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Old 10-25-2015
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Default Vit D and bone resorption

Hey
can anyone explain this to me or tell me if I'm correct
Vit D will cause Bone resorption it will take Ca and ph from bone and at the same time will increase Ca absorption in kidney and intestine .
So if it does so how it will mineralize bone ?
is it from the absorbed Ca from bone which will be recycled for minerlization of new bone ?? (Ionization )
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Vit D doesnot resorb bone!!!!

This is the classic confusion between PTH and Vit D who does what and how!!! So lets get it sorted once and for all...

Before that lets talk about some basic things:

1. Bone Formation: Bone formation occurs when there is either rise in Ca2+ or Phosphorus ion and the solubility product of both is greater than what requires to form bone(layman = when conc of either one of them rises bone is formed) and Osteoblasts are responsible for this! They form osteoid which is nothing but bone!

2. Bone destruction: Bone is usually destroyed by the osteoclasts when osteoid is broken down to release Ca2+ and Phosphorus.

Now who does what:

PTH:
  • Increases Ca2+ reabosorbtion from Kidney
  • Decreases Phosphorus reabsorbtion from Kidney
  • Activates Vit D via 1-alpha hydroxylation in the kidney
  • PTH binds to osteoblast which releases IL-1, IL-1 activates osteoclast which breaks down bone

VIT D:
  • Increases Ca2+ reabosorbtion from Kidney
  • Increases Phosphorus reabsorbtion from Kidney
  • Increases Ca2+ reabsorbtion from the gut
  • Vit D binds to osteoblast and stimulates bone formation

Look that both hormones bind to Osteoblasts but one increases bone formation other decreases it!

Only Calcitonin hormone binds to osteoclasts directly and inhibits osteoclasts!

LOGIC:

Vit D forms bone...Vit D raises Ca2+ level to form bone...
PTH job is to maintain Ca2+ level and it doesnot care about bone and breaks it!

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hey

thanks this is a good explanation but I'm still confused with this point

one of the function of vit D :
Bone : increase resorption of Ca and Phosphate

(Pathoma) pg 192
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I seriously think he meant KIDNEY!!!

Kidney : increase resorption of Ca and Phosphate

or what he might have meant is that it increases deposition of Ca2+ and PO43-

there is no such thing as VIT D destroying bone and reabsorbing Ca2+
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yeah thats what confused me
even in FA
Vit D increase bone resorption of CA & PO

maybe its from old bone so it will be mineralized from the new absorbed Ca from Kid and ints !!

any one can Help !
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Default Vit D toxicity

The times I've come across vitamin D causing bone resorption is vitamin D toxicity. When the vitamin D concentration exceeds 10 times the RDA, then vitamin D would cause bone resorption rather than osteoid mineralization. Maybe that's what they meant.

Source. Kaplan biochemistry.
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Yep it all depends on the dose given or the plasma level of both. i.e low doses of PTH given in pulsatile doses facilitate bone formation. That's why the drug teripartide, a PTH analogue, is used for osteoporosis. Also high doses of Vit D facilitates bone resorption. But at low doses, what cleverfox has said works pretty well.
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