What are the Most Important Functions of Vitamin D?

Functions of Vitamin D (Calcitriol )

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  • Vitamin D acts on target organs like bones, kidneys & intestinal mucosa to regulate  calcium & phosphate metabolism.
  • Calcitriol is the physiological active form of Vitamin D.
  • It increases the blood calcium level.

1. Intestinal Absorption of Calcium & Phosphate

  • Calcitriol (active form of Vitamin D) increases the absorption of Calcium from the brush border surface of the intestine (passive). 
  • From the intestinal cell to plasma it is transported by using energy  “active transport”.
  • It is either by the Sodium-Calcium exchange mechanism or by pumping out the Calcium-Calbindin Complex.
  • The action of Calcitriol is like a steroid hormone.
  • It enters the cell and binds to a specific nuclear receptor called VDR (Vitamin D Receptor) which binds to VDRE to form a dimer to activate Transcription of specific genes that code for Calbindin.
  • Due to the increased availability of calcium binding protein, the absorption of calcium is increased.

2. Mineralization of Bones

  • Calcitriol increases the activity of osteoblasts, releasing phosphatase leading to mineralization of bones.
  • It has its action on all 3 types of cells namely the osteoblasts, osteoclasts and osteocytes.
  • It is responsible for increased production of bone matrix proteins such as collagen, osteocalcein and osteopontin.

3. Other Effects

  • Vitamin D helps in the renal re-absorption of calcium & phosphorus.
  • It lowers the pH in certain parts of gut and increases urinary pH.
  • It also helps in the Signal transduction to brain and the immune system.
  • Recent research has proved that most tissues possess Vitamin D receptor and several tissues have the enzymes to generate calcitriol. 
  • 1, 25-DHCC has been found to have a modulatory effect on immune-hematopoietic system.
  • Therapeutic doses given to children with rickets have been found to correct the anemia, hypocellularity of the bone marrow & increased susceptibility to infection.
  • It has also been found to reduce the risk of cancer & coronary vascular disease.
  • Beneficial effects have been observed in patients with AIDS.
  • Calcitriol exerts its pleiotropic actions via the specific receptor VDR, an important transcription factor.
  • Vitamin D has been found to be a modulator of immune response.

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