Vitamin D deficiency is common worldwide. It is a common cause of rickets and osteomalacia, but these disorders may also result from other conditions, such as chronic kidney disease, various renal tubular disorders, familial hypophosphatemic (vitamin D–resistant) rickets, chronic metabolic acidosis, hyperparathyroidism, hypoparathyroidism, inadequate dietary calcium, and disorders or drugs that impair the mineralization of bone matrix.
Vitamin D deficiency causes hypocalcemia, which stimulates production of parathyroid hormone (PTH), causing hyperparathyroidism. Hyperparathyroidism increases absorption, bone mobilization, and renal conservation of calcium but increases excretion of phosphate. As a result, the serum level of calcium may be normal, but because of hypophosphatemia, bone mineralization is impaired.
Vitamin D has 2 main forms:
Vitamin D3 is synthesized in skin by exposure to direct sunlight (ultraviolet B radiation) and obtained in the diet chiefly in fish liver oils and salt water fish (see table Sources, Functions, and Effects of Vitamins). In some developed countries, milk and other foods are fortified with vitamin D. Human breast milk is low in vitamin D, containing an average of only 10% of the amount in fortified cow’s milk.
Vitamin D levels may decrease with age because skin synthesis declines. Sunscreen use and dark skin pigmentation also reduce skin synthesis of vitamin D.
Vitamin D is a prohormone with several active metabolites that act as hormones. Vitamin D is metabolized by the liver to 25(OH)D (calcifediol, calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D), which is then converted by the kidneys to 1,25-dihydroxyvitamin D (1,25-dihydroxycholecalciferol, calcitriol, or active vitamin D hormone). 25(OH)D, the major circulating form, has some metabolic activity, but 1,25-dihydroxyvitamin D is the most metabolically active. The conversion to 1,25-dihydroxyvitamin D is regulated by its own concentration, parathyroid hormone (PTH), and serum concentrations of calcium and phosphate.
Vitamin D affects many organ systems (see table Actions of Vitamin D and Its Metabolites), but mainly it increases calcium and phosphate absorption from the intestine and promotes normal bone formation and mineralization.
Vitamin D and related analogs may be used to treat psoriasis, hypoparathyroidism, and renal osteodystrophy. Vitamin D's usefulness in preventing leukemia and breast, prostate, colon, or other cancers has not been proved, nor has its efficacy in treating various other nonskeletal disorders in adults (1–3). Vitamin D supplementation does not effectively treat or prevent depression or cardiovascular disease (4, 5), nor does it prevent fractures or falls. Some evidence, however, suggests that taking the combined recommended daily allowance of both vitamin D and calcium reduces the risk of hip fractures (6).
(See also Overview of Vitamins.)
Actions of Vitamin D and Its Metabolites
1. Autier P, Mullie P, Macacu A, et al: Effect of vitamin D supplementation on non-skeletal disorders: A systematic review of meta-analyses and randomised trials. Lancet Diabetes Endocrinol 5 (12):986–1004, 2017. doi: 10.1016/S2213-8587(17)30357-1
2. Manson JE, Cook NR, Lee IM, et al: Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med 380(1):33-44, 2019. doi: 10.1056/NEJMoa1809944
3. LeFevre ML, LeFevre NM: Vitamin D screening and supplementation in community-dwelling adults: Common questions and answers. Am Fam Physician 97(4):254-260, 2018.
4. Okereke OI, Reynolds CF 3rd, Mischoulon D, et al: Effect of long-term vitamin D3 supplementation vs placebo on risk of depression or clinically relevant depressive symptoms and on change in mood scores: A randomized clinical trial. JAMA 324(5):471-480, 2020. doi: 10.1001/jama.2020.10224
5. Barbarawi M, Kheiri B, Zayed Y, et al: Vitamin D supplementation and cardiovascular disease risks in more than 83,000 individuals in 21 randomized clinical trials: A meta-analysis [published correction appears in JAMA Cardiol 2019 Nov 6]. JAMA Cardiol 4(8):765-776, 2019. doi: 10.1001/jamacardio.2019.1870
6. Yao P, Bennett D, Mafham M, et al: Vitamin D and calcium for the prevention of fracture: A systematic review and meta-analysis. JAMA Netw Open 2(12):e1917789, 20019. doi:10.1001/jamanetworkopen.2019.17789
Vitamin D deficiency may result from the following:
Inadequate direct sunlight exposure or sunscreen use and inadequate intake usually occur simultaneously to result in clinical deficiency. Susceptible people include
Inadequate vitamin D stores are common among the elderly, particularly those who are housebound, institutionalized, or hospitalized or who have had a hip fracture.
Recommended direct sunlight exposure is 5 to 15 minutes (suberythemal dose) to the arms and legs or to the face, arms, and hands, at least 3 times a week. However, many dermatologists do not recommend increased sunlight exposure because risk of skin cancer is increased.
Malabsorption can deprive the body of dietary vitamin D; only a small amount of 25(OH)D is recirculated enterohepatically.
Vitamin D deficiency may result from defects in the production of 25(OH)D or 1,25-dihydroxyvitamin D. People with chronic kidney disease may develop rickets or osteomalacia because renal production of 1,25-dihydroxyvitamin D is decreased and phosphate levels are elevated. Hepatic dysfunction can also interfere with production of active vitamin D metabolites.
Type I hereditary vitamin D–dependent rickets is an autosomal recessive disorder characterized by absent or defective conversion of 25(OH)D to 1,25-dihydroxyvitamin D in the kidneys. X-linked familial hypophosphatemia reduces vitamin D synthesis in the kidneys.
Many antiseizure drugs and use of glucocorticoids increase the need for vitamin D supplementation.
Type II hereditary vitamin D–dependent rickets has several forms and is due to mutations in the 1,25-dihydroxyvitamin D receptor. This receptor affects the metabolism of gut, kidney, bone, and other cells. In this disorder, 1,25-dihydroxyvitamin D is abundant but ineffective because the receptor is not functional.
Vitamin D deficiency can cause muscle aches, muscle weakness, and bone pain at any age.
Vitamin D deficiency in a pregnant woman causes deficiency in the fetus. Occasionally, deficiency severe enough to cause maternal osteomalacia results in rickets with metaphyseal lesions in neonates.
In young infants, rickets causes softening of the entire skull (craniotabes). When palpated, the occiput and posterior parietal bones feel like a ping pong ball.
In older infants with rickets, sitting and crawling are delayed, as is fontanelle closure; there is bossing of the skull and costochondral thickening. Costochondral thickening can look like beadlike prominences along the lateral chest wall (rachitic rosary).
In children 1 to 4 years, epiphyseal cartilage at the lower ends of the radius, ulna, tibia, and fibula enlarge; kyphoscoliosis develops, and walking is delayed.
In older children and adolescents, walking is painful; in extreme cases, deformities such as bowlegs and knock-knees develop. The pelvic bones may flatten, narrowing the birth canal in adolescent girls.
Tetany is caused by hypocalcemia and may accompany infantile or adult vitamin D deficiency. Tetany may cause paresthesias of the lips, tongue, and fingers; carpopedal and facial spasm; and, if very severe, seizures. Maternal deficiency can cause tetany in neonates.
Osteomalacia predisposes to fractures. In the elderly, hip fractures may result from only minimal trauma.
Vitamin D deficiency may be suspected based on any of the following:
X-rays of the radius and ulna plus serum levels of calcium, phosphate, alkaline phosphatase, parathyroid hormone (PTH), and 25(OH)D are needed to differentiate vitamin D deficiency from other causes of bone demineralization.
Assessment of vitamin D status and serologic tests for syphilis can be considered for infants with craniotabes based on the history and physical examination, but most cases of craniotabes resolve spontaneously. Rickets can be distinguished from chondrodystrophy because the latter is characterized by a large head, short extremities, thick bones, and normal serum calcium, phosphate, and alkaline phosphatase levels.
Tetany due to infantile rickets may be clinically indistinguishable from seizures due to other causes. Blood tests and clinical history may help distinguish them.
Bone changes, seen on x-rays, precede clinical signs. In rickets, changes are most evident at the lower ends of the radius and ulna. The diaphyseal ends lose their sharp, clear outline; they are cup-shaped and show a spotty or fringy rarefaction. Later, because the ends of the radius and ulna have become noncalcified and radiolucent, the distance between them and the metacarpal bones appears increased. The bone matrix elsewhere also becomes more radiolucent. Characteristic deformities result from the bones bending at the cartilage-shaft junction because the shaft is weak. As healing begins, a thin white line of calcification appears at the epiphysis, becoming denser and thicker as calcification proceeds. Later, the bone matrix becomes calcified and opacified at the subperiosteal level.
In adults, bone demineralization, particularly in the spine, pelvis, and lower extremities, can be seen on x-rays; the fibrous lamellae can also be seen, and incomplete ribbonlike areas of demineralization (pseudofractures, Looser lines, Milkman syndrome) appear in the cortex.
Because levels of serum 25(OH)D reflect body stores of vitamin D and correlate with symptoms and signs of vitamin D deficiency better than levels of other vitamin D metabolites, the best way to diagnose vitamin D deficiency is generally considered to be
Target 25(OH)D levels are > 20 to 24 ng/mL (about 50 to 60 nmol/L) for maximal bone health; whether higher levels have other benefits remains uncertain, and higher absorption of calcium may increase risk of coronary artery disease.
If the diagnosis is unclear, serum levels of 1,25-dihydroxyvitamin D and urinary calcium concentration can be measured. In severe deficiency, serum 1,25-dihydroxyvitamin D is abnormally low, usually undetectable. Urinary calcium is low in all forms of the deficiency except those associated with acidosis.
In vitamin D deficiency, serum calcium may be low or, because of secondary hyperparathyroidism, may be normal. Serum phosphate usually decreases, and serum alkaline phosphatase usually increases. Serum PTH may be normal or elevated.
Type I hereditary vitamin D–dependent rickets results in normal serum 25(OH)D, low serum 1,25-dihydroxyvitamin D and calcium, and normal or low serum phosphate.
Calcium deficiency (which is common) and phosphate deficiency should be corrected.
As long as calcium and phosphate intake is adequate, adults with osteomalacia and children with uncomplicated rickets can be cured by giving vitamin D3 40 mcg (1600 units) orally once a day. Serum 25(OH)D and 1,25-dihydroxyvitamin D begin to increase within 1 or 2 days. Serum calcium and phosphate increase and serum alkaline phosphatase decreases within about 10 days. During the 3rd week, enough calcium and phosphate are deposited in bones to be visible on x-rays. After about 1 month, the dose can usually be reduced gradually to the usual maintenance level of 15 mcg (600 units) once/day.
If tetany is present, vitamin D should be supplemented with IV calcium salts for up to 1 week (see Hypocalcemia).
Some elderly patients need vitamin D3 25 to > 50 mcg (1000 to ≥ 2000 units) daily to maintain a 25(OH)D level > 20 ng/mL (> 50 nmol/L); this dose is higher than the recommended daily allowance for people < 70 years (600 units) or > 70 years (800 units). The current upper limit for vitamin D is 4000 units/day. Higher doses of vitamin D2 (eg, 25,000 to 50,000 units every week or every month) are sometimes prescribed; because vitamin D3 is more potent than vitamin D2, it is now preferred.
Because rickets and osteomalacia due to defective production of vitamin D metabolites are vitamin D–resistant, they do not respond to the doses usually effective for rickets due to inadequate intake. Endocrinologic evaluation is required because treatment depends on the specific defect. When 25(OH)D production is defective, vitamin D3 50 mcg (2000 units) once a day increases serum levels and results in clinical improvement. Patients with kidney disorders often need 1,25-dihydroxyvitamin D (calcitriol) supplementation.
Type I hereditary vitamin D–dependent rickets responds to 1,25-dihydroxyvitamin D 1 to 2 mcg orally once a day. Some patients with type II hereditary vitamin D–dependent rickets respond to very high doses (eg, 10 to 24 mcg/day) of 1,25-dihydroxyvitamin D; others require long-term infusions of calcium.
Dietary counseling is particularly important in communities whose members are at risk of vitamin D deficiency. Fortifying unleavened chapati flour with vitamin D (125 mcg/kg) has been effective among Indian immigrants in Britain. The benefits of sunlight exposure for vitamin D status must be weighed against the increased skin damage and skin cancer risks.
All breastfed infants should be given supplemental vitamin D 10 mcg (400 units) once a day from birth to 6 months; at 6 months, a more diversified diet is available. Any benefit of doses higher than the recommended daily allowance is unproved.
Vitamin D deficiency is common and results from inadequate exposure to sunlight and inadequate dietary intake (usually occurring together) and/or from chronic kidney disease.
The deficiency can cause muscle aches and weakness, bone pain, and osteomalacia.
Suspect vitamin D deficiency in patients who have little exposure to sunlight and a low dietary intake, typical symptoms and signs (eg, rickets, muscle aches, bone pain), or bone demineralization seen on x-rays.
To confirm the diagnosis, measure the level of 25(OH)D (D2+D3).
To treat vitamin D deficiency, correct deficiencies of calcium and phosphate and give supplemental vitamin D.
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