Common Questions
What causes vitamin D deficiency?
Vitamin D deficiency is the most common cause of osteomalacia. Several mechanisms can cause a deficiency in vitamin D levels, including decreased skin synthesis, gastrointestinal losses, and impaired metabolism.
Decreased skin synthesis. Endogenous production of vitamin D begins when previtamin D3 is produced in the skin in response to sunlight (UVB radiation) exposure. Lack of adequate sunlight exposure is seen in:
- Nursing home, institutionalized, or homebound patients
- Veiled or heavily clothed persons
- Residents of northern latitudes
Decreased efficiency in skin production of previtamin D3 occurs with aging, dark skin pigmentation and the use of sunscreens.
Decreased absorption. Dietary absorption of vitamin D may occur in malabsorption syndromes, biliary fistulas, chronic steatorrhea, gastrectomy, pancreatic insufficiency, cystic fibrosis, inflammatory bowel disease, and gastric or intestinal bypass.
Impaired vitamin D metabolism. Previtamin D3 in the skin must be converted to the active form, 1,25-(OH)2 vitamin D3. This is accomplished by 25-hydroxylation in the liver followed by further 1-hydroxylation in the kidney. Impaired hepatic 25-hydroxylation can be seen in severe liver disease (e.g., primary biliary cirrhosis, alcoholic cirrhosis) and with anti-convulsant therapy and other drugs. Impaired renal 1-alpha-hydroxylation of 25-hydroxyvitamin D occurs in chronic renal failure, hypoparathyroidism, hereditary vitamin D-dependent rickets type 1, and oncogenic disease that suppresses synthesis of 1,25-(OH)2 vitamin D3.
Loss of vitamin D binding protein. Vitamin D levels are suppressed when binding protein levels are low. This can occur in the nephrotic syndrome and peritoneal dialysis.
- Wilson JD, Foster DW, Kronenberg HM, Williams RH. Williams Textbook of Endocrinology 1998; 9th ed.
- Goldman L, Bennet JC. Cecil Textbook of Medicine 2000; 21st ed.