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 very 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 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, vitamin D 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 Deficiency

• The most common cause is lack of exposure to sunlight, 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 results in a complete recovery.

Vitamin D deficiency is common. Most commonly, it occurs when the skin is not exposed to enough sunlight. Almost no one consumes enough vitamin D from foods to prevent vitamin D deficiency when exposure to sunlight is inadequate.

In vitamin D deficiency, calcium and phosphate levels in the blood decrease because vitamin D is necessary for absorption of these minerals. 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 a pregnant woman, vitamin D deficiency causes the deficiency in the fetus, and the newborn has a high risk of rickets. Occasionally, the deficiency is severe enough to cause osteomalacia in the woman. Vitamin D deficiency makes osteoporosis worse. To try to increase the low calcium level in blood caused by vitamin D deficiency, 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.

Did You Know... Lack of exposure to sunlight can cause vitamin D deficiency. Most older people need vitamin D supplements.

Causes

The most common cause is inadequate exposure to sunlight. 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.

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 (see Malabsorption). They also cannot absorb vitamin D because it is a fat-soluble vitamin, which is normally absorbed with fats in the small intestine.
• The body may not be able to convert vitamin D to an active form. Certain kidney and liver disorders and several rare hereditary disorders interfere with this conversion, as do certain drugs, such as some anticonvulsants and rifampin.

Spotlight on Aging Older people are likely to develop vitamin D deficiency for several reasons: Their requirements are higher than younger persons. They tend to spend less time outdoors, or stay indoors more in the winter, and thus are not exposed to enough sunlight. They may not be exposed to enough sunlight because they are housebound, live in long-term care facilities, or need to stay in the hospital for a long time. When exposed to sunlight, their skin does not form as much vitamin D. They consume so little vitamin D in their diet that even taking vitamin D supplements in low doses (such as 400 units per day) does not prevent the deficiency. They may have disorders or take drugs that interfere with the processing of vitamin D. New studies suggest that older adults may need more vitamin D than the current recommended dietary allowance or even the recommended upper limits. In fact, they may need 1000 to 2000 IU (or more) daily, but taking such high amounts should be done only after consulting a doctor. Older people who take high amounts of vitamin D supplements need to have periodic blood tests to check their levels of calcium, vitamin D, and parathyroid hormone.

Symptoms

Vitamin D deficiency can cause muscle aches, weakness, and bone pain in people of all ages. Muscle spasms, which are caused by a low calcium level, may be the first sign of rickets in infants.

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 may result from only slight jarring or a minor fall.

Diagnosis

Doctors suspect vitamin D deficiency when people report an inadequate diet or exposure to sunlight. Doctors also suspect the deficiency in older adults, especially in those with decreased bone density (for example, with osteoporosis) or broken bones. Blood tests to measure vitamin D can confirm the deficiency. X-rays may also be taken. 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.

Prevention and Treatment

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. The diet rarely contains enough vitamin D to compensate for lack of sunlight. Many multiple vitamins contain little or no vitamin D, so most people need to take vitamin D supplements. These supplements are particularly important for people who are at risk (such as people who are older, housebound, or living in long-term care facilities). Commercially available liquid milk (but not cheese or yogurt) is fortified 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. Commercial infant formulas contain enough vitamin D.

Treatment involves taking high doses of vitamin D by mouth or by injection daily or weekly 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 Excess

Taking very high daily doses of vitamin D—for example, 50 or more times the recommended daily allowance (RDA)—over several months can cause toxicity and a high calcium level in the blood (hypercalcemia—see Minerals and Electrolytes: Hypercalcemia).

Early symptoms are loss of appetite, nausea, and vomiting, followed by excessive thirst, weakness, nervousness, 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.

Vitamin D excess is usually diagnosed when blood tests detect a high calcium level in a person who takes high doses of vitamin D. The diagnosis is confirmed by measuring the level of vitamin D in the blood.

Treatment consists of the following:

• Stopping vitamin D supplements
• Following a low-calcium diet for a while to offset the effects of a high calcium level in the body
• Taking drugs (such as corticosteroids or bisphosphonates) to suppress the release of calcium from the bones

Last full review/revision August 2007 by Larry E. Johnson, MD, PhD