Question

Calcitriol, commonly known as Vitamin D, is known to be important for healthy bone formation and maintenance. What did you learn in the previous question that clarifies why this statement isn’t completely correct?

The previous question was to describe the function of the three main hormones involved in calcium regulation. (parathyroid, calcitriol, calcitonin).

All the sources I am provided with, say that calcitriol is the most active form of vitamin D, and is important for bone formation and maintenance. So why isn't the statement completely correct?

Answer 1

The statement is not completely correct because Vitamin D is available as cholecalciferol (D3) or ergosterol ( D2) which are not biological active. It has to be metabolized to 1,25-dihydroxycholecalciferol to show it's biological action which happens in two steps in liver and then in kidney. Vitamin D alone is not important for bone formation and maintenance. Vitamin D helps in intestinal absorption of calcium. In the absence of vitamin D, calcium is not absorbed properly. Vitamin D stimulates several proteins to transport calcium. Hence for proper bone formation Vitamin D has to be converted to biological active form 1,25-dihydroxycholecalciferol which then aids in calcium absorption for bone formation and maintenance.