Vitamin D is a fat-soluble vitamin found in certain foods. It is also formed in the skin as a result of direct exposure to sunlight. The liver and kidneys convert vitamin D from food sources or sunlight to an active form called calcitriol.

Vitamin D helps the body maintain healthy levels of calcium and phosphorus. The body can only absorb calcium, when vitamin D is present. Vitamin D is also needed to build and maintain healthy bones.

People who are deficient in vitamin D are at risk for rickets, a metabolic disease that causes soft, weak bones. Rickets used to be quite common, but because milk products are now fortified with vitamin D, the disease is now rare.

Getting the proper amount of vitamin D can help prevent a number of serious health conditions.

Osteoporosis

Adequate amounts of vitamin D throughout one's life (in combination with exercise, proper nutrition, calcium, and magnesium) is necessary for preventing bone loss. Low levels of vitamin D and insufficient sunlight exposure (fewer 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 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 make sure they are getting enough vitamin D through fortified milk and dairy products, or taking a vitamin D supplement or a multivitamin containing vitamin D.

Arthritis

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. Clinical studies evaluating the use of vitamin D for osteoarthritis have found that it prevents the breakdown of cartilage.

Abnormalities of the Parathyroid Hormones

The four parathyroid glands are located in the neck. They produce parathyroid hormone (PTH). PTH helps the body store and use calcium and phosphorus. Vitamin D is often used to treat disorders of the parathyroid gland.

High Blood Pressure

Clinical studies have suggested a link between low levels of vitamin D and high blood pressure. Low levels of vitamin D may play role in the development of high blood pressure in those with kidney disease or an overactive parathyroid gland (hyperparathyroidism). One clinical study 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

Laboratory studies indicate that vitamin D may have anti-cancer effects. However, no human studies have directly measured how vitamin D impacts cancer risk.

Clinical study findings on vitamin D and specific cancers such as colorectal cancer have been inconsistent. However, a review of the scientific literature shows strong evidence that vitamin D may reduce the risk of colorectal cancer. It is best to obtain calcium and vitamin D from the diet. Supplements would most likely be needed to reach the suggested amounts for the prevention and treatment of colorectal cancer (800 IU/day of vitamin D and 1,800 mg/day of calcium). 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 research 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 several servings of fruit each day is associated with lower rates of prostate cancer.

Synthetic forms of vitamin D3 show promise in the treatment of various cancers, including breast and skin. However, this research is still in the experimental stages.

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 clinical 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

Research shows that supplementing infants and children with doses of 2,000 IU per day of vitamin D and higher may protect against the development of type 1 diabetes. A clinical 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 as high as 4,000 - 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 likely to get 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. Fish is a rich source of vitamin D (see Dietary Sources). The clinical studies have suggested that vitamin D from foods and sunlight may help protect against this condition. However, this does not mean that vitamin D supplements will help prevent or treat MS in people. Further human studies are needed.

Other

Vitamin D supplementation may also be helpful for people who:

• Take corticosteroids or anti-seizure drugs, as these are both known to reduce bone mass.
• Follow a strict vegan diet.
• 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. People with psoriasis may have lower levels of vitamin D . More research is needed.

There are two dietary forms of vitamin D:

• Cholecalciferol
• Ergocalciferol

These 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:

• Cod liver oil (best source). Cod liver oil often contains very high levels of vitamin A which can be toxic over time. Ask your health care provider.
• Fatty fish such as salmon, mackerel, tuna, sardines, herring
• Vitamin D-fortified milk and cereal
• Eggs

Vitamin D is manufactured by the skin when it is exposed to the ultraviolet (UV) rays of the sun. The color of your skin has an effect on the skin's production of vitamin D. A fair-skinned person can meet their vitamin D needs by exposing the face and arms to bright sunlight for as little as 45 minutes a week, while a person with dark skin may need up to 3 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 cooler, northern climates, it is difficult to get enough vitamin D from sunlight during the winter. People who live in such climates may need supplementation

Vitamin D is included in many multivitamins. It can be found alone in over-the-counter preparations in strengths from 50 - 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 - 8 years: 5 mcg (200 IU)
• Children 9 - 13 years: 5 mcg (200 IU)
• Adolescents 14 - 18 years: 5 mcg (200 IU)

Adult

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

Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.

Taking too much vitamin D (more than 1,000 IU daily) can cause a number of side effects including:

• Excessive thirst
• Metal taste in mouth
• Poor appetite
• Weight loss
• Bone pain
• Tiredness
• Sore eyes
• Itchy skin
• Vomiting
• Diarrhea
• Constipation
• A frequent need to urinate
• 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 vitamin D is a result of taking supplements in too high a dose.

People with the following conditions should be cautious 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 health care 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, 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 -- 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 doctors 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 (also known as alli), 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 now requires that vitamin D and other fat-soluble vitamins (namely, A, E, and K) be added to food products containing olestra. How well the body absorbs and uses vitamin D from such food products is not clear. In addition, physicians who prescribe orlistat add a multivitamin with fat soluble vitamins to the regimen.

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