Vitamin D Biochemistry
Vitamin D is a fat-soluble vitamin that exists in a number of different forms. Ergocalciferol (D2) is a less potent form of vitamin D found in plants, whereas cholecalciferol (D3) is a more potent form synthesized in the skin after exposure to sunlight. In addition, dairy products may be fortified with vitamin D3.
After ingestion or synthesis, vitamin D (calciferol) undergoes hydroxylation in the liver to form 25-hydroxy Vitamin D (calcidiol), the inactive storage form of vitamin D. Later, calcidiol undergoes further hydroxylation in the kidney, forming the active form: 1,25-dihydroxy vitamin D (calcitriol). **Errata: the video mentions "peritubular capillaries" in the kidney. 1,25 alpha-hydroxylase acts in the proximal tubules of the kidney. For examination purposes, knowing the kidney as the site of activation is sufficient.
Active vitamin D operates at the kidneys, GI tract, and bone to control calcium and phosphate levels. At low levels, it leads to increased bone mineralization. At high levels, it leads to increased bone resorption, thereby increasing calcium and phosphate levels.
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