(See also Overview of Vitamins.)
Two forms of vitamin D are important for nutrition:
Vitamin D2 (ergocalciferol): This form is synthesized from plants and yeast precursors. It is also the form used in high-dose supplements.
Vitamin D3 (cholecalciferol): This form is the most active form of vitamin D. It is formed in the skin when the skin is exposed to direct sunlight. The most common food source is fortified foods, mainly cereals and dairy products. Vitamin D3 is also present in fish liver oils and fatty fish.
Human breast milk contains only small amounts of vitamin D.
Vitamin D is stored mainly in the liver. Vitamin D2 and D3 are not active in the body. Both forms must be processed (metabolized) by the liver and kidneys into an active form called active vitamin D or calcitriol. This active form promotes absorption of calcium and phosphorus from the intestine. Calcium and phosphorus, which are minerals, are incorporated into bones to make them strong and dense (a process called mineralization). Thus, calcitriol is necessary for the formation, growth, and repair of bones.
Vitamin D also enhances immune function and improves muscle strength.
Requirements for vitamin D increase as people age.
Vitamin D is a fat-soluble vitamin. It dissolves in fat and must be eaten with some fat to be absorbed.
The most common cause is lack of exposure to sunlight, usually when the diet is deficient in vitamin D, but certain disorders can also cause the deficiency.
Without enough vitamin D, muscle and bone weakness and pain occur.
Infants develop rickets: The skull is soft, bones grow abnormally, and infants are slow to sit and crawl.
Blood tests and sometimes x-rays are done to confirm the diagnosis.
From birth, breastfed infants should be given vitamin D supplements because breast milk contains little vitamin D.
Vitamin D supplements taken by mouth or given by injection usually result in a complete recovery.
Vitamin D deficiency is common. Most commonly, it occurs when the skin is not exposed to enough sunlight. Natural (unfortified) foods alone rarely provide enough vitamin D to prevent deficiency. Foods that are fortified with vitamin D and supplements of vitamin D can help prevent deficiency when exposure to sunlight is inadequate.
In vitamin D deficiency, the body absorbs less calcium and phosphate. Because not enough calcium and phosphate are available to maintain healthy bones, vitamin D deficiency may result in a bone disorder called rickets in children or osteomalacia in adults. In osteomalacia, the body does not incorporate enough calcium and other minerals into bones, resulting in weak bones. In a pregnant woman, vitamin D deficiency causes the deficiency in the fetus, and the newborn has a high risk of developing rickets. Occasionally, the deficiency is severe enough to cause osteomalacia in the woman. Vitamin D deficiency makes osteoporosis worse.
Vitamin D deficiency results in a low calcium level in blood. To try to increase the low calcium level, the body may produce more parathyroid hormone. However, as the parathyroid hormone level becomes high (a condition called hyperparathyroidism), the hormone draws calcium out of bone to increase the calcium level in blood. Thus, bones are weakened.
The most common cause of vitamin D deficiency is
Thus, vitamin D deficiency occurs mainly among people who do not spend much time outdoors: older people and people who live in an institution such as a nursing home. The deficiency can also occur in the winter at northern and southern latitudes or in people who keep their bodies covered, such as Muslim women.
Because breast milk contains only small amounts of vitamin D, breastfed infants who are not exposed to enough sunlight are at risk of the deficiency and rickets.
Some experts recommend that the arms and legs or the face, arms, and hands should be exposed to direct sunlight for 5 to 15 minutes at least 3 times a week, but some people, such as those who have darker skin or are older, may need more exposure to sunlight. However, many dermatologists do not recommend increased sunlight exposure because risk of skin cancer is increased.
Vitamin D deficiency usually occurs in people who are not exposed to sunlight and who do not consume enough vitamin D in their diet.
When the skin is exposed to enough sunlight, the body usually forms enough vitamin D. However, certain circumstances increase the risk of vitamin D deficiency even when there is exposure to sunlight:
The skin forms less vitamin D in response to sunlight in certain groups of people. They include people with darker skin (particularly blacks), older people, and people who use sunscreen.
The body may not be able to absorb enough vitamin D from foods. In malabsorption disorders, people cannot absorb fats normally. They also cannot absorb vitamin D because it is a fat-soluble vitamin, which is normally absorbed with fats in the small intestine. Less vitamin D may be absorbed from the intestine as people age.
The body may not be able to convert vitamin D to an active form. Certain kidney and liver disorders and several rare hereditary disorders (such as hypophosphatemic rickets) interfere with this conversion, as do certain drugs, such as some anticonvulsants and rifampin.
Vitamin D deficiency can cause muscle aches, weakness, and bone pain in people of all ages.
Muscle spasms (tetany) may be the first sign of rickets in infants. These spasms may also occur in adults. They are caused by a low calcium level in the blood in people with severe vitamin D deficiency. If pregnant women have vitamin D deficiency, their newborn may have spasms. The spasms may affect the face, hands, and feet. If the spasms are severe, they may cause seizures.
In young infants who have rickets, the entire skull may be soft.
Older infants may be slow to sit and crawl, and the spaces between the skull bones (fontanelles) may be slow to close.
In children aged 1 to 4 years, bone growth may be abnormal, causing an abnormal curve in the spine and bowlegs or knock-knees. These children may be slow to walk.
For older children and adolescents, walking is painful. The pelvic bones may flatten, narrowing the birth canal in adolescent girls.
In adults, the bones, particularly the spine, pelvis, and leg bones, weaken. Affected areas may be painful to touch, and fractures may occur.
In older people, bone fractures, particularly hip fractures, may result from only slight jarring or a minor fall.
Doctors suspect vitamin D deficiency in the following people:
Blood tests to measure vitamin D can confirm the deficiency.
X-rays may also be taken. The characteristic changes in bone may be seen on x-rays before symptoms become noticeable.
The diagnosis of rickets or osteomalacia is based on symptoms, the characteristic appearance of bones on x-rays, and a low level of vitamin D in the blood.
Many people need to take vitamin D supplements. Getting enough exposure to sunlight may be difficult, especially because the skin also needs to be protected from sun damage. Natural foods rarely contains enough vitamin D to compensate for lack of sunlight.
Vitamin D supplements are particularly important for people who are at risk (such as people who are older, housebound, or living in long-term care facilities). To prevent deficiency, older people should usually take 800 IU of vitamin D daily in supplements.
Commercially available liquid milk (but not cheese or yogurt) is fortified with vitamin D in the United States and Canada. Many other countries do not fortify milk with vitamin D. Breakfast cereals may also be fortified.
In breastfed infants, starting vitamin D supplements at birth is particularly important because breast milk contains little vitamin D. Supplements are given until infants are 6 months old, when they begin to eat a more varied diet. For formula-fed infants, commercial infant formulas contain enough vitamin D.
Treatment of vitamin D deficiency involves taking high doses of vitamin D, usually daily by mouth, for 1 to 2 months or longer.
If muscle spasms are present or calcium is thought to be deficient, calcium supplements are also given. If phosphate is deficient, phosphate supplements are given. Usually, this treatment leads to a complete recovery.
People with a chronic liver or kidney disorder may require special formulations of vitamin D supplements.
Vitamin D toxicity causes high levels of calcium in the blood.
People with vitamin D toxicity may lose their appetite, feel nauseated, vomit, excessive thirst, need to urinate frequently, and feel weak and nervous.
Doctors diagnose the deficiency by measuring levels of calcium and vitamin D in the blood.
Treatment involves stopping vitamin D supplements and giving the person fluids and sometimes drugs.
Taking very high daily doses of vitamin D—for example, 60 to 100 or more times the recommended daily allowance (RDA)—over several months can cause toxicity and a high calcium level in the blood (hypercalcemia). Levels of calcium become high because when levels of vitamin D are high, the following occur:
More bone is broken down than is reformed. (Normally, bones are continuously broken down and reformed—in a process called remodeling—to adjust to the changing demands placed on them.) As a result, calcium is released from the bone into the bloodstream.
More calcium is absorbed from food in the intestine.
People with vitamin D toxicity may have symptoms due to the high levels of calcium in the blood.
Early symptoms of vitamin D toxicity are loss of appetite, nausea, and vomiting, followed by excessive thirst, increased urination, weakness, nervousness, itching, and high blood pressure.
Because the calcium level is high, calcium may be deposited throughout the body, particularly in the kidneys, blood vessels, lungs, and heart. The kidneys may be permanently damaged and malfunction, resulting in kidney failure.
Treatment of vitamin D toxicity involves stopping vitamin D supplements for a while to offset the effects of a high calcium level in the blood. Fluids are given intravenously as needed. Rarely, a low-calcium diet is recommended.
Drugs, such as corticosteroids or bisphosphonates, are given to suppress the release of calcium from the bones.