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Vitamin D metabolism

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

In the skin, sunlight and ultraviolet rays act on 7-dehydrocholesterol to form vitamin D3.

  • Vitamin D2 (known as ergocalciferol) and D3 are obtained from the diet.
    • Moderate amounts of bile salts and fat are required for absorption. An alpha-globulin, vitamin D transport protein, binds to the gut- and skin-derived vitamin D3 and transports it to the liver.

Vitamin D3 has several forms:

  • cholecalciferol (calciol) is an inactive, unhydroxylated form of vitamin D3
  • calcidiol (25-hydroxyvitamin D3) is measured in the blood to assess vitamin D status
    • in the liver, vitamin D3 is hydroxylated at the 25' position to produce 25-hydroxycholecalciferol (calcidiol). This is then bound to another selective transport protein for transit to the kidneys.
    • most abundant form of vitamin D, but is less active than calcitriol
  • calcitriol (also called 1,25-dihydroxyvitamin D3) is the active form of D3
    • produced in the kidneys through the action of 1-hydroxylase enzyme
    • Calcitriol is more potent than the other hydroxylation products in its actions on mineral physiology. Hence, by varying the activity of the 1-hydroxylase enzyme, the ratio of calcitriol to other hydroxylated products can be varied, and so the degree to which calcium and phosphate physiology is affected
      • renal hydroxylation of calcidiol to calcitriol by 1-alpha-hydroxylase is tightly regulated (stimulated by either parathyroid hormone or hypophosphatemia) and serves as the major control point in production of the most active circulating hormone calcitriol (1,25-dihydroxyvitamin D3)

The activity of the 1-hydroxylase enzyme is increased by:

  • hypocalcaemia
  • hypophosphataemia
  • parathyroid hormone
  • growth hormone

Calcidiol within the liver is the main storage form of vitamin D; it has a half life of about 14 days.


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