What is Vitamin D?
Vitamin D is a fat-soluble vitamin that's formed when skin is exposed to the sun's ultraviolet rays. Vitamin D is also found in food and dietary supplements.
There are two major types of vitamin D. Vitamin D3, also called cholecalciferol, is the type made in the body in response to sun exposure and found in certain foods. Vitamin D2 (ergocalciferol) is the most common form used in supplements. It's also used to fortify certain foods, such as milk.
Both types of vitamin D must be converted in the liver and kidneys to the active form, 1,25 dihydroxyvitamin D, to be useful to the body.
What Does Vitamin D Do?
The main function of vitamin D is to maintain normal levels of calcium and phosphorus in the blood to support bone mineralization (hardening of bones), cell functions, and proper nerve and muscle function. Vitamin D acts as a hormone, enhancing the absorption of calcium and phosphorus in the small intestine.
Vitamin D is needed for normal growth. Without it, bones become weak and deformed, resulting in rickets in children and a condition called osteomalacia in adults.
Vitamin D deficiency can lead to the development of osteoporosis because it reduces calcium absorption. In older people, low vitamin D levels have been associated with an increased risk of falling. Higher vitamin D levels have been associated with stronger bones (greater bone mineral density).
In most studies, the effects of vitamin D and calcium can't be separated. An analysis of previously published studies found that supplementing with vitamin D3 (more than 700 IU per day) and calcium had a small but beneficial effect on bone mineral density compared to a placebo, and reduced the risk of falls.
Preliminary evidence suggests vitamin D may aid in cancer prevention by blocking cell growth and differentiation (cells mature and take on a specialized form and function).
Since the late 1980s, the risk of developing and dying from breast, prostate, ovarian, and other cancers has been found to be increased in geographic areas located at higher latitudes, where there's less sun exposure, and with vitamin D deficiency.
A study published in the American Journal of Clinical Nutrition found that vitamin D may reduce the risk of breast and other cancers.
Because vitamin D is thought to influence the immune system, there is some evidence it may help with autoimmune disease.
Sources of Vitamin D
The main source of vitamin D comes from exposure to the sun's UVB rays. The American Academy of Dermatology advises that we obtain vitamin D from foods and supplements rather than UV exposure, because of the risk of skin cancer.
Vitamin D occurs naturally in certain types of fish, such as herring, mackerel, salmon, and sardines, and in egg yolks. Milk is fortified with vitamin D. Some juice products, breakfast cereals, and other foods may also be fortified with vitamin D.
Vitamin D supplements are also available. Multivitamins and calcium supplements provide vitamin D, but the amount varies widely so it's important to read labels. Vitamin D3 (cholecalciferol) is the preferred form.
Why Do People Use Vitamin D Supplements?To ensure that they are getting enough vitamin D, many people may opt to use supplements. Supplementation is common for:
- Osteoporosis prevention and treatment
- Cancer prevention
- Diabetes prevention
- Polycystic ovary disease
- Gum disease prevention
- Seasonal affective disorder
Recommended Vitamin D Intake
In the United States, the Institute of Medicine's recommendations for vitamin D intake are as follows:
- Birth to 50 years -- 200 IU (5 mcg)
- Between 51 and 70 years -- 400 IU (10 mcg)
- Over 71 years -- 600 IU (15 mcg)
- Pregnant and nursing women -- 200 IU (5 mcg)
These recommendations were set in 1997. There is growing consensus that vitamin D reference intakes need to be reassessed, based on increasing evidence that vitamin D insufficiency is widespread in the general population and that vitamin D plays a key role in bone health and the prevention of chronic health conditions.
In a recent study that found a 7% reduction in mortality, the average intake was about 500 IU per day.
If you don't think you're getting the recommended amount of vitamin D, ask your doctor whether a vitamin D supplement might be appropriate for you.
Who May Require Extra Vitamin D?
The amount of vitamin D formed from exposure to the sun is affected by season and latitude. In general, in northern cities such as Boston or New York, there aren't enough UVB rays to form vitamin D during the fall and winter months.
People who are homebound, women who wear robes and head coverings for religious reasons, and people whose jobs or work hours limit their exposure to sunlight are likely not obtaining significant amounts of vitamin D from sunlight.
People who use sunscreen
The use of sunscreen prevents the formation of vitamin D, too. Even sunscreen with an SPF of 8, an amount found in many daily moisturizers, can greatly reduce vitamin D production. In one research study, nearly half of people in Springfield, Illinois, who always wore sunscreen while outdoors had a vitamin D deficiency.
People with darker skin have more melanin, the pigment that gives skin its color. Melanin absorbs UV rays, which reduces the skin's ability to produce vitamin D. The more pigment in a person's skin, the more important it is that he or she consume enough vitamin D.
Older people are at greater risk of vitamin D deficiency, as the ability to produce vitamin D dwindles with age. In fact, vitamin D levels in elderly subjects have been found to be approximately 30% of the levels found in young adults.
It's estimated that over 50% of adults over age 50 in the United States and Europe are at risk of vitamin D deficiency. One study found that in Boston at the end of the summer, 30% of whites, 42% of Hispanics, and 84% of African-Americans had vitamin D deficiency.
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