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.
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.
Here are the guidelines for vitamin D intake:
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:
Symptoms of toxicity:
- Poor appetite
- Weight loss
- High blood pressure
Raised levels of calcium in the blood, which can cause:
- 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:
- Orange juice
- 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
- Egg yolks
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.
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.
- Reviewer: Brian Randall, MD
- Review Date: 08/2011 -
- Update Date: 03/06/2013 -