Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's liver and fatty tissues. Vitamin D acts as both a vitamin and a hormone.

Vitamin D is found in some foods, but the main sources are vitamin D-fortified milk and sunlight. The ultraviolet rays of the sun react with cholesterol present on the skin and create previtamin D3. This compound goes through a series of reactions in the kidneys and the liver, and the final product is vitamin D.

Functions

Vitamin D's functions:

    
  • Plays a crucial role in the growth and maintenance of strong, healthy bones
  • Maintains normal blood levels of calcium and phosphorus
  • In children with low vitamin D levels, the supplement can improve bone mineral density. While the evidence does not give a clear answer, it has also been suggested that vitamin D supplementation may reduce the risk of osteoporosis and seasonal flu in children, and high blood pressure and some forms of cancer.

    Vitamin D has also been found to improve pain symptoms in patients with low vitamin D levels.

    Recommended Intake

    Here are the guidelines for vitamin D intake:

    Age GroupRecommended Dietary Allowance or Adequate Intake (IU/Day)
    0-12 months400
    1-70 years600
    71+ years800
    Pregnant or nursing women600

    IU: international units

    The American Academy of Pediatrics recommends supplementation for all children who do not receive at least 400 IU of vitamin D daily. Breastfed babies may require a supplement within the first few days of life. Bottle-fed babies who do not consume enough vitamin-D fortified formula may also need the supplement, as well as any child who does not get plenty of vitamin D in their diet.

    As seen above, requirements for pregnant women are the same as for healthy adults, though some believe that pregnant mothers should take more vitamin D than recommended. Furthermore, some experts believe that people at highest risk for vitamin D deficiency, such as older adults or those with limited sun exposure during the winter months, should take 1,000 IU or more daily. However, since the risk of vitamin D toxicity increases with higher doses, such recommendations ought to be discussed individually with a physician.

    Vitamin D Deficiency

    Symptoms of overt vitamin D deficiency are rare today, but can include the following:

        
  • Rickets —in children, a disease in which the bones become soft and weak
  • Osteomalacia —in adults, a disease in which the bones become soft and weak
  • Muscle weakness
  • More mild vitamin D deficiency is thought to be relatively common, especially in higher latitudes, and may lead to increased risk of osteoporosis.

    Vitamin D Toxicity

    Since vitamin D is stored in the body and not excreted in the urine like most water-soluble vitamins, it is possible for it to accumulate and reach toxic levels. Here are safe upper level intakes for vitamin D:

    Age GroupUpper Level Intake (IU/Day)
    0-6 months1,000
    7-12 months1,500
    1-3 years2,500
    4-8 years3,000
    9 years and older4,000
    Pregnant or nursing women4,000

    Symptoms of toxicity:

        
  • Nausea
  • Vomiting
  • Poor appetite
  • Constipation
  • Weakness
  • Weight loss
  • High blood pressure
  • Raised levels of calcium in the blood, which can cause:     
  • Confusion
  • Heart rhythm abnormalities
  • Deposits of calcium in soft tissues, like the kidney, heart, and lungs
  • It is unlikely for sunlight and diet to cause vitamin D toxicity.

    If you have problems with any symptoms related to vitamin D toxicity, take less vitamin D supplements.

    Major Food Sources

    Fortified foods provide the most vitamin D. Examples of foods that may be fortified with vitamin D are:

        
  • Milk
  • Cereal
  • Orange juice
  • Yogurt
  • Margarine
  • Soy beverages
  • There are not many foods that are natural sources of vitamin D. Of those foods that have vitamin D naturally are (most to least):

        
  • Fatty fish, such as salmon, tuna, and mackerel
  • Beef liver
  • Cheese
  • Egg yolks
  • Mushrooms
  • A relatively small amount of sun exposure can provide adequate vitamin D. In a study of naval personnel in submarines, six days of sun exposure proved capable of supplying enough vitamin D for 49 sunless days. However, the actual synthesis of vitamin D through sunlight is affected by season, latitude, time of day, cloud cover, smog, use of sunblock, and skin pigmentation.

    Health Implications

    Populations at Risk for Vitamin D Deficiency

    The following populations may be at risk for vitamin D deficiency and may require a supplement:

        
  • Breastfeed babies—Human milk does not have enough vitamin D. Breastfed babies should receive a 400 IU vitamin supplement each day to make up for this.
  • Older adults—Studies suggest that adults over age 65 have less ability to synthesize vitamin D through sunlight exposure than adults aged 20-30. They are also likely to spend less time out in the sun. Elderly women with vitamin D deficiency may benefit from taking D3 supplements. Talk to your doctor, though, before taking these.
  • Locales with limited sun exposure—People who live above latitudes of approximately 40°N and below latitudes of approximately 40°S are at risk for deficiency during most of the winter months.
  • People with dark skin—Those with darker skin are less able to make vitamin D from the sun.
  • People who are obese —Body fat can bind to some vitamin D preventing it from getting into the blood where it can be used by the body.
  • People with a reduced ability to absorb dietary fat—Because vitamin D is a fat-soluble vitamin, fat is required for its absorption from foods. Some conditions that can cause fat malabsorption include Crohn's disease , cystic fibrosis , celiac disease , pancreatic enzyme deficiency, and liver disease.
  • Tips For Increasing Your Vitamin D Intake

    Here are tips to help increase your intake of vitamin D:

        
  • If you take a vitamin supplement, make sure it contains vitamin D.
  • Drink vitamin D-fortified milk.
  • Get sun exposure, but be careful to watch for sunburn. Sunlight is a major cause of skin cancer .