Vitamin D is an endogenously synthesized hormone in our body. It is derived from 7 - dehydrocholesterol, which undergoes subsequent hydroxylation in the liver and kidney to form 1, 25 - dihroxycholecalciferol (Calcitriol) - Vit D. 

Synthesis:

  • Vitamin D synthesis starts from the skin, the precursor molecule 7 - dehydrocholesterol is converted to Cholecalciferol ( Known as Cholecalciferol).
  • Cholecalciferol is then transported to the liver by binding to a  plasma protein called D - Binding protein (DBP).
  • In the liver it undergoes hydroxylation at the 25th position to form 25 - hydroxy cholecalciferol.
  • The above molecule is then transported to the kidney to be hydroxylated at the 1st position (Predominant) to form the final product - 1, 25 - dihydroxy cholecalciferol ( Calcitriol ). This is the active form of vitamin D which exerts it’s action on different sites of the body, this is described later in this ar ticle.
    Vitamin D synthesis in a nutshell
Biological Functions of Vitamin D:

Now that we know how Vitamin D is synthesised, let’s see the actions that Vitamin D exerts in our body.

  • Effect on Bone:
Calcitriol acts on the osteoblasts and increases it’s activity, this causes mineralization of bones and it’s remodelling. The deficiency of vitamin D hence causes diseases like Rickets and osteomalacia.

  • Vitamin D and Intestine:
Calcitriol here in the intestine acts on the epithelium as a steroid hormone. It binds to the receptor for calcitriol and forms a complex, this Calcitriol - receptor complex acts intranuclear on the specific gene sequence which codes for CALBINDIN. CALBINDIN is then transcribed and helps in passive absorption of calcium. Note the movement of calcium from the epithelium to the blood is an active process.
  • Effect on renal tubules:
The renal tubules under the action of calcitriol reabsorbs Calcium and phosphates from the lumen of the tubules.

All these effects lead to an increase in the level of calcium and phosphates in the blood. This mechanism is kept under check by the serum levels of calcium, phosphates, parathormone and calcitonin.

Deficiency state of Vitamin D:

Deficiency of Calcitriol in the blood is not directly measured in a lab on a routine basis, hence we measure the intermediate in the synthesis of vitamin D - 25 hydroxy cholecalciferol. The optimum levels of 25Hydroxy cholecalciferol is 20 - 30 ng/ml. Any value of < 20 ng/ml is diagnostic of Vitamin D deficiency. 
Vitamin D deficiency in children produces RICKETS and in adults causes OSTEOMALACIA. Further discussion of these will be available in the Pathology section.

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