The effect of vitamin D on osteoblasts and osteoclasts

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Abstract
Vitamin D, when administered for severe vitamin D deficiency or in large doses, stimulates osteoclastic bone resorption, presumably through a molecular interaction between recently identified receptor activator of NF-κB ligand/osteoprotegerin ligand on marrow stromal cells and receptor activator of NF-κB on osteoclast precursors. In contrast, small pharmacologic doses of active vitamin D that do not cause hypercalcemia, when administered in vitamin D–sufficient animals with high bone turnover, can suppress bone resorption while maintaining or even stimulating bone formation. In patients with severe vitamin D deficiency, the anabolic effect of vitamin D is mediated primarily through increased serum calcium, whereas active vitamin D can stimulate bone formation in vitamin D–replete animals, independently of serum calcium or parathyroid hormone level. Therefore, the effects of vitamin D on bone depend on its dosage and on the conditions under which it is administered.