D: What is it?
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 sun (1,2). 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. (3).
The major biologic function
of vitamin D is to maintain normal blood levels of calcium and phosphorus (4).
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 (5,6).
are the sources of vitamin D?
Fortified foods are the
major dietary sources of vitamin D (4). 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 US (7).
One cup of vitamin D fortified
milk supplies about one-fourth of the estimated daily need for this vitamin
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 oils (4). The table of selected food sources
of vitamin D suggests dietary 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 (7,8). Season, latitude, time of day, cloud
cover, smog, and suncreens affect UV ray exposure (8). 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.
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 (4). 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) are:
mcg* or 200 IU
mcg* or 200 IU
or 400 IU
mcg* or 400 IU
mcg* or 600 IU
mcg* or 600 IU
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 fortified food (4).
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
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 (5,6,7).
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 deficiency(9).
The ability of skin to convert vitamin D to its active form decreases as we
age (4, 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 supplement.
It is important for
individuals with limited sun exposure to include good
sources of vitamin D in their diets (8, 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, Crohns 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 (17-20). 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.
What are some current
issues and controversies about vitamin D?
D and osteoporosis:
It is estimated that over
25 million adults in the United States have, or are at risk of developing
osteoporosis (21). 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.
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 (4,9,10-12),
has been associated with greater incidence of hip fractures (22). A greater
vitamin D intake from diet and supplements has been associated with less bone
loss in older women (23). 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 (22) 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. (24) Your physician
will discuss your need for vitamin D supplementation as part of an overall
plan to prevent and/or treat osteoporosis when indicated.
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 cancer (25-27). Another dietary survey associated
a higher calcium and vitamin D intake with a lower incidence of colon cancer
(28). 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:
are often prescribed to reduce inflammation from a variety of medical problems.
These medicines may be essential for a persons medical treatment, but
they have potential side effects, including decreased calcium absorption (29,30).
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 (30). 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 Alzheimers Disease:
Adults with Alzheimers
disease have increased risk of hip fractures (31). This may be because many
Alzheimers patients are homebound, and frequently sunlight deprived.
Alzheimers 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
(4,10-12). One study of women with Alzheimers disease found that decreased
bone mineral density was associated with a low intake of vitamin D and inadequate
sunlight exposure (32). Physicians evaluate the need for vitamin D supplementation
as part of an overall treatment plan for adults with Alzheimers disease.
is the health risk of too much vitamin D?
There is a high health risk
associated with consuming too much vitamin D (33). Vitamin D toxicity can cause
nausea, vomiting, poor appetite, constipation, weakness, and weight loss (34).
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 (4).
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.
Food Sources of vitamin D (4, 36, 37)
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 D.
If you want more information
about building a healthful diet, refer to the Dietary Guidelines for Americans
and the Food Guide Pyramid.
of Selected Food sources of Vitamin D
Cod Liver Oil, 1 Tbs.
Salmon, cooked, 3
Mackerel, cooked, 3 1/2
Sardines, canned in oil, drained,3 1/2
Eel, cooked, 3 1/2
Milk, nonfat, reduced fat, and whole, vitamin D fortified,
Margarine, fortified, 1 Tbs.
Cereal grain bars, fortified w/ 10% of the DV, 1 each
c prepared from mix and made with vitamin D fortified milk
Dry cereal, Vit D fortified w/10%* of DV, 3/4
* Other cereals may be fortified with more
or less vitamin D
Liver, beef, cooked, 3 1/2
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 very much of a specific nutrient. The DV for vitamin D is 400 IU.
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.