D: What is it? | What are the sources of Vitamin
[Recommended Dietary Allowance]
[When can vitamin D deficiency occur?]
[Who may need extra vitamin D to prevent a deficiency]
[Vitamin D issues and controversies | Too
much vitamin D]
[Selected food sources for Vitamin D | References]
D: What is it?
The major biologic function of vitamin D is to maintain normal blood levels
of calcium and phosphorus4. Vitamin D aids in the absorption
of calcium, helping to form and maintain strong bones. It promotes bone
mineralization in concert with a number of other vitamins, minerals, and
hormones. Without vitamin D, bones can become thin, brittle, soft, or
misshapen. Vitamin D prevents rickets in children and osteomalacia in
adults, which are skeletal diseases that result in defects that weaken
Vitamin D, calciferol,
is a fat-soluble vitamin. It is found in food, but also can be made in
your body after exposure to ultraviolet rays from the sun1,2.
Vitamin D exists in several forms, each with a different activity. Some
forms of vitamin D are relatively inactive in the body, and have limited
ability to function as a vitamin. The liver and kidney help convert vitamin
D to its active hormone form3.
are the sources of vitamin D?
sources | Exposure to sunlight]
Fortified foods are the major dietary sources of vitamin D4.
Prior to the fortification of milk products in the 1930s, rickets (a
bone disease seen in children) was a major public health problem in
the United States. Milk in the United States is fortified with 10 micrograms
(400 IU) of vitamin D per quart, and rickets is now uncommon in the
One cup of vitamin
D fortified milk supplies about one-fourth of the estimated daily need
for vitamin D for adults. Although milk is fortified with vitamin D,
dairy products made from milk such as cheese, yogurt, and ice cream
are generally not fortified with vitamin D. Only a few foods naturally
contain significant amounts of vitamin D, including fatty fish and fish
oils4. The table of selected food sources
of vitamin D suggests dietary sources of vitamin D.
Exposure to sunlight is an important source of vitamin D. Ultraviolet
(UV) rays from sunlight trigger vitamin D synthesis in the skin7,8.
Season, latitude, time of day, cloud cover, smog, and suncreens affect
UV ray exposure8. For example, in Boston
the average amount of sunlight is insufficient to produce significant
vitamin D synthesis in the skin from November through February. Sunscreens
with a sun protection factor of 8 or greater will block UV rays that
produce vitamin D, but it is still important to routinely use sunscreen
whenever sun exposure is longer than 10 to 15 minutes. It is especially
important for individuals with limited sun exposure to include good
sources of vitamin D in their diet.
the Recommended Dietary Allowance for vitamin D for adults?
The Recommended Dietary Allowance (RDA) for Vitamin D is the average daily
dietary intake level that is sufficient to meet the nutrient requirements
of nearly all (97-98%) healthy individuals in each life-stage and gender
group4. There is insufficient evidence to
establish a RDA for vitamin D. Instead, an Adequate Intake (AI), a level
of intake sufficient to maintain healthy blood levels of an active form
of vitamin D, has been established. The 1998 AIs4 for vitamin
D for adults, in micrograms (mcg) and International Units (IUs) are:
5 mcg* or 200
mcg* or 200 IU
mcg* or 400 IU
10 mcg* or 400
or 600 IU
mcg* or 600 IU
mcg vitamin D + 40 International Units (UI)
Estimates of vitamin
D intake in the United States are not available because dietary surveys
do not assess vitamin D intake. Dietary intake of vitamin D is largely
determined by the intake of fortified food4.
can vitamin D deficiency occur?
A deficiency of vitamin D can occur when dietary
intake of vitamin D is inadequate, when there is limited exposure to sunlight,
when the kidney cannot convert vitamin D to its active form, or when someone
cannot adequately absorb vitamin D from the gastrointestinal tract7.
The classic vitamin
D deficiency diseases are rickets and osteomalacia. In children, vitamin
D deficiency causes rickets, which results in skeletal deformities. In
adults, vitamin D deficiency can lead to osteomalacia, which results in
muscular weakness in addition to weak bones5,6,7.
may need extra vitamin D to prevent a deficiency?
Older Americans (greater than age 50) are thought to have a higher risk
of developing vitamin D deficiency9. The ability
of skin to convert vitamin D to its active form decreases as we age4,
10-12. The kidneys, which help
convert vitamin D to its active form, sometimes do not work as well when
people age. Therefore, some older Americans may need vitamin D from a
It is important for
individuals with limited sun exposure to include good sources of vitamin
D in their diets8, 13-15.
Homebound individuals, people living in northern latitudes such as in
New England and Alaska, women who cover their body for religious reasons,
and individuals working in occupations that prevent exposure to sunlight
are at risk of a vitamin D deficiency. If these individuals are unable
to meet their daily dietary need for vitamin D, they may need a supplement
of vitamin D.
Individuals who have
reduced ability to absorb dietary fat (fat malabsorption) may need extra
vitamin D because it is a fat soluble vitamin. Some causes of fat malabsorption
are pancreatic enzyme deficiency, Crohn's disease, cystic fibrosis, sprue,
liver disease, surgical removal of part or all of the stomach, and small
bowel disease6. Symptoms of fat malabsorption
include diarrhea and greasy stools16.
Vitamin D supplements
are often recommended for exclusively breast-fed infants because human
milk may not contain adequate vitamin D17-20.
The Institute of Medicine states that "With habitual small doses of sunshine
breast- or formula-fed infants do not require supplemental vitamin D."
Mothers of infants who are exclusively breastfed and have a limited sun
exposure should consult with a pediatrician on this issue. Since infant
formulas are routinely fortified with vitamin D, formula fed infants usually
have adequate dietary intake of vitamin D.
are some current issues and controversies about vitamin D?
| Cancer | Steroids | Alzheimer's
D and osteoporosis
It is estimated that over 25 million adults in the United States have,
or are at risk of developing osteoporosis21.
Osteoporosis is a disease characterized by fragile bones. It results
in increased risk of bone fractures. Having normal storage levels of
vitamin D in your body helps keep your bones strong and may help prevent
osteoporosis in elderly, non-ambulatory individuals, in post-menopausal
women, and in individuals on chronic steroid therapy.
that normal bone is constantly being remodeled (broken down and rebuilt).
During menopause, the balance between these two systems is upset, resulting
in more bone being broken down (resorbed) than rebuilt. Estrogen replacement,
which limits symptoms of menopause, can help slow down the development
of osteoporosis by stimulating the activity of cells that rebuild bone.
Vitamin D deficiency,
which occurs more often in post-menopausal women and older Americans4,9,10-12,
has been associated with greater incidence of hip fractures22.
A greater vitamin D intake from diet and supplements has been associated
with less bone loss in older women23. Since
bone loss increases the risk of fractures, vitamin D supplementation
may help prevent fractures resulting from osteoporosis.
In a group of women
with osteoporosis hospitalized for hip fractures, 50 percent were found
to have signs of vitamin D deficiency. Treatment of vitamin D deficiency22
can result in decreased incidence of hip fractures, and daily supplementation
with 20 mcg (800 IU) of vitamin D may reduce the risk of osteoporotic
fractures in elderly populations with low blood levels of vitamin D24.
Your physician will discuss your need for vitamin D supplementation
as part of an overall plan to prevent and/or treat osteoporosis when
D and cancer
Laboratory, animal, and epidemiologic evidence suggest that vitamin
D may be protective against some cancers. Some dietary surveys have
associated increased intake of dairy foods with decreased incidence
of colon cancer25-27.
Another dietary survey associated a higher calcium and vitamin D intake
with a lower incidence of colon cancer28.
Well-designed clinical trials need to be conducted to determine whether
vitamin D deficiency increases cancer risk, or if an increased intake
of vitamin D is protective against some cancers. Until such trials are
conducted, it is premature to advise anyone to take vitamin D supplements
to prevent cancer.
D and steroids
medications are often prescribed to reduce inflammation from a variety
of medical problems. These medicines may be essential for a person's
medical treatment, but they have potential side effects, including decreased
There is some evidence that steroids may also impair vitamin D metabolism,
further contributing to the loss of bone and development of osteoporosis
associated with steroid medications30.
For these reasons, individuals on chronic steroid therapy should consult
with their physician or registered dietitian about the need to increase
vitamin D intake through diet and/or dietary supplements.
D and Alzheimer's Disease
Adults with Alzheimer's disease have increased risk of hip fractures31.
This may be because many Alzheimer's patients are homebound, and frequently
sunlight deprived. Alzheimer's disease is more prevalent in older populations,
so the fact that the ability of skin to convert vitamin D to its active
form decreases as we age also may contribute to increased risk of hip
fractures in this group4,10-12.
One study of women with Alzheimer's disease found that decreased bone
mineral density was associated with a low intake of vitamin D and inadequate
sunlight exposure32. Physicians evaluate
the need for vitamin D supplementation as part of an overall treatment
plan for adults with Alzheimer's disease.
is the health risk of too much vitamin D?
There is a high health risk associated with consuming too much vitamin
D33. Vitamin D toxicity can cause nausea,
vomiting, poor appetite, constipation, weakness, and weight loss34.
It can also raise blood levels of calcium, causing mental status changes
such as confusion. High blood levels of calcium also can cause heart rhythm
abnormalities. Calcinosis, the deposition of calcium and phosphate in
soft tissues like the kidney can be caused by vitamin D toxicity4.
Consuming too much
vitamin D through diet alone is not likely unless you routinely consume
large amounts of cod liver oil. It is much more likely to occur from high
intakes of vitamin D in supplements. The Food and Nutrition Board of the
Institute of Medicine considers an intake of 25 mcg (1,000 IU) for infants
up to 12 months of age and 50 mcg (2,000 IU) for children, adults, pregnant,
and lactating women to be the tolerable upper intake level (UL). A daily
intake above the UL increases the risk of adverse health effects and is
Food Sources of vitamin D4, 36,
As the 2000 Dietary Guidelines for Americans state, "Different foods contain
different nutrients. No single food can supply all the nutrients in the
amounts you need"35. The following table
suggests dietary sources of vitamin D. As the table indicates, fortified
foods are a major source of vitamin D. Breakfast cereals, pastries, breads,
crackers, cereal grain bars and other foods may be fortified with 10%
to 15% of the DV for vitamin D. It is important to read the nutrition
facts panel of the food label to determine whether a food provides vitamin
If you want more
information about building a healthful diet, refer to the Food Guide Pyramid.
Table of Selected Food Sources
of Vitamin D
Cod liver oil,
3 1/2 oz.
3 1/2 oz.
in oil, drained, 3 1/2 oz.
3 1/2 oz.
reduced fat, and whole, vitamin D fortified, 1 c.
bars, fortified w/ 10% of the DV, 1 each
c. prepared from mix and made with vitamin D fortified milk
vitamin D fortified w/ 10%* of DV, 3/4 c.
*Other cereals may be fortified w/ more or less vitamin
cooked, 3 1/2 oz.
Egg, 1 whole
(vitamin D is present in the yolk)
DV = Daily Value. DVs are reference numbers based on the Recommended
Dietary Allowance (RDA). They were developed to help consumers determine
if a food contains a lot or a little of a specific nutrient. The DV
for vitamin E is 30 International Units (or 20 mg). The percent DV
(%DV) listed on the nutrition facts panel of food labels tells adults
what percentage of the DV is provided by one serving. Percent DVs
are based on a 2,000-calorie diet. Your Daily Values may be higher
or lower depending on your calorie needs. Foods that provide lower
percentages of the DV will contribute to a healthful diet.
This fact sheet was developed
by the Clinical Nutrition Service, Warren Grant Magnuson Clinical Center,
National Institutes of Health (NIH), Bethesda, MD, in conjunction with the
Office of Dietary Supplements (ODS) in the Office of the Director of NIH.
The mission of ODS is to strengthen knowledge and understanding of dietary
supplements by evaluating scientific information, stimulating and supporting
research, disseminating research results, and educating the public to foster
an enhanced quality of life and health for the U.S. population. The Clinical
Nutrition Service and the ODS would like to thank the expert scientific
reviewers for their role in ensuring the scientific accuracy of the information
discussed in this fact sheet.
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