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Vitamin D exists in two forms: D2 (ergocalciferol) and D3(cholecalciferol). Vitamin D3, “the sunshine vitamin,” is synthetizedin the human epidermis via ultraviolet irradiation, orit may be consumed in the form of oily fish or supplements.Vitamin D2 is found in plants, as a product of irradiationof ergosterol [1]. The vitamin is converted in the liver andkidney to calcidiol and calcitriol, respectively, and acts onspecific target tissues via vitamin D receptors. Calcitriol, theactive form of vitamin D, binds to vitamin D receptors in theintestines, bones, and kidneys to increase calcium absorptionfrom the intestines, promote calcium deposition in bones,and decrease parathyroid hormone concentrations (PTH).Its extraosseous effects are less known. Vitamin D receptorswere found in other tissues, as well, including the brain,cardiomyocytes, vascular smooth muscle cells, endothelialcells, pancreatic beta-cells, skeletal muscle, breast, prostate,colon, macrophages, and skin, exerting several pleiotropiceffects, and their expression decreases with age. The vitaminD receptor is closely related to the thyroid, retinoid,and peroxisome proliferator-activator receptors [2]. Recentstudies have found active 1 alpha hydroxylase in severalextra renal tissues, such as the heart and vascular smoothmuscle cells [3–5]. Activated vitamin D may influence cellulargrowth, proliferation and apoptosis, oxidative stress,membrane transport, matrix homeostasis, cell adhesion, and
immune system functions and may regulate a large number
of genes and healthy aging [6, 7].
Vitamin D insufficiency is a common public health
problem, very often unrecognized and untreated, associated
with rickets, dental caries, and growth retardation in children
and osteomalacia, osteopenia, osteoporosis, decreased
muscle strength, falls, and increased risk of fracture in adults.
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