calciferol (vitamin D-3), cholecalciferol, dihydrotachysterol (a synthetic vitamin D), ergocalciferol (vitamin D-2), ergosterol (provitamin D-2), 7-dehydrocholesterol (provitamin D-3), 22,23-dihydroergosterol (vitamin D-4 or provitamin D-4)

Vitamin D is a fat-soluble vitamin. That means it can dissolve in fats and oils. It's needed for bone growth and development. It also helps cell growth and nerve, muscle, and immune function. Lack of vitamin D may cause rickets. This is a disease that affects the bones. Because it's stored by the body, taking too much vitamin D can cause high levels to build up. This can cause health problems. Vitamin D is called the sunshine vitamin. This is because vitamin D is made by the body in response to sun exposure.

There are different forms of vitamin D. They include:

• Ergosterol (provitamin D-2). This type is found in plants.
• Ergocalciferol (vitamin D-2). This type is also found in plants.
• 7-dehydrocholesterol. This type is found in animals.
• Calciferol (vitamin D-3). This type is formed in the body from exposure to sunlight.

Vitamin D helps control calcium balance in the body. It maintains normal calcium levels and bone density. Vitamin D you get from the sun or in your diet increases the absorption of calcium and phosphorus from your intestines. It also aids in the reabsorption of phosphorus from the kidneys. Vitamin D is needed for normal bone growth. It's also needed for healing bones after a fracture. It's very important for babies, children, and teens.

Vitamin D helps treat:

• Low calcium levels
• Low phosphate levels
• Problems with bone growth
• Rickets
• Some types of abnormal muscle contractions
• Osteomalacia
• Vitamin D deficiency

Very strong, manmade forms of vitamin D have been used to treat renal rickets. This is a condition that can be caused by severe kidney disease. Or it can be caused by an inherited disorder of renal calcium and phosphorus absorption.

There may be benefits that have not yet been proven through studies.

Vitamin D may help control heart rate. It may prevent muscle weakness and improve arthritis. Studies are also looking at vitamin D's role in preventing and treating other health conditions. These include cancer, psoriasis, and diabetes.

Vitamin D is measured in micrograms (mcg). One international unit equals 0.025 micrograms (mcg) of vitamin D. The RDA is the recommended dietary allowance.

* Adequate Intake

Adults can meet the RDA for vitamin D without supplements. You can do this by exposing your face, hands, arms, or back, without sunscreen, to the sun for 10 to 15 minutes at least 2 times a week. Women who are pregnant or breastfeeding may need to take supplements. Talk to your doctor first. If you are breastfeeding, ask your doctor if your baby should get vitamin D supplements.

People who are found to be deficient in vitamin D or have low levels of vitamin D (vitamin D insufficiency) should take higher doses of vitamin D. If you are found to have vitamin D insufficiency or deficiency, your doctor will determine the correct supplement and dosage for you.

You can also get vitamin D through your diet. These foods contain vitamin D:

Vitamin D is stable in heat. It doesn't need to be refrigerated. It's only slightly sensitive to light.

Freezing foods high in vitamin D content doesn't reduce their vitamin D content. This includes foods, such as frozen salmon or mackerel. Vitamin D content stays high even when foods are cooked.

People over the age of 50 may be at increased risk of vitamin D deficiency. As people age, skin can't make vitamin D as well. And the kidneys are less able to turn vitamin D into its active hormone form. For these reasons, older adults may need vitamin D supplements.

Melanin is the pigment that gives skin its color. It can reduce the skin's ability to make vitamin D. People with darker skin are less able to make vitamin D from sunlight. They need to get vitamin D from foods or supplements.

More vitamin D is needed by those who live in subpolar and polar regions. This is because long winter nights reduce sun exposure.

Low vitamin D levels are common in people who are lactose intolerant or have a milk allergy, as well as those that consume a vegan or ovo-vegetarian diet.

Other people at risk for low vitamin D are people who are homebound, women who often wear long robes and head coverings, and people with jobs that limit sun exposure.

People who have a malabsorption syndrome may need more vitamin D. This is often the case if a person has a lot of fat loss through stool (steatorrhea). This includes people with any of these:

• Lactose intolerance
• Tropical and nontropical sprue
• Celiac disease
• Cystic fibrosis
• Ulcerative colitis
• Crohn's disease
• Pancreatitis

Other things that can cause vitamin D deficiency include:

• Liver diseases, such as cirrhosis
• Kidney failure
• Eating a lot of foods that have the fat substitute olestra
• Chronic intake of mineral oil

When a baby or child doesn't get enough vitamin D, their bones don't grow normally. This leads to nutritional rickets, the most common cause of rickets. It rarely occurs in tropical areas. But rickets was common in children in northern cities of the U.S. and in African-American children until milk was fortified with vitamin D. Low levels of vitamin D in babies can also lead to soft skull (craniotabes).

Low levels of vitamin D in an adult can lead to loss of calcium and the softening of bones (osteomalacia). It can also lead to thinning of the bones (osteoporosis).

Older adults are at increased risk for vitamin D insufficiency. This is because older skin can't make vitamin D as well. Older adults also spend more time indoors, away from sunlight. And they may have diets low in vitamin D.

Signs and symptoms of rickets include:

• Delayed closure of the soft spot (anterior fontanel). It may take until after the second year of life.
• A larger than normal head (macrocephaly)
• Defects in the enamel of forming teeth
• Knobby growths at the points where the ribs join the sternum (rachitic rosary)
• Thickening of the ankles and wrists
• Curvature of the spine (lordosis or scoliosis)
• Bowing of the legs
• Greenstick bone fractures
• Muscle weakness
• Delayed motor development

Some of the symptoms of osteomalacia include:

• Bone pain, often in the hips
• Muscle weakness

Some of the symptoms of osteoporosis include:

• Back pain
• Loss of height as the vertebrae become compressed
• Broken bones that occur easily

Vitamin D in large amounts is toxic. Symptoms in children and adults often occur after several months of heavy use. They include:

• Constipation
• Decreased muscle tone (hypotonia)
• Joint pain
• Irritability
• Being thirstier than normal
• Making more urine than normal
• Loss of appetite
• Vomiting
• High blood pressure (hypertension)

Too much vitamin D can also damage the valves in the heart and the kidneys. This is due to calcium building up in these organs.

You shouldn't take vitamin D if you have any of these:

• Too much calcium in the blood (hypercalcemia)
• Signs of vitamin D toxicity
• Hypervitaminosis D
• Increased sensitivity to the effects of vitamin D
• Lowered kidney function

Use vitamin D with caution if you have any of these:

• Arteriosclerosis
• Hyperphosphatemia
• Kidney problems
• Sarcoidosis
• Heart problems

Vitamin D can interact with certain medicines. Talk with your doctor if you take any of these:

• Antacids, especially those that contain magnesium
• Digitalis glycosides, such as digoxin and digitoxin
• Verapamil
• Cholestyramine
• Phenytoin
• Thiazide diuretics
• Barbiturates
• Mineral oil

When taking vitamin D supplements, don't use magnesium, phosphorus, or calcium unless your doctor says to.

Because vitamin D is fat soluble, it's best to take vitamin D supplements with food that has some fat in it for enhanced absorption.