General Description
Vitamin A is a fat-soluble nutrient involved in a wide variety of
bodily functions. In its pure, isolated form, vitamin A appears
as a yellow, crystalline substance known as retinol or preformed
vitamin A. The body readily converts retinol into the active forms
retinal (or retinaldehyde) and retinoic acid. Beta-carotene, a pigment
found in green, orange, and yellow vegetables, can be metabolized
in the body to yield retinol and is the best nonanimal source of
the vitamin. For this reason, beta-carotene is often described as
a provitamin A carotenoid.
Food sources
Good sources of preformed vitamin A include liver, fish liver
oils, eggs, and dairy products. Rich sources of beta-carotene
include carrots, spinach, sweet potatoes, bell peppers, squash,
broccoli, and apricots.
Health applications
Functions and uses
Vitamin A is known to play an essential role in eye function, and
vitamin A deficiency is a major cause of blindness in children,
especially in developing countries.1 Vitamin
A is also required for normal growth and development of the body,
hormone production, immune system function, and for maintaining
healthy skin. It is also important for maintaining the integrity
of the mucous membranes in the respiratory, urinary, and digestive
tracts. Symptoms of vitamin A deficiency include night blindness
and other eye disorders, stunted growth, skin disorders, reproductive
failure, and increased susceptibility to infection.
Vision
Vitamin A is a key component of rhodopsin, a pigment needed by
the eye to initiate biochemical responses to visual stimuli. Low
serum vitamin A levels have been associated with night blindness,2-4
and numerous studies have shown night blindness to improve with
vitamin A supplementation.3-5 Other eye disorders
such as cataract formation and macular degeneration do not appear
to be related to vitamin A status.
Immune function
Vitamin A appears to play a significant role in immune system
function, particularly in children. Studies in developing nations
where vitamin A deficiency is common have shown that vitamin A
supplementation led to a reduced number of infections, especially
those of the upper respiratory tract.6-8 In
one study, children with upper respiratory tract infections who
were given 200,000 IU of vitamin A per day recovered faster than
children who did not take the supplement.9
Other research indicates that vitamin A can increase lymphocyte
activity, an effect which may play a role in the prevention of
some forms of cancer.10,11 However, a direct
effect for vitamin A on cancer progression has not been demonstrated.
Skin health
There is some evidence that vitamin A may play a useful role in
the treatment of some skin conditions such as chronic acne (acne
vulgaris) and psoriasis. Research has shown that high doses of
vitamin A (300,000 to 500,000 IU daily) can be very effective
in treating acne vulgaris; however,12 doses
in this range are generally not recommended because of concerns
about toxic accumulations in the liver. Synthetic retinol derivatives
(retinoids), which have less potential for toxicity and have been
shown to be effective in the treatment of acne13-15
and psoriasis,16 are available by prescription.
Dosage/toxicity
Vitamin A has traditionally been measured in international units
(IU), but can also be measured in micrograms (mcg) or retinol
equivalents (RE). Although optimum intake of vitamin A may vary
with age and gender, 5,000 IU per day is considered adequate for
most people. Because vitamin A is a fat-soluble compound that
is stored in the liver, long-term, high-dose supplementation (over
50,000 IU per day for adults, over 25,000 IU per day for children)
can result in toxic accumulations. For adult men and post-menopausal
women, up to 25,000 IU per day is generally considered safe. Because
high retinol intake has been associated with increased risk for
birth defects, women who are or may become pregnant should not
consume more than 10,000 IU per day.17 Beta-carotene
may be a safer source of vitamin A for these women because it
is not associated with birth-defect risk.18,19
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