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Vitamin D–Deficient Bone Showing Its Age
Vitamin D, which is sometimes called the “sunshine vitamin” because humans can synthesize it in the presence of sunlight, has long been associated with prevention of bone disease. Vitamin D is required for proper absorption of calcium and its uptake into bone, and a lack of vitamin D is known to cause rickets and osteomalacia—diseases in which bone is too soft because of excessive collagenous matrix and its inadequate mineralization. Now, Busse and co-authors provide some evidence that the reverse is also partially true, and vitamin D deficiency can result in areas of overly dense mineralization in the bone.
To study the effects of vitamin D deficiency, Busse and colleagues used samples of bone from 30 apparently healthy people. Half of these subjects were deficient in vitamin D, defined by low concentration of vitamin D in the blood and altered macroscopic characteristics of the bone. Through detailed analysis of bone structure and functional tests measuring the bones’ resistance to cracking, the authors characterized the ways in which vitamin D–deficient bone differs from normal. As expected, they found that bones from vitamin D–deficient subjects had a much thicker layer of unmineralized osteoid coating the surface of mineralized bone. However, they also demonstrated that the bone underneath this osteoid layer was more heavily mineralized than normal and had structural characteristics of older and more brittle bone. They explained this phenomenon by noting that osteoclasts, cells that normally remodel the bone, cannot get through the thick osteoid layer. As a result, the areas of bone hidden underneath the osteoid continue to age and mineralize even as the overall bone mineral content progressively decreases.
These interesting and unexpected findings about human bone emphasize the negative consequences of vitamin D deficiency, which is all too common, especially at northern latitudes. Additional work will be needed to translate this knowledge into clinical practice, but the detailed understanding of human bone structure may provide some insight into more effective ways to prevent or treat fractures in patients with vitamin D deficiency.
- Copyright © 2013, American Association for the Advancement of Science
