Vitamin D is a nutrient required to build and maintain strong bones, the frame for the body. The presence of vitamin D signals the intestines to absorb more of the minerals calcium and phosphorus from food into the blood. Along with vitamin D, these two minerals are involved in bone remodeling—the constant breaking down and rebuilding of bone cells.

Other roles of the vitamin are being investigated, including maintaining the immune and nervous systems.

The body can develop a deficiency of vitamin D. Children with insufficient vitamin D may develop rickets, a condition associated with skeletal abnormalities. Skeletal diseases such as osteomalacia may occur in adults, resulting in soft, painful bones. Osteoporosis—where the bones become thin, brittle and more easily fractured—may also develop.

Unlike most other vitamins, the body is capable of producing all the vitamin D it needs. It is created when exposure to sunlight causes a chemical reaction in the skin that changes a product of cholesterol into an inactive form of vitamin D. The inactive form moves to the liver and the kidneys, where it undergoes additional changes to become a form of vitamin D the body can use.

Inactive forms of vitamin D are also available in certain foods and supplements. Significant dietary sources include cod liver oil, fatty fish and fortified foods (such as milk, margarine, cereals and fruit juices). Similar to vitamin D from sunlight, the inactive forms of vitamin D from food must also be processed by the liver and kidneys to become active.

People are advised to use vitamin D supplements under the guidance of a physician. Excessive amounts are hazardous and have the potential to cause kidney damage, hardening of blood vessels, and damage to the lungs or heart, which can be potentially fatal.

Vitamin D, also known as calciferol, is a fat-soluble nutrient that the body is able to produce through exposure to sunlight. Like all fat-soluble vitamins, vitamin D requires specific proteins to allow it to move through the body and interact with cells and tissues. It can also be stored in the body, with excess amounts found in the liver and fat tissues.

For many people, the body makes enough vitamin D to fulfill its needs. Ultraviolet radiation from sunlight (specifically UVB radiation) causes skin cells to change a product of cholesterol into the inactive vitamin D3. This vitamin undergoes chemical changes in the liver and kidneys to become the active form of vitamin D. Dietary sources also provide inactive forms of vitamin D, which undergo the same conversions to the active form in the liver and kidneys.

Once active, vitamin D behaves as a hormone and helps regulate the amount of calcium and phosphorus in the blood. The release of vitamin D causes more of these minerals to be absorbed by the intestines and less to be excreted by the kidneys. When it is needed, activated vitamin D causes calcium and phosphorus to move from the blood into bones. Both of these minerals are needed to keep bones strong.

Scientists continue to investigate possible roles of vitamin D in other parts of the body. There is evidence of its activity in the immune, nervous and reproductive systems, as well as the pancreas, skin and muscles.

The U.S. government has established dietary reference intakes (DRIs) for various nutrients. For vitamin D, the types of DRIs include:

• Adequate intakes (AI). This is the daily amount of vitamin D believed to be necessary to maintain health, but there is some scientific uncertainty. For infants (0 to 12 months old), AI expresses the average intake for healthy, breastfed children.
  
  
• Tolerable upper intake levels (UL). This is the maximum daily intake of vitamin D thought to be taken safely without risking potential health damage.

Recent research suggests that many Americans are not getting enough vitamin D. An Institute of Medicine committee is currently reviewing the research to re-evaluate and update DRI values and upper limits. The new recommendations are expected in 2010. Also reflecting new research, the American Academy of Pediatrics is now recommending that infants, children and adolescents increase their daily intake of vitamin D to at least 400 IU—double the current DRI. In the meantime, current daily vitamin D recommendations are as follows:

Vitamin D DRIs in micrograms (mcg) and International Units (IU) per day

Among the vitamins, vitamin D is unique because the body can make enough to satisfy its needs. This requires adequate nutritional status and external conditions, (such as sun exposure). Otherwise, vitamin D is also available from supplements and some food sources.

Compared to most vitamins, relatively few foods naturally contain significant amounts of vitamin D. Cod liver oil and fatty fish (including salmon, mackerel and tuna) are among the few sources that provide large amounts. Small quantities are also found in eggs, cheese and liver. Other dietary options include fortified foods, such as milk and margarine, which have inactive forms of vitamin D added during processing. For vegans and others who avoid eating animal products, fortified breakfast cereals and fruit juices are also available.

For many people, sunlight provides most or all the vitamin D the body needs and dietary intake is less important. For light-skinned people, exposing the arms and legs for about 10 minutes a day may provide sufficient vitamin D. The amount of exposure required increases with darker skin color, due to higher levels of the skin pigment melanin. In addition, the skin’s ability to produce vitamin D diminishes with age, and people living in the northern areas of the United States do not get adequate sunlight to produce vitamin D in the winter.

Under certain conditions a physician may recommend supplements. Vitamin D is available individually, coupled with calcium or as part of many multivitamins. It’s best to check with your doctor before taking vitamin D supplements due to the health risks that may occur if you take too much over a period of time.

Vitamin D is known to help maintain healthy bones. It has been shown to reduce the incidence of osteoporosis and hip fractures when taken with calcium. In addition, physicians recommend vitamin D supplements to treat a number of other conditions. Some examples include:

• Psoriasis. This disorder causes red, scaly patches to occur on the skin. Topical prescription medications containing vitamin D may reduce symptoms.
  
  
• Familial hypophosphatemia. This inherited disease prevents the body from using phosphate, resulting in skeletal abnormalities similar to those caused by rickets. Physicians may prescribe both oral vitamin D and phosphate supplements to treat this disorder.

Research continues to investigate possible roles of vitamin D in the prevention and treatment of other conditions. These include:

• High blood pressure. There is some evidence that increasing vitamin D intake with supplements or sun exposure may help reduce blood pressure. Recent research shows an association between low blood levels of vitamin D and increased risk of heart attack, stroke and other cardiovascular events.
  
  
• Cancer. Whether from the sun or diet, increased vitamin D has been associated with lower risk of developing some cancers, including colorectal cancer and possibly breast cancer. Study results are mixed, and the amount of vitamin D required is unknown, so supplementation for this reason is not recommended due to the uncertainty of benefits and the potential dangers of vitamin D overdoses. Although sun exposure is important for vitamin D, excessive sun exposure is a leading cause of skin cancer, the most common form of cancer in the United States.
  
  
• Multiple sclerosis. Some studies have found a link between use of vitamin D supplements and a lower incidence of multiple sclerosis. Further research is required before the vitamin can be recommended for this use.
  
  
• Other conditions. Scientists also continue to examine the use of vitamin D to reduce the risk of diabetes, rheumatoid arthritis and other diseases.

Studies continue to find new evidence of additional possible benefits of vitamin D. For example, one study found that some postmenopausal women who take a combination of calcium and vitamin D supplements may gain less weight than women who do not.

Another study found that people with higher levels of vitamin D in their blood have a reduced risk of developing the early stages of age-related macular disease, the most common cause of blindness in older U.S. adults.

At the proper levels, vitamin D is necessary for the human body to function. Too little or too much stresses cells and tissues and may lead to serious, life-threatening diseases. The diseases related to overdoses or deficiencies of vitamin D largely reflect the relationship between this vitamin and the mineral calcium.

Diseases related to vitamin D deficiency are often characterized by weakened bones. They include:

• Rickets. This disease slows growth and causes skeletal abnormalities in children. Bones soften and bend when they support weight, leading to bowed legs and arms, and spinal deformities. In addition, the rib cage may be lined with bumps that resemble beads. The child’s head may also become large and abnormally shaped. Rickets may lead to permanent disfigurement without treatment.
  
  
• Osteomalacia. The adult form of vitamin D deficiency causes soft, painful bones and muscle weakness. In many patients, this leads to a stooped posture as the spine bends and legs become bowed. Patients also face an increased risk of fractures. It is most likely to occur in older women with low vitamin D intake and little exposure to the sun.
  
  
• Osteoporosis. This loss of bone density increases the risk of fractures. It is often associated with insufficient calcium intake, but may also result from vitamin D deficiency which reduces calcium absorption. Osteoporosis occurs most commonly among postmenopausal women.

These diseases can usually be treated by providing vitamin D and in some cases, additional calcium and phosphorus.

There are several factors that can increase the risk of developing vitamin D deficiency. Any factor that reduces exposure to sunlight limits the skin’s ability to synthesize vitamin D and can lead to deficiency. Possible contributors include:

• Dark skin. Melanin is a pigment that protects the skin from the harmful effects of the sun, but it can also reduce the rate of vitamin D synthesis, especially in northern climates. Studies have shown that African-American children are at greater risk than their peers for having low levels of vitamin D.
  
  
• Geographic location. During the winter months, sunlight is not strong enough to cause skin to create vitamin D in people living above about 35 degrees north latitude (approximately a line connecting Los Angeles and Columbia, SC). People who live at higher latitudes also may be at risk of vitamin D deficiency because the angle of the sun’s rays may not be appropriate for producing adequate levels of vitamin D in the skin.
  
  
• Urban environment. The smoke and smog often present in large cities blocks the ultraviolet light necessary for vitamin synthesis.
  
  
• Skin cover. Clothing and sunscreen (SPF 8 or higher) used to reduce exposure to the sun also interfere with the process of vitamin D production. However, for many people the skin only requires sun exposure on the extremities for several minutes a day several times per week for adequate vitamin D. Protecting the skin from UV radiation with sunscreen is important for skin cancer prevention.

Other risk factors include:

• Age. People who are 50 and older are believed to be at increased risk of developing vitamin D deficiency. As people age, the ability of the skin, liver and kidneys to make and activate vitamin D decreases. This may occur in conjunction with reduced exposure to the sun and limited dietary intake of vitamin D.
  
  
• Breastfeeding. Infants fed a diet consisting exclusively of breast milk after six months of age may develop vitamin D deficiency, particularly during winter in a northern climate. This can be prevented by including fortified formula or vitamin supplements in the infant’s diet.
  
  
• Fat malabsorption. Vitamin D is a fat-soluble nutrient. Any condition that limits fat absorption may also reduce the amount of vitamin D usable from foods. Diseases that reduce the body’s ability to absorb fats include Crohn’s disease, cystic fibrosis, pancreatic enzyme deficiency, liver disease and gluten intolerance. Fat absorption may also be limited by surgical removal of parts of the stomach or intestines and medications to treat obesity.
  
  
• Liver and kidney disease. These organs are critical to converting inactive forms of vitamin D to forms the body can use. Any disease that impacts liver or kidney function can limit the amount of active vitamin D in the body.
  
  
• Gastric bypass. Surgery performed to reduce the size of the stomach (bariatric surgery) can increase the risk of vitamin D deficiency.
  
  
• Hereditary vitamin D resistance. An extremely rare genetic disease that prevents the body from using vitamin D. It causes deficiency symptoms to occur within the first years of life.

While vitamin D deficiency can lead to serious diseases, taking too much of the vitamin from supplements can also cause significant problems. Excessive levels of vitamin D in the body can cause too much calcium to enter the bloodstream, a condition known as hypercalcemia. Initial symptoms include:

• Dehydration
• Nausea
• Vomiting
• Anorexia
• Constipation
• Muscle weakness
• Weight loss
• Confusion
• Lethargy
• Fatigue

Over time, toxic levels of vitamin D in the body can cause serious disease. Calcium may leave the blood and build up in soft tissues. Kidney stones are one result of this process. Damage may occur in the kidneys, blood vessels, heart and lungs.

Overuse of supplements is the primary cause of vitamin D toxicity, but this rarely occurs. Staying in the sun too long does not lead to vitamin D toxicity. Excess exposure to sunlight degrades the early, inactive forms of vitamin D in the skin, limiting production of the active form. However, too much sun increases your risk for skin cancer.

Preparing questions in advance can help patients have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to vitamin D:

1. Am I at risk for vitamin D deficiency or toxicity?
   
   
2. How can I learn more about sources of vitamins and how much I need each day?
   
   
3. Should I consult a registered dietitian or nutritionist to reduce my risk of vitamin D deficiency or overdose? Can you recommend one?
   
   
4. What are early symptoms of vitamin D deficiency?
   
   
5. What are the treatment options for vitamin D deficiency?
   
   
6. Can my body make too much vitamin D?
   
   
7. What are the health benefits of using vitamin D supplements?
   
   
8. What are the risks of consuming excess vitamin D?
   
   
9. How are vitamin D and calcium related? And how does my vitamin D intake influence my risk of developing osteoporosis?
   
   
10. How should I change my vitamin D intake as I get older?