Vitamin D
Dietary Supplement Fact Sheet: Vitamin D
Office of Dietary Supplements • NIH Clinical Center • National Institutes
of Health
What is vitamin D?
Vitamin D is a fat soluble vitamin that is found in food and can also be
made in your body after exposure to ultraviolet (UV) rays from the sun.
Sunshine is a significant source of vitamin D because UV rays from
sunlight trigger vitamin D synthesis in the skin [1-2].
Vitamin D exists in several forms, each with a different level of
activity. Calciferol is the most active form of vitamin D. Other forms are
relatively inactive in the body. The liver and kidney help convert vitamin
D to its active hormone form [3]. Once vitamin D is produced in the skin
or consumed in food, it requires chemical conversion in the liver and
kidney to form 1,25 dihydroxyvitamin D, the physiologically active form of
vitamin D. Active vitamin D functions as a hormone because it sends a
message to the intestines to increase the absorption of calcium and
phosphorus [3].
The major biologic function of vitamin D is to maintain normal blood
levels of calcium and phosphorus [3-4]. By promoting calcium absorption,
vitamin D helps to form and maintain strong bones. Vitamin D also works in
concert with a number of other vitamins, minerals, and hormones to promote
bone mineralization. Without vitamin D, bones can become thin, brittle, or
misshapen. Vitamin D sufficiency prevents rickets in children and
osteomalacia in adults, two forms of skeletal diseases that weaken bones
[5-6].
Research also suggests that vitamin D may help maintain a healthy immune
system and help regulate cell growth and differentiation, the process that
determines what a cell is to become [3,7,8].
What are the sources of vitamin D?
Food Sources
Fortified foods are common sources of vitamin D [4]. In the 1930s, rickets
was a major public health problem in the United States (U.S.). A milk
fortification program was implemented to combat rickets, and it nearly
eliminated this disorder in the U.S. [4,9]. About 98% to 99% of the milk
supply in the U.S. is fortified with 10 micrograms (ìg) (equal to 400
International Units or IU) of vitamin D per quart. One cup of vitamin D
fortified milk supplies one-half of the recommended daily intake for
adults between the ages of 19 and 50, one-fourth of the recommended daily
intake for adults between the ages of 51 and 70, and approximately 15% of
the recommended daily intake for adults age 71 and over.
Although milk is fortified with vitamin D, dairy products made from milk,
such as cheese and ice creams, are generally not fortified with vitamin D
and contain only small amounts. Some ready-to-eat breakfast cereals may be
fortified with vitamin D, often at a level of 10% to 15% of the Daily
Value*. There are only a few commonly consumed foods that are good sources
of vitamin D
Sun exposure
Sun exposure is perhaps the
most important source of vitamin D because exposure to sunlight provides
most humans with their vitamin D requirement [13]. UV rays from the sun
trigger vitamin D synthesis in skin [13-14]. Season, geographic latitude,
time of day, cloud cover, smog, and sunscreen affect UV ray exposure and
vitamin D synthesis [14]. For example, sunlight exposure from November
through February in Boston is insufficient to produce significant vitamin
D synthesis in the skin. Complete cloud cover halves the energy of UV
rays, and shade reduces it by 60%. Industrial pollution, which increases
shade, also decreases sun exposure and may contribute to the development
of rickets in individuals with insufficient dietary intake of vitamin D
[15]. Sunscreens with a sun protection factor (SPF) of 8 or greater will
block UV rays that produce vitamin D, but it is still important to
routinely use sunscreen to help prevent skin cancer and other negative
consequences of excessive sun exposure. An initial exposure to sunlight
(10 -15 minutes) allows adequate time for Vitamin D synthesis and should
be followed by application of a sunscreen with an SPF of at least 15 to
protect the skin. Ten to fifteen minutes of sun exposure at least two
times per week to the face, arms, hands, or back without sunscreen is
usually sufficient to provide adequate vitamin D [14]. It is very
important for individuals with limited sun exposure to include good
sources of vitamin D in their diet.
See also:
Nutrition Fruits & Vegetables
Source and more info at
Vitamin and Mineral Supplement Fact Sheets
from the Office
of Dietary Supplements
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