Vitamin D, calciferol, is a
fat-soluble vitamin. It is made in your body after exposure to ultraviolet
rays from the sun, and also can be found in food. Vitamin D exists in several
forms, each with a different activity. Some forms 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 form.
The major biologic function of vitamin D is to
maintain normal blood levels of calcium and phosphorus. 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 bones.
are the sources of vitamin D?
Exposure to sunlight
Exposure to sunlight is an important source of vitamin D. Ultraviolet
(UV) rays from sunlight trigger vitamin D synthesis in the skin. Season,
latitude, time of day, cloud cover, smog, and suncreens affect UV ray
exposure. 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.
Is there a Recommended
Dietary Allowance for vitamin D for adults?
The Recommended Dietary Allowance (RDA) 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 group.
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 AIs (4)
for vitamin D for adults, in micrograms (mcg) and International Units (IUs)
mcg* or 200 IU
mcg* or 200 IU
mcg* or 400 IU
mcg* or 400 IU
mcg* or 600 IU
mcg* or 600 IU
*1 mcg vitamin D = 40
International Units (IU)
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
When can vitamin D
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 tract.
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 bones .
Who 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 deficiency(9). The ability of skin to convert
vitamin D to its active form decreases as we age. 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 diets. 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 disease (6).
Symptoms of fat malabsorption include diarrhea and greasy stools.
Vitamin D supplements are often recommended
for exclusively breast-fed infants because human milk may not contain adequate
vitamin D. 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?
Vitamin D and osteoporosis
It is estimated that over 25 million adults in the United States have, or
are at risk of developing osteoporosis. 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
Researchers know 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 Americans, has been associated with greater
incidence of hip fractures. A greater vitamin D intake from diet and
supplements has been associated with less bone loss in older women. 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 deficiency 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 D. Your physician will discuss your need for
vitamin D supplementation as part of an overall plan to prevent and/or treat
osteoporosis when indicated.
Vitamin D and steroids
Corticosteroid 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 calcium absorption. 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 medications. 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.
Vitamin D and Alzheimer’s
Adults with Alzheimer’s disease have increased risk of hip
fractures. 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 group. 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 exposure. Physicians evaluate the need for
vitamin D supplementation as part of an overall treatment plan for adults with
What is the health risk of
too much vitamin D?
There is a high health risk associated with consuming too much
vitamin D. Vitamin D toxicity can cause nausea, vomiting, poor appetite,
constipation, weakness, and weight loss. 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 toxicity.
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 not advised.
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.