10 (14): 2799–804. doi:10.1021/bi00790a023. PMID 4326883.
Wolf G (June 2004). "The discovery of vitamin D: the contribution of Adolf Windaus." The Journal of Nutrition. 134 (6): 1299–302. PMID 15173387.

Vitamin D3