When you ingest Vitamin D, you can take it as D2 (found in some vitamins and many fortified foods like milk). Vitamin D3 is bioidentical to the D that humans make when we get sun.. D2 is called ergocalciferol, and D3 is called cholecalciferol. They are so named because ergocalciferol is synthesized by fungi (from ergosterol, the fungus version of cholesterol) and cholecalciferol is synthesized from cholesterol (the animal version of the fungal ergosterol).
Mostly, if you supplement, you'll find Vit D3 nowadays and it's probably a good idea to use it. The main issue is that it's not vegan - it's made from lanolin usually.
Once in the body, cholecalciferol is hydroxylated in the liver to form 25-hydroxycholecalciferol. This is then hydroxylated in the kidney to form 1,25-dihydroxycholecalciferol. This last one is the active ingredient. If your kidneys do not function, you can't do this - so your doc would give you calcitriol (same thing) and bypass it. Otherwise, your parathyroid hormone would shoot up and pull most of the calcium out of your bones (which would be bad).
If you have normal kidney and liver function you should not need calcitriol. The body will make it as needed from the stores of vitamin D3 that you consumed or made by sitting in the sun.
Mostly, if you supplement, you'll find Vit D3 nowadays and it's probably a good idea to use it. The main issue is that it's not vegan - it's made from lanolin usually.
Once in the body, cholecalciferol is hydroxylated in the liver to form 25-hydroxycholecalciferol. This is then hydroxylated in the kidney to form 1,25-dihydroxycholecalciferol. This last one is the active ingredient. If your kidneys do not function, you can't do this - so your doc would give you calcitriol (same thing) and bypass it. Otherwise, your parathyroid hormone would shoot up and pull most of the calcium out of your bones (which would be bad).
If you have normal kidney and liver function you should not need calcitriol. The body will make it as needed from the stores of vitamin D3 that you consumed or made by sitting in the sun.