Topic Resources
Inadequate exposure to
sunlight predisposes to vitamin D deficiency. Deficiency impairs bone
mineralization, causing rickets in children and osteomalacia in adults
and possibly contributing to osteoporosis. Diagnosis involves
measurement of serum 25(OH)D (D2 + D3). Treatment usually consists of
oral vitamin D; calcium and phosphate are supplemented as needed.
Prevention is often possible. Rarely, hereditary disorders cause
impaired metabolism of vitamin D (dependency).
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.
Physiology
Vitamin D has 2 main forms:
-
D2 ( ergocalciferol)
-
D3 ( cholecalciferol): The naturally occurring form and the form used for low-dose supplementation
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, and colon cancers has not been
proved, nor has its efficacy in treating various other nonskeletal
disorders (1, 2) or preventing falls (3, 4, 5)
in the elderly. Whether vitamin D supplementation is useful in
preventing fractures in the frail or healthy elderly is under study (6).
General references
-
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. Cummings SR, Kiel DP, Black DM: Vitamin D supplementation and increased risk of falling: A cautionary tale of vitamin supplements retold. JAMA Intern Med 176 (2):171–172, 2016.
-
4. Uusi-Rasi K, Patil R, Karinkanta S, Kannus P, et al: Exercise and vitamin D in fall prevention among older women: A randomized clinical trial. JAMA Intern Med 75 (5):703–711, 2015.
-
5. LeBlanc ES, Chou R: Vitamin D and falls—Fitting new data with current guidelines. JAMA Intern Med 175 (5):712–713, 2015.
-
6. Zhao JG, Zeng XT, Wang J, Liu L: Association between calcium or vitamin D supplementation and fracture incidence in community-dwelling older adults: A systematic review and meta-analysis. JAMA 318:2466–2482, 2017.
Etiology
Vitamin D deficiency may result from the following:
-
Inadequate exposure to sunlight
-
Inadequate intake of vitamin D
-
Reduced absorption of vitamin D
-
Abnormal metabolism of vitamin D
-
Resistance to the effects of vitamin D
Inadequate exposure or intake
Inadequate direct sunlight exposure or sunscreen use and
inadequate intake usually occur simultaneously to result in clinical
deficiency. Susceptible people include
-
The elderly (who are often undernourished and are not exposed to enough sunlight)
-
Certain communities (eg, women and children who are confined to the home or who wear clothing that covers the entire body and face)
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.
Reduced absorption
Malabsorption can deprive the body of dietary vitamin D; only a small amount of 25(OH)D is recirculated enterohepatically.
Abnormal metabolism
Vitamin D deficiency may result from defects in the production of
25(OH)D or 1,25-dihydroxyvitamin D. People with chronic kidney disease
commonly 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.
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.
Resistance to effects of vitamin D
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.
Symptoms and Signs
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.
Diagnosis
-
Levels of 25(OH)D (D2 + D3)
Vitamin D deficiency may be suspected based on any of the following:
-
A history of inadequate sunlight exposure or dietary intake
-
Symptoms and signs of rickets, osteomalacia, or neonatal tetany
-
Characteristic bone changes seen on x-ray
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.
X-rays
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.
Laboratory tests
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
-
25(OH)D (D2 + D3) levels
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.
Treatment
-
Correction of calcium and phosphate deficiencies
-
Supplemental vitamin D
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.
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.
Prevention
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.
Key Points
-
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.
Posts
0 Comments