Physiological Roles of Vitamin D

It has been found that Vitamin D is converted to active derivatives in the body which are 25-hydroxycholecalcifero and 1, 25 dihydroxycalciferol in the liver which then enters the blood stream where it represent the major circulating metabolite of cholecalciferol. 25-hydroxycalciferol is then further hydroxylated to 1,25 dihydroxycalciferol which is now regarded the biologically active form of vitamin D. Absence of the enzyme catalyzing the conversion of Cholecalciferol to 25-hydroxycholecalciferol results in vitamin D resistant rickets; there patients respond to 25-hydroxycholecalciferol

The roles of Vitamin D in Body are as follows:

1. Vitamin D causes an increased absorption of Ca and P from Digestive Tract. Vitamin G greatly increases the amount of the protein responsible for transporting Ca across the mucosa of the duodenum and the upper small intestine. This leads an increased rate of absorption of Ca; the increased absorption of P is probably secondary to that of Ca.
   
   
2. Vitamin D leads to an efficient utilization of Ca and P in bone formation even if these mineral may be available in lesser amounts. Bone contains about 1% citrate. Vitamin D increase the citrate Level of bones which in some way also helps in bone formation.
   
   
3. It decreases P Excretion in Urine in the intact animal by increasing P re-absorption in renal tubules. This caused by a suppression of parathyroid activity brought about by a raised blood Ca level. This is in spite of the fact that the direct action of vitamin D is an increased P excretion in the urine; this effect is however less pronounced than that exerted through the parathyroid glands described above.
   
   
4. Vitamin D appear to have a direct effect of increasing tubular re-absorption of amino acids since aminoaciduria is seen in patients suffering from rickets.
   
   
5. It has been found that Vitamin D is converted to active derivatives in the body which are 25-hydroxycholecalcifero and 1, 25 dihydroxycalciferol in the liver which then enters the blood stream where it represent the major circulating metabolite of cholecalciferol. 25-hydroxycalciferol is then further hydroxylated to 1,25 dihydroxycalciferol which is now regarded the biologically active form of vitamin D. Absence of the enzyme catalyzing the conversion of Cholecalciferol to 25-hydroxycholecalciferol results in vitamin D resistant rickets; there patients respond to 25-hydroxycholecalciferol.