Vitamin D is a fat-soluble vitamin that plays a role in many important body functions. It is best known for working with calcium in your body to help build and maintain strong bones. Vitamin D is also involved in regulating the immune system and cells, where it may help prevent cancer.

Your body stores vitamin D and makes vitamin D when your skin is exposed to sunlight. Vitamin D is also found in some foods, mostly ones like milk that have been fortified with vitamin D. There are two forms of vitamin D: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Some research suggests that cholecalciferol is better at raising levels of vitamin D in the blood.

In children, a vitamin D deficiency can cause rickets, a disease that results in soft, weak bones. In adults, many people may not be getting enough vitamin D, especially those who live in northern areas (like the northern half of the U.S.) and the elderly. People with dark skin do not absorb sunlight as easily as those with light skin, so their risk of low vitamin D is even higher. One study suggests that three quarters of adults in the U.S. have low levels of vitamin D.

That's important because researchers are beginning to find that low levels of vitamin D may be linked to other diseases, including breast and colon cancer, prostate cancer, high blood pressure, depression, and obesity. The evidence doesn't prove that too little vitamin D causes these conditions, but that people with higher levels of vitamin D are less likely to get these diseases.

Getting the proper amount of vitamin D may help prevent several serious health conditions.

Osteoporosis

Vitamin D helps your body absorb and use calcium, which you need for strong bones. Getting enough vitamin D throughout your life is important, since most bone is formed when you are young. For post-menopausal women who are at higher risk of osteoporosis, taking vitamin D along with calcium supplements can reduce the rate of bone loss, help prevent osteoporosis, and may reduce the risk of fractures.

Other Bone Disorders

Vitamin D protects against rickets and osteomalacia, softening of the bones in adults. Seniors who live in northern areas, and people who do not get direct sunlight for at least 45 minutes per week, should make sure they get enough vitamin D through fortified milk and dairy products. Alternatively, they can take a vitamin D supplement or a multivitamin with vitamin D.

Prevention of Falls

People who have low levels of vitamin D are at greater risk of falling, and studies have found that taking a vitamin D supplement (700 to 1000 IU daily) may reduce that risk. In seniors, vitamin D may reduce falls by 22%.

Parathyroid Problems

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

High Blood Pressure (Hypertension)

In population studies, people with low levels of vitamin D seem to be at greater risk of developing high blood pressure compared to those with higher levels of vitamin D. However, there is no proof that low levels of vitamin D cause high blood pressure in healthy people.

Evidence about vitamin D and blood pressure has been mixed. When people with kidney disease or an overactive parathyroid gland (hyperparathyroidism) develop high blood pressure, low levels of vitamin D may play a role. But another large study found that taking vitamin D and calcium supplements did not lower blood pressure significantly or reduce the risk of developing high blood pressure in postmenopausal women. If you have high blood pressure, ask your doctor.

Cancer

There is some evidence that getting enough vitamin D may lower your risk of certain cancers, especially of the colon, breast, prostate, skin, and pancreas. This evidence is based mostly on studies of large population studies, and doesn't prove a connection between taking vitamin D and reduced cancer risk. Laboratory studies show that vitamin D may have anticancer effects in test tubes. Population-based and laboratory studies also suggest that higher vitamin D levels are associated with a lower risk of skin cancer.

Some research suggests that postmenopausal women who take calcium and vitamin D supplements may have a lower risk of developing cancer of any kind compared to those who did not take these supplements. But a large study of more than 36,000 postmenopausal women found that taking calcium and vitamin D supplements did not lower their risk of breast cancer.

One study suggests that taking calcium and vitamin D supplements together may reduce the risk of colon cancer. Population studies suggest that people who have higher levels of vitamin D in the blood have a lower risk of developing colorectal cancer. But again, the evidence isn't entirely clear. Another study examining postmenopausal women who took calcium and vitamin D supplements found that they did not have a lower risk of colorectal cancer. In addition, researchers don't know how much vitamin D you would need to take to get a benefit.

Seasonal Affective Disorder (SAD)

SAD is a type of depression that happens during the winter months when there's not much sunlight. It is often treated with photo (light) therapy. A few studies suggest that the mood of people with SAD improves when they take vitamin D. Talk to your doctor.

Diabetes

Population studies suggest that people who have lower levels of vitamin D are more likely to develop type 2 diabetes than people who have higher levels of vitamin D. But there is no evidence that taking vitamin D can help prevent or treat type 2 diabetes.

One study found that giving infants doses of 2,000 IU per day of vitamin D during the first year of life may help protect them from developing type 1 diabetes when they are older.

Heart Disease

Population studies suggest that people with low levels of vitamin D have a greater risk of developing heart disease, including heart attack, stroke, and heart failure compared to people with higher levels of vitamin D. Low levels of vitamin D may increase the risk of calcium build up in the arteries. Calcium build up is part of the plaque that forms in arteries when you have atherosclerosis and can lead to a heart attack or stroke. Other population studies show that people with lower levels of vitamin D are more likely to have risk factors for heart disease, including high blood pressure, type 2 diabetes, obesity, and high cholesterol.

However, one large clinical study found that taking 200 IU of vitamin D along with 500 mg of calcium twice per day did not reduce the risk of heart attack or stroke.

Multiple Sclerosis (MS)

Population studies show that women who take at least 400 IU of vitamin D daily have a lower their risk of developing MS. Higher levels of vitamin D in the blood seem to be associated with a lower risk of developing MS in white men and women, although the same may not be true of African American and Hispanic men and women. However, this does not mean that vitamin D supplements will help prevent or treat MS in people. More research is needed.

Obesity

Population studies show that people who have lower levels of vitamin D are more likely to be obese compared to people with higher levels of vitamin D. One study also found that postmenopausal women who took 400 IU vitamin D plus 1,000 mg calcium daily for 3 years were less likely to gain weight than those who took placebo, although the weight difference was small. Women who were not getting enough calcium to start with (less than 1,200 mg per day) saw the most benefit.

Other Conditions

Scientists think vitamin D has immune boosting effects. Preliminary research suggests vitamin D supplementation may help prevent and treat respiratory infections.

Overall Mortality

Population studies suggest that people with lower levels of vitamin D have a higher risk of dying from any cause.

There are two dietary forms of vitamin D:

• Cholecalciferol (D3)
• Ergocalciferol (D2)

Some studies suggest that cholecalciferol (vitamin D3) is better absorbed by the body than ergocaliciferol (vitamin D2). Both forms are naturally found in foods and are added to milk. Not all yogurt and cheese are fortified with vitamin D. Food 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 about this source of vitamin D.
• Fatty fish such as salmon, mackerel, tuna, sardines, herring
• Vitamin D-fortified milk and cereal
• Eggs

Your body makes vitamin D when your skin is exposed to the sun. The color of your skin affects the production of vitamin D. A fair-skinned person may need only about 45 minutes of sunlight a week to get enough vitamin D while a person with dark skin may need up to 3 hours.

Clouds, smog, clothing, sunscreen, and window glass all reduce the amount of sunlight that actually reaches the skin. Most physicians maintain that wearing sunscreen is an important step in preventing skin cancer.

In northern areas, it is hard to get enough vitamin D from sunlight during the winter, so people living there may need to take vitamin D supplements. In the U.S., people who live above a line running from Los Angeles to South Carolina may not get enough vitamin D in winter.

Vitamin D is included in many multivitamins. It can be found alone as softgel capsules, tablets, and liquid in over-the-counter strengths from 50 to 1,000 IU. Higher doses are also available, but it is best to ask your doctor to recommend the safest, most effective dose. For those who have trouble digesting fat, vitamin D injections are also available by prescription.

Recommended dietary allowances for vitamin D are listed below. Seniors and people who do not get exposed to much sunlight may need to take supplements. Seniors may be at risk of developing vitamin D deficiency because, as we age, the body does not make as much vitamin D from sunlight, and it has a harder time converting vitamin D into a form it can use.

If you are concerned about your vitamin D levels, ask your doctor whether you should take a supplement, and how much.

Pediatric

• Infants 0 to 12 months: 400 IU (adequate intake)
• Children 1 to 18 years: 600 IU (recommended dietary allowance)

Note: The American Academy of Pediatrics (AAP) recommends that infants consume 400 IU of vitamin D daily. Since breastfed infants, or infants who consume insufficient amounts of fortified formula, typically do not receive adequate vitamin D, supplementation is often necessary. Continue with supplementation until the child is weaned and drinking at least 1 liter of whole milk or formula fortified with vitamin D. Speak with your physician about the type and amount of vitamin D that is appropriate for your infant. The AAP also recommends that children and teens who drink less than 1 liter of milk a day take 400 IU of vitamin D.

Ask your doctor before giving a vitamin D supplement to a child.

Adult

• 19 to 50 years: 600 IU (recommended dietary allowance)
• 70 years and older: 800 IU (recommended dietary allowance)
• Pregnant and breastfeeding females: 600 IU (recommended dietary allowance)

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 can cause several side effects. However, scientists disagree about how much is too much. The National Institutes of Health has set the maximum tolerable upper limit at:

• 1,000 IU daily for infants 0 to 6 months
• 1,500 IU daily for infants 6 months to 1 year
• 2,500 IU daily for children 1 to 3 years
• 3,000 IU daily for children 4 to 8 years
• 4,000 IU daily for anyone over 9.

Ask your doctor to determine the right dose for you or your child.

Side effects may include:

• Being very thirsty
• 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

You cannot get too much vitamin D from sunlight, and it would be very hard to get too much from food. Generally, too much vitamin D is a result of taking supplements in too high a dose.

People with the following conditions should be careful when considering taking vitamin D supplements:

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

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.

Atorvastatin (Lipitor). Taking vitamin D may reduce the amount of Lipitor absorbed by the body, making it less effective. If you take Lipitor or any statin (drugs used to lower cholesterol), ask your doctor before taking vitamin D.

Calcipotriene (Dovonex). It is possible that taking vitamin D supplements and using calcipotriene, a medication applied to the skin for psoriasis, could cause calcium levels to get dangerously high in the blood.

Calcium channel blockers. Vitamin D may interfere with these medications, used to treat high blood pressure and heart conditions. If you take any of these medications, do not take vitamin D without first asking your doctor. Calcium channel blockers include:

• Nifedipine (Procardia)
• Verapamil (Calan)
• Nicardipine (Cardene)
• DiltiaZem (Cardizem, Dilacor)
• Amlodipine (Norvasc)

Corticosteroids (prednisone). Taking corticosteroids long term can cause bone loss, leading to osteoporosis. Supplements of calcium and vitamin D can help maintain bone strength. If you take corticosteroids for 6 months or more, ask your doctor about taking a calcium and vitamin D supplement.

Digoxin (Lanoxin). A medication used to treat irregular heart rhythms. Taking vitamin D if you take digoxin may cause levels of calcium to get dangerously high in the blood.

These drugs may raise the amount of vitamin D in the blood:

Estrogen. Hormone replacement therapy with estrogen seems to raise vitamin D levels in the blood, which may have a positive effect on calcium and bone strength. In addition, taking vitamin D supplements along with estrogen replacement therapy (ERT) increases bone mass more than ERT alone. However, that may not be true if you also take progesterone.

Isoniazid (INH). A medication used to treat tuberculosis.

Thiazide. A diuretic or water pill that helps your body get rid of too much fluid. It can increase vitamin D activity and lead to high calcium levels in the blood.

Vitamin D levels may be lowered by the following medications.

If you take any of these medications, ask your doctor if you need more vitamin D:

Antacids. Taking certain antacids for long periods of time may alter the levels, metabolism, and availability of vitamin D.

Antiseizure medications. These medications include:

• Phenobarbital
• Phenytoin (Dilantin)
• Primidone (Mysoline)
• Valproic acid (Depakote)

Bile acid sequestrants. Used to lower cholesterol. These medications include:

• Cholestyramine (Questran, Prevalite)
• Cholestipol (Colestid)

Rifampin. Used to treat tuberculosis.

Mineral oil. Mineral oil also interferes with absorption of vitamin D.

Orlistat (Alli). A medication used for weight loss that prevents your body from absorbing fat. Because of how it works, orlistat may also prevent the absorption of fat-soluble vitamins, such as vitamin D. Doctors who prescribe orlistat tell their patients to take a multivitamin with fat-soluble vitamins.

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