Vitamin D is a fat-soluble vitamin found in certain foods and is manufactured in the skin as a result of direct exposure to sunlight. The liver and kidneys convert vitamin D from food sources or sunlight to its active form, calcitriol. Vitamin D helps the body maintain healthy levels of calcium and phosphorus and is therefore essential for building and maintaining healthy bones. Calcium, which is the principal element in bone, can be absorbed by the body only when vitamin D is present. Rickets, a vitamin D deficiency disease responsible for softening and weakening bones in children, used to be quite common but now is rarely seen in countries that have adopted the practice of fortifying milk products. Vitamin D and calcium are involved in many body functions, including keeping the immune and nervous systems healthy.

Getting adequate vitamin D can help prevent a number of serious health conditions, including those listed below:

Osteoporosis
Adequate amounts of vitamin D, throughout one's life (in conjunction with exercise, proper nutrition, calcium, and magnesium) is necessary for preventing bone loss. Low levels of vitamin D and insufficient sunlight exposure (less than 20 minutes per day) are associated with osteoporosis. Vitamin D is needed to properly absorb calcium. Calcium, together with vitamin D, can help heal bone fractures from osteoporosis and decrease the risk of future bone breaks.

Other Bone Disorders
Vitamin D protects against the preventable bone diseases rickets and osteomalacia (softening of the bones in adults caused by inability to properly deposit calcium). Seniors in northern climates and people who do not receive direct sunlight for at least 45 minutes per week should take a vitamin D supplement or a multivitamin containing vitamin D.

Osteoarthritis
Arthritis and its associated symptoms occur because of breakdown of cartilage in the joints. Just as vitamin D is needed for bone health, it is also needed to maintain healthy cartilage. Low intakes of vitamin D may be linked to an increased risk of arthritis of the hip in older women and to joint changes seen on x-rays of both men and women. Studies evaluating vitamin D use for osteoarthritis have found that it prevents the breakdown of cartilage.

Abnormalities of the Parathyroid Hormones
The parathyroids are four glands located in the neck and produce parathyroid hormone, which is responsible for regulating the metabolism of calcium and phosphorous. Low levels of parathyroid hormone (called hypoparathyroid) leads to low levels of calcium and vitamin D. Vitamin D by prescription, along with calcium, is the mainstay of treatment for this condition.

On the other hand, low levels of vitamin D can lead to what is called secondary hyperparathyroidism. In other words, the parathyroid glands begin to over produce parathyroid hormone in response to the low levels of vitamin D -- trying to increase the amount of this essential nutrient in the body. The treatment for this type of hyperparathyroidism is also vitamin D.

High Blood Pressure
Low levels of vitamin D may play role in the development of high blood pressure in those with kidney disease and/or hyperparathyroidism. One study, for example, suggested that supplementation with vitamin D and calcium helped to lower blood pressure in older women with low levels of vitamin D and high levels of parathyroid hormone. Your doctor will determine if vitamin D may be helpful for preventing or treating high blood pressure.

Cancer
Researchers have found that people who eat adequate amounts of vitamin D have a lower risk of developing colon cancer than those who do not eat enough. Although it is best to obtain calcium and vitamin D from the diet, in order to get the suggested amounts for the prevention and treatment of colorectal cancer (800 IU/day of vitamin D and 1800 mg/day of calcium) one would most likely require supplementation. Some population-based evidence suggests that the same amount of vitamin D may improve survival rates in those with a history of breast cancer. More information is needed.

There is some evidence that a certain form of vitamin D inhibits growth of prostate cancer cells in laboratory tests. Fructose, the sugar found in fruits, stimulates the production of this type of vitamin D. Eating lots of fruits (several servings each day), in turn, is associated with lower rates of prostate cancer.

Vitamin D3 analogs (synthetic forms of this vitamin) show promise in the treatment of various cancers, including breast and skin. However, there remain a number of problems to be worked out before such therapy moves beyond the experimental stage.

Seasonal Affective Disorder (SAD)
SAD is a form of depression that occurs during the winter months because of lack of sunlight. This condition is often treated with photo (light) therapy. A few studies suggest that the mood of those with SAD improves when taking vitamin D supplements. Talk to your doctor about whether this is a safe and appropriate addition to your treatment regimen.

Diabetes
A recent study conducted in northern Finland (where the annual exposure to sunlight is very limited) found that infants given at least 2,000 IU of vitamin D per day (generally from cod liver oil) for the first year of life were significantly less likely to develop type 1 diabetes over a 30-year time course than infants who were fed lower amounts of vitamin D.

Interestingly, the recommended daily amount of vitamin D in Finland was once as high as 4,000 to 5,000 IU in the early 1960s. In 1964, it was reduced to 2,000 IU per day, in 1975 to 1,000 IU, and 400 IU in 1992. At the same time, incidence of type 1 diabetes has been on the rise in Finland, particularly throughout the 1980s.

Tuberculosis
Low levels of vitamin D may make a person more susceptible to contracting tuberculosis. Preliminary reports suggest that vitamin D may be a worthwhile addition to the usual treatment for this infection.

Atherosclerosis
Low levels of vitamin D may increase the risk of calcium build-up in the arteries, a significant component of atherosclerotic plaque. Atherosclerotic plaque build up in blood vessels can lead to a heart attack or stroke. More research is needed to understand the practical implications of this possible relationship between low vitamin D levels and atherosclerosis.

Multiple Sclerosis (MS)
Scientists have observed that MS rates are significantly lower in areas that receive a lot of sunlight and where people eat a lot of fish, which is rich in vitamin D (see Dietary Sources). This, combined with results from animal studies, suggests that vitamin D from foods and sunlight may help protect against this condition. This does not imply, however, that vitamin D supplements will help prevent or treat MS in people. Further studies are needed.

Other
Vitamin D supplementation may also be helpful for those who:

• Are taking anticonvulsant drug therapy for seizures or corticosteroids, as these are both known to reduce bone mass.
• Follow a strict vegan diet because these people do not eat foods that are sources of vitamin D such as fish, fortified dairy products, or eggs
• Are overweight or obese. Obesity appears to alter the body's ability to produce vitamin D in the skin and to absorb it through the intestines. As a result, obesity is associated with vitamin D insufficiency.
• Have psoriasis because levels of vitamin D may be lower if you have this skin disorder. More research is needed before conclusions can be drawn.

Two forms of vitamin D, cholecalciferol and ergocalciferol, are naturally found in foods and are added to milk. It is important to note that the milk used to make yogurt and cheese is usually not vitamin D fortified. Foods sources of vitamin D include the following.

• Cod liver oil (best source)
• Fatty fish such as salmon, mackerel, tuna, sardines, herring
• Vitamin D-fortified milk and cereal
• Eggs

Another form of vitamin D, cholecalciferol, is manufactured by the skin when it is exposed to the ultraviolet (UV) rays of the sun. Skin pigmentation has an effect on the skin's production of vitamin D. While a fair-skinned person can meet his or her vitamin D needs by exposing the face and arms to bright sunlight for as little as 45 minutes a week, a person with dark skin may need up to three hours of exposure to get the same benefit.

Clouds, smog, clothing, sunscreen, and window glass all decrease the amount of sunlight that actually reaches the skin.

In northern climates such as New England, it is difficult to get enough vitamin D from sunlight during the winter and supplementation may be necessary.

Vitamin D is included in many, if not most, multivitamins. It can be found alone in over-the-counter preparations in strengths from 50 IU to 1,000 IU as softgel capsules, tablets, and liquid. Higher-dose preparations are also available by prescription. For those who have trouble digesting fat, vitamin D injections are also available by prescription.

Adequate daily intakes for dietary vitamin D are listed below. Use of supplements to obtain these recommended amounts may be appropriate for the elderly and those with limited sun exposure. The elderly, in particular, may be at risk of developing vitamin D deficiency since the body's ability to manufacture vitamin D from sunlight as well as the kidneys' ability to convert vitamin D into usable form, decline with age.

Pediatric

• Infants birth to 12 months: 5 mcg (200 IU)
• Children 1 to 8 years: 5 mcg (200 IU)
• Children 9 to 13 years: 5 mcg (200 IU)
• Adolescents 14 to 18 years: 5 mcg (200 IU)

Adult

• 19 to 50 years: 5 mcg (200 IU)
• 51 to 70 years: 10 mcg (400 IU)
• 70 years and older: 15 mcg (600 IU)
• Pregnant and breastfeeding females: 5 mcg (200 IU)

Because of the potential for side effects and interactions with medications, dietary supplements should be taken only under the supervision of a knowledgeable healthcare provider.

Taking too much vitamin D (more than 1,000 IU daily) can cause a number of adverse effects including excessive thirst, metal taste, poor appetite, weight loss, bone pain, tiredness, sore eyes, itching skin, vomiting, diarrhea or constipation, a need to urinate, and muscle problems.

Getting too much sunlight, however, will not provide an excess of vitamin D, nor is one likely to get too much vitamin D from food sources alone. Generally, excess of vitamin D is a result of taking supplements in too high a dose.

People with the following conditions should exercise caution when considering taking vitamin D supplements:

• High blood calcium or phosphorus levels
• Heart problems
• Kidney disease

If you are currently being treated with any of the following medications, you should not use vitamin D supplements without first talking to your healthcare provider.

Vitamin D levels may be increased by the following medications:

• Estrogen: Hormone replacement therapy with estrogen appears to increase vitamin D levels in the blood; this may have a beneficial effect on calcium and bone metabolism. In addition, use of vitamin D supplements in conjunction with estrogen replacement therapy (ERT) increases bone mass more than ERT alone. However, this benefit may be lost with the addition of progesterone.
• Isoniazid (INH): INH, a medication used to treat tuberculosis, may raise blood levels of vitamin D.
• Thiazide: Diuretics in this class (such as hydrochlorothiazide) increase the activity of vitamin D and can lead to inappropriately high calcium levels in the blood.

Vitamin D levels may be decreased, or its absorption may be reduced, by the following medications:

• Antacids: Taking certain antacids for long periods of time may alter the levels, metabolism, and availability of vitamin D.
• Calcium-channel blockers (such as verapamil): These medications are used to treat high blood pressure and heart conditions may decrease the production of vitamin D by the body.
• Cholestyramine: This cholesterol-lowering medication, known as a bile acid sequestrant, interferes with the absorption of vitamin D (as well as other fat soluble vitamins).
• Phenobarbital, phenytoin, and other anticonvulsant medications: These medications may accelerate the body's use of vitamin D.
• Mineral oil also interferes with absorption.

In addition, Vitamin D may enhance the effects of doxorubicin, a medicine used to treat a variety of cancers. More research is needed.

Some clinicians recommend following calcium levels closely if vitamin D is taken with digoxin, a medication used to treat irregular heart rhythms. This is because vitamin D improves absorption of calcium. Calcium, in turn, can increase the likelihood of a toxic reaction from this medication.

Weight Loss Products
Orlistat, a medication used for weight loss and olestra, a substance added to certain food products, are both intended to bind to fat and prevent the absorption of fat and the associated calories. Because of their effects on fat, orlistat and olestra may also prevent the absorption of fat-soluble vitamins such as vitamin D. Given this concern and possibility, the Food and Drug Administration (FDA) now requires that vitamin D and other fat soluble vitamins (namely, A, E, and K) be added to food products containing olestra. How well vitamin D from such food products is absorbed and used by the body is not clear. In addition, physicians who prescribe orlistat add a multivitamin with fat soluble vitamins to the regimen.

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