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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.