Vitamin D is produced when the skin is exposed to sunlight.
4 UV-B photons act on pro–vitamin D
3, a precursor in the cholesterol biosynthetic pathway, in the plasma membrane of epidermal cells to form pre–vitamin D
3.
1 Pre–vitamin D
3 is rapidly transformed into vitamin D
3 and transferred to the extracellular space where it binds to vitamin D–binding protein.
1 From there it is transported to the liver, where it is hydroxylated into 25(OH)D.
5 Serum concentration of vitamin 25(OH)D is the best indicator of its status.
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Dietary sources of vitamin D include fatty fish (salmon, tuna, sardines, swordfish), cod liver oil, egg yolks, portabella mushrooms, beef liver, and fortified foods such as breakfast cereal, milk (dairy and nondairy), infant formula, cheese, and orange juice.
3,6 Fortified foods in the United States use synthetic vitamin D
2 (ergocalciferol) derived from irradiation of ergosterol found in plants, mold ergot, or plankton. Dietary vitamin D is absorbed in the small intestine, incorporated into chylomicrons, and then transported to the liver bound to vitamin D–binding protein.
5,7 From the liver, 25(OH)D travels to the kidney bound to vitamin D–binding protein.
5,7 The kidney further hydroxylates 25(OH)D to 1,25-dihydroxyvitamin D (1,25[OH]
2D), the most active form.
4 Once in the active form, 1,25(OH)
2D (calcitriol) travels to the rest of the body and any cells with vitamin D receptors.
5 Vitamin 1,25(OH)
2D has a shorter half-life and is not as good of an indicator of vitamin D status in the absence of advanced renal disease.
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