Vitamin D is a nutrient your body needs for building and maintaining healthy bones. That's because your body can only absorb calcium, the primary component of bone, when vitamin D is present. Vitamin D also regulates many other cellular functions in your body. Its anti-inflammatory, antioxidant and neuroprotective properties support immune health, muscle function and brain cell activity.

Vitamin D isn't naturally found in many foods, but you can get it from fortified milk, fortified cereal, and fatty fish such as salmon, mackerel and sardines. Your body also makes vitamin D when direct sunlight converts a chemical in your skin into an active form of the vitamin (calciferol).

The amount of vitamin D your skin makes depends on many factors, including the time of day, season, latitude and your skin pigmentation. Depending on where you live and your lifestyle, vitamin D production might decrease or be completely absent during the winter months. Sunscreen, while important to prevent skin cancer, also can decrease vitamin D production.

Many older adults don't get regular exposure to sunlight and have trouble absorbing vitamin D. If your doctor suspects you're not getting enough vitamin D, a simple blood test can check the levels of this vitamin in your blood.

Taking a multivitamin with vitamin D may help improve bone health. The recommended daily amount of vitamin D is 400 international units (IU) for children up to age 12 months, 600 IU for people ages 1 to 70 years, and 800 IU for people over 70 years.

Research on vitamin D use for specific conditions shows:

• Cancer. Findings on the benefits of vitamin D for cancer prevention are mixed. More studies are needed to determine whether vitamin D supplementation may reduce the risk of certain cancers.
• Cognitive health. Research shows that low levels of vitamin D in the blood are associated with cognitive decline. However, more studies are needed to determine the benefits of vitamin D supplementation for cognitive health.
• Inherited bone disorders. Vitamin D supplements can be used to help treat inherited disorders resulting from an inability to absorb or process vitamin D, such as familial hypophosphatemia.
• Multiple sclerosis. Research suggests that long-term vitamin D supplementation reduces the risk of multiple sclerosis.
• Osteomalacia. Vitamin D supplements are used to treat adults with severe vitamin D deficiency, resulting in loss of bone mineral content, bone pain, muscle weakness and soft bones (osteomalacia).
• Osteoporosis. Studies suggest that people who get enough vitamin D and calcium in their diets can slow bone mineral loss, help prevent osteoporosis and reduce bone fractures. Ask your doctor if you need a calcium and vitamin D supplement to prevent or treat osteoporosis.
• Psoriasis. Applying vitamin D or a topical preparation that contains a vitamin D compound called calcipotriene to the skin can treat plaque-type psoriasis in some people.
• Rickets. This rare condition develops in children with vitamin D deficiency. Supplementing with vitamin D can prevent and treat the problem.

Taken in appropriate doses, vitamin D is generally considered safe.

However, taking too much vitamin D in the form of supplements can be harmful. Children age 9 years and older, adults, and pregnant and breastfeeding women who take more than 4,000 IU a day of vitamin D might experience:

• Nausea and vomiting
• Poor appetite and weight loss
• Constipation
• Weakness
• Confusion and disorientation
• Heart rhythm problems
• Kidney stones and kidney damage

Possible interactions include:

• Aluminum. Taking vitamin D and aluminum-containing phosphate binders, which may be used to treat high serum phosphate levels in people with chronic kidney disease, might cause harmful levels of aluminum in people with kidney failure in the long term.
• Anticonvulsants. The anticonvulsants phenobarbital and phenytoin (Dilantin, Phenytek) increase the breakdown of vitamin D and reduce calcium absorption.
• Atorvastatin (Lipitor). Taking vitamin D might affect the way your body processes this cholesterol drug.
• Calcipotriene (Dovonex, Sorilux). Don't take vitamin D with this psoriasis drug. The combination might increase the risk of too much calcium in the blood (hypercalcemia).
• Cholestyramine (Prevalite). Taking vitamin D with this cholesterol-lowering drug can reduce your absorption of vitamin D.
• Cytochrome P-450 3A4 (CYP3A4) substrates. Use vitamin D cautiously if you're taking drugs processed by these enzymes.
• Digoxin (Lanoxin). Avoid taking high doses of vitamin D with this heart medication. High doses of vitamin D can cause hypercalcemia, which increases the risk of fatal heart problems with digoxin.
• Diltiazem (Cardizem, Tiazac, others). Avoid taking high doses of vitamin D with this blood pressure drug. High doses of vitamin D can cause hypercalcemia, which might reduce the drug's effectiveness.
• Orlistat (Xenical, Alli). Taking this weight-loss drug can reduce your absorption of vitamin D.
• Thiazide diuretics. Taking these blood pressure drugs with vitamin D increases your risk of hypercalcemia.
• Steroids. Taking steroid mediations such as prednisone can reduce calcium absorption and impair your body's processing of vitamin D.
• Stimulant laxatives. Long-term use of high doses of stimulant laxatives can reduce vitamin D and calcium absorption.
• Verapamil (Verelan, Calan SR). Taking high doses of vitamin D with this blood pressure drug can cause hypercalcemia, and might also reduce the effectiveness of verapamil.