Vitamin D Biochemistry



Vitamin D (calciferol) 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 upon 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 biologically-active form: 1,25-dihydroxy vitamin D (calcitriol). **Errata: 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 (calcitriol) 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.

Key Points

  • Vitamin D (calciferol)
    • Group of fat-soluble compounds
      • D2 (ergocalciferol)
        • ingested from plants
      • D3 (cholecalciferol)
        • consumed in dairy products
        • formed in sun-exposed skin (at stratum basale)
        • 25-OH D3 (calcidiol)
          • storage (inactive) form, formed by first hydroxylation at liver
        • 1,25-OH D3 (calcitriol)
          • active form, created by second hydroxylation at kidney
            • may be deficient in renal failure
    • Functions
      • Increases GI absorption of calcium and phosphate
      • Affects bone mineralization
        • mineralization at low levels
        • resorption at high levels

Find Vitamin D Biochemistry and other Vitamins among Pixorize's visual mnemonics for the USMLE Step 1 and NBME shelf exams.