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Vitamin D acts in a way which to me is counterintuitive. It functionally supplemets Parathormone. It in the intestinal tract steps up calcium absorption by altering nuclear gene expression and also prevents calcium excretion in kidneys. All of this is understandable. But it also, like parathormone, steps up osteoclast action in bone (actually steps up both osteoclast and osteoblast, but the osteoclast action is increased more to result in net bone resorption). This means that Vitamin D increases blood calcium level by increasing bone resorption.

Then how does Vitamin D help in improving bone density, bone strength and prevent rickets or osteoporosis? All of these would require bone deposition rather than resorption.

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yes there are some colflicting theories.. This article says that Vit-D actually inhibits osteoclastogenesis. What I can say about it is that the action of Vit-D should be studied in a systems perspective. It may be the effect of dynamics. It allows calcium absorption, and its deficiency would cause reduced calcium leading to reduced bone-mineralization as seen in rickets. However, it also affects resorption and active modeling of the bone. It is important to take other factors (hormones etc) into consideration. –  WYSIWYG Jan 9 '14 at 6:26
and thats where mathematical models are useful –  WYSIWYG Jan 9 '14 at 9:30
The one which is yet to be built :P –  WYSIWYG Jan 10 '14 at 7:11
@WYSIWYG Won't be long. I am already learning Python! –  Satwik Pasani Jan 10 '14 at 8:04
well.. i would suggest that you learn Matlab as well.. The numpy module of python does support numerical computations but matlab has optimized solvers for different types of equations. Having said that, I love python too :) –  WYSIWYG Jan 10 '14 at 9:18

1 Answer 1

There are two pieces to this question:

a) How does bone resorption (movement of Ca/Phos out of bone into the blood) result in net improvement in bone structure?

Bones are constantly remodeling, primarily in response to mechanical stressors. Although you clearly already realize this, I will make it explicit: osteoblasts are the cells that create new bone; osteoclasts break down (resorb) bone.

Quoting Harrison’s Internal Medicine1:

Radioisotope studies indicate that as much as 18% of the total skeletal calcium is deposited and removed each year. Thus, bone is an active metabolizing tissue.…The cycle of bone resorption and formation is a highly orchestrated process carried out by the basic multicellular unit, which is composed of a group of osteoclasts and osteoblasts

Bone density can not be equated with bone quality. (This is the difference between osteoporosis – deficient bone density – and rickets – poor bone architecture.) Thus, vitamin D’s stimulation of both osteoblast and osteoclast activity is beneficial to bone structure.

b) How does vitamin D lead to overall increased bone density?

In addition to the effects above, you mentioned that vitamin D increases serum calcium by several mechanisms. This is true. Both this increased serum calcium as well as the direct effects of Vitamin D on the parathyroid gland suppress PTH* production. As PTH has powerful actions to increase bone resorption, the suppression on its excretion applied both directly and indirectly by Vitamin D results in a net increase in bone density.

Thus, both by its effects on bone quality and quantity, vitamin D is a net boon for the bones.

*Parathyroid hormone. a.k.a. parathormone

1. Bringhurst F, Demay M.B., Krane S.M., Kronenberg H.M. (2012). Chapter 352. Bone and Mineral Metabolism in Health and Disease. In Longo D.L., Fauci A.S., Kasper D.L., Hauser S.L., Jameson J, Loscalzo J (Eds), Harrison's Principles of Internal Medicine, 18e.

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