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 While the evidence does not give a clear answer, it has been suggested that vitamin D supplementation may reduce the risk of osteoporosis, seasonal flu in children, high blood pressure, and some forms of cancer.

Recommended Intake

According to most authorities, the recommended intakes for vitamin D are:

Age Group (years) Adequate Intake
FemalesMales
1-50200 IU200 IU
51-70400 IU400 IU
70 +600 IU600 IU

NOTE : A growing number of experts have expressed concerns that the standard intake recommendations may be too low. One review of studies published in The Journal of the American Medical Association in 2005 found that while 700-800 IU daily may reduce the risk of hip fractures, 400 IU is insufficient, and 600 IU is less than optimal.

Infants that are breastfed only may require additional supplementation with vitamin D starting within the first two months of life. The recommended dose of vitamin D in normal infants to prevent a deficiency is 200 units per day. 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. The recommended tolerable upper intake level (UL) for vitamin D from dietary sources and supplements combined is 2,000 IU.

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

FoodServing size Vitamin D content
(IU)
Cod liver oil1 Tbs.1,360
Salmon, cooked3 ½ ounces360
Mackerel, cooked3 ½ ounces345
Sardines, canned in oil3 ½ ounces270
Milk, vitamin D-fortified1 cup98
Margarine, fortified1 Tbs.60
Liver, beef, cooked3 ½ ounces30
Egg1 large25

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:

  • Adults 65 and older—Studies suggest that adults over age 65 have less ability to synthesize vitamin D through sunlight exposure than adults aged 20 to 30. They are also likely to spend less time out in the sun.
  • 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 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.