This site is intended for healthcare professionals

Go to /sign-in page

You can view 5 more pages before signing in

Go to /pro/cpd-dashboard page

This page is worth 0.05 CPD credits. CPD dashboard

Go to /account/subscription-details page

This page is worth 0.05 CPD credits. Upgrade to Pro

Vitamin D3 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.

Reference


Related pages

Create an account to add page annotations

Annotations allow you to add information to this page that would be handy to have on hand during a consultation. E.g. a website or number. This information will always show when you visit this page.