Osteomalacia, rickets, and vitamin D insufficiency

Published on 02/03/2015 by admin

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Last modified 02/03/2015

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CHAPTER 11

Osteomalacia, rickets, and vitamin D insufficiency

1. What are osteomalacia and rickets?

2. Why is it important to know about osteomalacia and rickets?

In the United States at the beginning of the 20th century, rickets due to a deficiency of vitamin D was common in urban areas. In the 1920s, rickets was virtually eliminated by an appreciation of the antirachitic properties of sunlight and the use of cod liver oil (which contains vitamin D). However, with the development of effective treatments for previously fatal diseases that affect vitamin D metabolism (such as chronic renal failure) and with an improved understanding of both vitamin D and mineral metabolism, many additional syndromes with osteomalacia or rickets as a feature have emerged. Many later studies have demonstrated that undiagnosed vitamin D deficiency or insufficiency is common in the United States, and for a significant number of adult women with osteoporosis, vitamin D insufficiency may be an unsuspected component of their bone loss.

3. List the causes of osteomalacia and rickets

The primary abnormality of bone in patients with either osteomalacia or rickets is defective mineralization of the bone matrix. The major mineral in bone is hydroxyapatite—Ca10(PO4)6(OH)2. Thus, any disease that results in decreased availability to bone of either calcium or phosphorus may result in osteomalacia or rickets (Table 11-1). Causes of osteomalacia and rickets fall into three categories: (1) disorders associated with abnormalities of vitamin D metabolism or action that limit the availability of calcium for mineralization of bone, (2) disorders associated with abnormalities of phosphorus metabolism, and (3) a small group of disorders in which there is normal vitamin D and mineral metabolism.

4. Describe how vitamin D is synthesized and metabolized.

Serum vitamin D comes from two sources: dietary intake and conversion by ultraviolet (UV) irradiation of 7-dehydrocholesterol or ergosterol in the skin. Vitamin D is then transported through the blood to the liver, where it is converted to 25-hydroxyvitamin D (25-OHD) by the hepatic 25-hydroxylase enzyme. The 25-OHD is then converted in the kidney to the active hormone, 1,25-dihydroxyvitamin D, by the renal 1α-hydroxylase enzyme. This active vitamin D metabolite has effects in many tissues, including the intestine (increases calcium absorption), the kidney (increases calcium reabsorption), the parathyroid glands (decreases parathyroid hormone [PTH] secretion), and bone (stimulates osteoblast maturation and bone matrix synthesis) (Fig. 11-1). Studies have now suggested possible other roles for vitamin D in cardiovascular and neurologic diseases, insulin resistance and diabetes, malignancies, autoimmune conditions, and infections. From an understanding of how vitamin D is metabolized, it is apparent that even when dietary intake and UV-mediated vitamin D synthesis are normal, vitamin D deficiency may occur in association with severe malabsorptive, renal, or liver disease.

5. Discuss the disease processes that interfere with the metabolism of vitamin D.

6. List genetic disorders that interfere with vitamin D synthesis or action.

7. What conditions associated with abnormalities of phosphate metabolism result in osteomalacia or rickets?

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