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Other common
name(s): none
Scientific/medical
name(s): Retinol, retinoic acid, retinoids
Description
Vitamin A is a nutrient that is vital to growth and
development. It is obtained in the diet from animal sources, and is
also derived from beta carotene in plant foods. Beta carotene is
changed into vitamin A in the small intestine. Vitamin A is stored in
the liver until needed by the body. Vitamin A and closely related
molecules are also known as retinoids.
Overview
Vitamin A supplements have not been proven to be effective in
preventing cancer in humans. However, further clinical studies are
being done to explore the role of vitamin A and other retinoids in
cancer prevention and treatment. High doses of vitamin A are toxic, and
long-term use of high-dose supplements may increase the risk of lung
cancer among people at high risk, such as smokers.
How is it promoted for use?
Vitamin A is essential for normal growth, bone development,
reproduction, vision, the maintenance of healthy skin and mucous
membranes (which line the nose and mouth), and protection against
infections in the respiratory, digestive, and urinary tracts. Vitamin A
is obtained in two ways: as vitamin A from animal sources such as
liver, fish oils, and dairy products, and as beta carotene, which the
body converts to retinol (a type of vitamin A), from many fruits and
vegetables, including carrots, broccoli, spinach, squash, peaches, and
apricots.
Some research suggests that vitamin A and some other retinoids
are able to change cancer cells and can also prevent normal cells from
becoming cancerous. Retinoids are given as a conventional medical
prescription medication for some rare types of cancer or precancerous
conditions.
Some proponents say that vitamin A supplements prevent cancer
in general.
What does it involve?
Vitamin A is absorbed from dietary animal fats (especially
liver, fish, egg yolks, and milk fat), from dietary supplements, and in
the form of beta carotene, which is found in some fruits and vegetables
and is converted to vitamin A by the body. Vitamin A is stored in the
liver until needed by the body, so it does not need to be consumed
every day. The best way to get this vitamin is to eat a well-balanced
diet. People who eat a balanced diet of fruits, vegetables, dairy
products, and animal fats usually obtain enough vitamin A and beta
carotene for good health, although supplements are available. The
recommended daily allowance (RDA) of vitamin A is 2,310 IU (0.7
milligrams) per day for women (slightly more for women who are pregnant
or breastfeeding) and 3,000 IU (.9 milligrams) per day for men.
What is the history behind it?
The discovery of vitamin A dates back to research from the
early twentieth century. Over the past twenty years, vitamin A has been
extensively studied as a cancer-fighting nutrient in laboratory,
animal, and population studies. In addition to studies of vitamin A and
cancer prevention, related retinoids have also been evaluated in
laboratory studies and in human clinical trials.
Several studies based on dietary surveys of large numbers of
people have concluded that eating foods rich in vitamin A is associated
with a lower risk of several forms of cancer. However, it is not clear
whether the protective effect was due to vitamin A or to other helpful
substances in these foods.
What is the evidence?
Vitamin A deficiency is rare in developed countries. It can
cause a lowered resistance to infection, poor night vision or even
blindness, poor growth in children, weak bones and teeth, inflamed
eyes, diarrhea, and poor appetite.
Some animal studies have found that vitamin A and other
retinoids may enhance the immune system, slow tumor growth, decrease
the size of tumors, and increase the effectiveness of some cancer
treatments. Some laboratory, animal, and human studies have found that
certain retinoids may also inhibit cancer development.
Studies of vitamin A's possible role in cancer prevention have
been generally disappointing. Clinical trials have found that vitamin A
supplements do not lower the risk for lung cancer in smokers and
actually increased their risk for dying of lung cancer and heart
disease. Studies of overall health and longevity found no benefit to
taking vitamin A supplements, and the U.S. Institute of Medicine does
not recommend use of these supplements by the general public. Studies
on vitamin A and other types of cancer have been mixed, but there have
been no consistent findings showing a decreased risk of cancers of the
stomach, intestines, skin, breast, cervix, bladder, or prostate due to
vitamin A in the diet.
The use of vitamin A supplements has also not been proven to
be effective in reducing cancer risk in humans. It appears that the
combination of micronutrients in fruits, vegetables, legumes, and
grains is more likely to be helpful than individual vitamins.
A 2007 review of 68 studies of vitamin supplements concluded
that people taking beta carotene or vitamin A supplements had a shorter
life expectancy than those who did not take these supplements.
Synthetic retinoids that are more potent than natural vitamin
A or beta carotene have shown some ability to reverse pre-malignancies
in the cervix, mouth, throat, and skin. They also may help prevent new
tumors in people who have already been treated for these forms of
cancer. However, further clinical research is needed. Several clinical
trials involving retinoids have been completed and others are ongoing.
Retinoids are not currently used as a cancer treatment, with
one notable exception. A relatively rare type of leukemia,
promyelocytic leukemia, often responds to a combination of retinoic
acid (a retinoid) and chemotherapy. Patients with this form of leukemia
receive high doses of retinoic acid under the supervision of a
hematologist and/or oncologist. Treatment of promyelocytic leukemia
does not include use of non-prescription vitamin A supplements or
dietary changes intended to increase intake of this vitamin.
Studies of other types of cancer such as lung cancer, head and
neck cancer, and melanoma found that vitamin A supplements are not
helpful. In addition, some oncologists are concerned that vitamin A and
other vitamins that act as antioxidants may make chemotherapy and
radiation therapy less effective when taken during treatment. For this
reason, many oncologists recommend that their patients not take such
antioxidant supplements until their treatment is complete.
Are there any possible problems or
complications?
This product is sold as a
dietary supplement in the United States. Unlike companies that produce
drugs (which must provide the FDA with results of detailed testing
showing their product is safe and effective before the drug is approved
for sale), the companies that make supplements do not have to show
evidence of safety or health benefits to the FDA before selling their
products. Supplement products without any reliable scientific evidence
of health benefits may still be sold as long as the companies selling
them do not claim the supplements can prevent, treat, or cure any
specific disease. Some such products may not contain the amount of the
herb or substance that is written on the label, and some may include
other substances (contaminants). Though the FDA has written new rules
to improve the quality of manufacturing processes for dietary
supplements and the accurate listing of supplement ingredients, these
rules do not take full effect until 2010. And, the new rules do not
address the safety of supplement ingredients or their effects on health
when proper manufacturing techniques are used.
Most such
supplements have not been tested to find out if they interact with
medicines, foods, or other herbs and supplements. Even though some
reports of interactions and harmful effects may be published, full
studies of interactions and effects are not often available. Because of
these limitations, any information on ill effects and interactions
below should be considered incomplete.
High doses of vitamin A supplements can cause nausea,
vomiting, diarrhea, loss of appetite, tiredness, headaches, dizziness,
blurred vision, poor muscle coordination, itchiness and scaling of the
skin, bone pain, hair loss, irregular menstruation in women, temporary
or permanent liver damage, and can cause birth defects if taken during
pregnancy. Relying on this type of treatment alone and avoiding or
delaying conventional medical care for cancer may have serious health
consequences.
Additional Resources
More information from your American Cancer
Society
The following information on complementary and alternative
therapies may also be helpful to you. These materials may be found on
our Web site (www.cancer.org)
or ordered from our toll-free number (1-800-ACS-2345).
References
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Accessed June 16, 2008.
Note: This information may not cover
all possible claims, uses, actions, precautions, side effects or
interactions. It is not intended as medical advice, and should not be
relied upon as a substitute for consultation with your doctor, who is
familiar with your medical situation.
Last Medical Review: 11/01/2008
Last Revised: 11/01/2008
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