Functions | Recommended Intake | Vitamin D Deficiency | Vitamin D Toxicity | Major Food Sources | Health Implications | Tips For Increasing Your Vitamin D Intake

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 involving 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

Vitamin D supplementation appears to decrease the risk of fractures. 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, seasonal flu in children, 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.

Requirements for pregnant women are the same as for healthy adults. 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 (eg, older adults, 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

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 (eg, 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

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

Tips For Increasing Your Vitamin D Intake

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