Vitamin A: What is it?
What foods provide
vitamin A?
What is the Recommended
Dietary Allowance
for vitamin A?
Table 1: Recommended
Dietary Allowances for vitamin A for children and adults
Table 2: Adequate
Intake for vitamin A for infants
When can vitamin A deficiency
occur?
What is the association
between vitamin A, beta carotene, and cancer?
Can an excess
intake of vitamin A promote osteoporosis?
What is the health
risk of too much vitamin A?
Table 3: Tolerable
Upper Intake Levels for preformed vitamin A for children and adults
What is the health
risk of too many carotenoids?
Selected food sources
of vitamin A
Vitamin A:
What is it?
Vitamin A is a family
of fat-soluble vitamins. Retinol is one of the most active, or usable,
forms of vitamin A, and is found in animal foods such as liver and eggs
and in some fortified food products.
Retinol is often called preformed vitamin
A. It can be converted to retinal and retinoic acid, other active forms
of the vitamin A family (1-4).
Some plant foods contain darkly colored
pigments called provitamin A carotenoids that can be converted to vitamin
A. In the U.S., approximately 26% and 34% of vitamin A consumed by men
and women is provided by provitamin A carotenoids (1).
Beta-carotene is a provitamin A
carotenoid that is more efficiently converted to retinol than other carotenoids
(1-4).
For example, alpha-carotene and b-cryptoxanthin
are also converted to vitamin A, but only half as efficiently as beta-carotene
(1). Lycopene, lutein, and zeaxanthin are other carotenoids commonly found
in food. They are not sources of vitamin A but may have other health promoting
properties. The Institute of Medicine (IOM) encourages consumption of
carotenoid-rich fruits and vegetables for their health-promoting benefits.
Vitamin A plays an important role
in vision, bone growth, reproduction, cell division and cell differentiation,
which is the process by which a cell decides what it is going to become
(1, 5-8).
It helps maintain the surface linings of
the eyes and the respiratory, urinary, and intestinal tracts (9). When
those linings break down, bacteria can enter the body and cause infection
(9). Vitamin A also helps maintain the integrity of skin and mucous membranes
that function as a barrier to bacteria and viruses (10-12).
Vitamin A helps regulate the immune system
(2, 5, 13). The immune system helps prevent or fight off infections by
making white blood cells that destroy harmful bacteria and viruses. Vitamin
A may help lymphocytes, a type of white blood cell that fights infections,
function more effectively.
Some carotenoids, in addition to serving
as a source of vitamin A, have been shown to function as antioxidants
in laboratory tests. However, this role has not been consistently demonstrated
in humans (1). Antioxidants protect cells from free radicals, which are
potentially damaging by-products of oxygen metabolism that may contribute
to the development of some chronic diseases (3, 14-16).
What foods
provide vitamin A?
Preformed vitamin A is found in animal
foods such as whole eggs, whole milk and liver.
Most fat free milk and dried nonfat milk
solids sold in the US are fortified with vitamin A to replace the vitamin
A lost when the fat is removed (17). Fortified foods such as fortified
breakfast cereals also provide vitamin A. Provitamin A carotenoids are
abundant in darkly colored fruits and vegetables. Tables 4 and 5 at the
end of this document list animal sources of vitamin A and a variety of
plant sources of provitamin A carotenoids (18).
It is important for you to regularly eat
foods that provide vitamin A or beta-carotene even though vitamin A is
stored in the liver (2). Stored vitamin A will help meet needs when intake
of provitamin A carotenoids or preformed vitamin A is low (19, 20).
What
is the Recommended Dietary Allowance for vitamin A for children and adults?
The latest recommendations for vitamin A are given in the Dietary
Reference Intakes developed by the Institute of Medicine.
Dietary Reference Intakes (DRIs) is the
umbrella term for a group of reference values used for planning and assessing
diets for healthy people.
One of those references values,
the Recommended Dietary Allowance (RDA), is the average daily dietary
intake level sufficient to meet the nutrient requirements of nearly all
(97-98%) healthy individuals in each age and gender group (1).
RDAs for vitamin A are listed as Retinol
Activity Equivalents (RAE) to account for the different activities of
retinol and provitamin A carotenoids. In the table below, RDAs are also
listed in International Units (IU) because food and some supplement labels
list vitamin A content in International Units (1 RAE in micrograms (ug)
= 3.3 IU). The 2001 RDAs for adults and children (21) in ug RAE and IUs
are:
Table
1: Recommended Dietary Allowances for vitamin A in micrograms (ug) Retinol
Activitiy Equivalents (RAE) and International Units (IUs) for children
and adults
Results of two national surveys, the third
National Health and Nutrition Examination Survey (NHANES III 1988-91)
(1, 21) and the Continuing Survey of Food Intakes by Individuals (CSFII
1994) (1, 22) suggested that dietary intakes of some Americans do not
meet recommended levels for vitamin A. These surveys highlight the importance
of encouraging all Americans to include dietary sources of vitamin A in
their daily diets.
There is no RDA for beta-carotene or other provitamin
A carotenoids. The Institute of Medicine report suggests that consuming
3 to 6 mg of beta-carotene daily will maintain plasma beta-carotene blood
levels in the range associated with a lower risk of chronic diseases (1).
A diet that provides five or more servings of fruits and vegetables per
day and includes some dark green and leafy vegetables and deep yellow
or orange fruits should provide recommended amounts of beta-carotene.
When can vitamin
A deficiency occur?
Vitamin A deficiency rarely occurs
in the United States, but it is still a major public health problem in
the developing world.
At least 3 million children develop xeropthalmia,
damage to the cornea of the eye, and 250,000 to 500,000 go blind each
year from a deficiency of vitamin A (1). Most of these children live in
developing countries. Night blindness is one of the first signs
of vitamin A deficiency. In ancient Egypt it was known that night
blindness could be cured by eating liver, which was later found to be
a rich source of vitamin A (2). Vitamin A deficiency contributes to blindness
by making the cornea very dry and promoting damage to the retina and cornea(23).
Vitamin A deficiency diminishes
the ability to fight infections.
In countries where immunization programs
are not widespread and vitamin A deficiency is common, millions of children
die each year from complications of infectious diseases such as measles.
(9). When there is not enough vitamin A, cells lining the lung lose their
ability to remove disease-causing microorganisms. This may contribute
to the pneumonia associated with vitamin A deficiency (2,10,11).
There is increased interest in subclinical
forms of vitamin A deficiency, described as low storage levels of vitamin
A that do not cause overt deficiency symptoms. This mild degree of vitamin
A deficiency may increase children’s risk of developing respiratory
and diarrheal infections, decrease growth rate, slow bone development,
and decrease likelihood of survival from serious illness (8, 23, 24, 25).
Children living in the United States who are considered to be at increased
risk for subclinical vitamin A deficiency include:
- toddlers and preschool age children,
- children living at or below the poverty level,
- children with inadequate health care or immunizations,
- children living in areas with known nutritional
deficiencies,
- recent immigrants or refugees from developing
countries with high incidence of vitamin A deficiency or measles, and
- children with diseases of the pancreas,
liver, intestines, or with inadequate fat digestion/absorption (9)
Vitamin A deficiency can occur
when vitamin A is lost through chronic diarrhea, and through an overall
inadequate intake, as is often seen with protein-calorie malnutrition.
Low plasma retinol concentrations indicate
depleted levels of vitamin A. This occurs with vitamin A deficiency but
also can result from an inadequate intake of protein, calories and zinc.
These nutrients are needed to make Retinol Binding Protein (RBP), which
is essential for mobilizing vitamin A from your liver and transporting
vitamin A to your general circulation (1).
Iron deficiency can also limit the metabolism of
vitamin A, and iron supplements provided to iron deficient individuals
may improve vitamin A nutriture as well as iron status (1).
Excess alcohol intake depletes vitamin A stores.
Also, diets high in alcohol usually do not provide recommended amounts
of vitamin A (1).
It is very important for anyone who consumes excessive
amounts of alcohol to include good sources of vitamin A in his or her
diet. However, Vitamin A supplementation may not be recommended for individuals
who abuse alcohol because alcohol may increase liver toxicity associated
with excess intakes of vitamin A (1,26 ). A medical doctor would need
to evaluate this situation and determine the need for vitamin A supplementation.
Who
may need extra vitamin A to prevent a deficiency?
Vitamin A deficiency rarely occurs in the United States, but the World
Health Organization (WHO) and the United Nations International Children’s
Emergency Fund (UNICEF) have issued joint statements about vitamin A and
children’s health. Both agencies recommend vitamin A administration
for all children diagnosed with measles in communities where vitamin A
deficiency is a serious problem and where death from measles is greater
than 1%. In 1994, the American Academy of Pediatrics recommended vitamin
A supplementation for two subgroups of children likely to be at high risk
for subclinical vitamin A deficiency. These subgroups were children 6-24
months of age who had been hospitalized with measles and hospitalized
children older than 6 months (27).
Fat malabsorption can promote
diarrhea and prevent normal absorption of vitamin A.
This is most often seen with cystic fibrosis,
sprue, pancreatic disorders, and after stomach surgery. Healthy adults
usually have a reserve of vitamin A stored in their livers and should
not be at risk of deficiency during periods of temporary or short term
fat malabsorption. Long-term problems absorbing fat, however, may result
in deficiency, and in these instances physicians may advise vitamin A
supplementation (9).
Vegetarians who do not consume
eggs and dairy foods need greater amounts of provitamin A carotenoids
to meet their need for vitamin A (1).
It is important for vegetarians to include
a minimum of five servings of fruits and vegetables daily and to regularly
choose dark green leafy vegetables and orange and yellow fruits to consume
recommended amounts of vitamin A.
What
is the association between vitamin A, beta carotene and cancer?
Surveys suggest an association between diets rich in beta-carotene
and vitamin A and a lower risk of some types of cancer (2, 28).
There is evidence that a higher intake
of green and yellow vegetables or other food sources of beta-carotene
and/or vitamin A may decrease the risk of lung cancer (29). However, a
number of studies that tested the role of beta-carotene supplements in
cancer prevention did not find it to be protective (30). In a study of
29,000 men, incidence of lung cancer was greater in the group of smokers
who took a daily supplement of beta-carotene (31).
The Carotene and Retinol Efficacy Trial,
a lung cancer chemoprevention trial that provided randomized subjects
with supplements of beta-carotene and vitamin A, was stopped after researchers
discovered that subjects receiving beta-carotene had a 46% higher risk
of dying from lung cancer than those who did not receive beta-carotene
(32). The Institute of Medicine (IOM) states that “beta-carotene
supplements are not advisable for the general population,” although
they also state that this advice “does not pertain to the possible
use of supplemental beta-carotene as a provitamin A source for the prevention
of vitamin A deficiency in populations with inadequate vitamin A nutriture”
(1).
Can
an excess intake of vitamin A promote osteoporosis?
Osteoporosis, a disorder characterized by porous, weak bones,
is a serious public health problem for more than 10 million Americans,
80% of whom are women.
Another 18 million Americans have decreased
bone density, which precedes the development of osteoporosis. Researchers
have identified many factors that increase the risk for developing osteoporosis,
including being female, thin, inactive, at advanced age, and having a
family history of osteoporosis. An inadequate dietary intake of calcium,
cigarette smoking and excessive intake of alcohol also increase the risk
of developing osteoporosis.
Researchers are now examining a potential new risk factor for
osteoporosis: an excess intake of vitamin A.
Animal, human, and laboratory research
suggest an association between greater vitamin A intake and weaker bones
(33, 34). Researchers have also noticed that worldwide, the highest incidence
of osteoporosis occurs in northern Europe, a population with a high intake
of vitamin A (35). However, decreased biosynthesis of vitamin D associated
with lower levels of sun exposure in this population may also contribute
to this finding.
One small study of nine healthy individuals in Sweden found that
the amount of vitamin A in one serving of liver may impair the ability
of vitamin D to promote calcium absorption (36).
To further test the association between
excess dietary intake of vitamin A and increased risk for hip fracture,
researchers in Sweden compared bone mineral density and retinol intake
in approximately 250 women with a first hip fracture to 875 age-matched
controls. They found that a dietary retinol intake greater than 1,500
mcg/day (more than twice the recommended daily intake for women) was associated
with reduced bone mineral density and increased risk of hip fracture as
compared to women who consumed less than 500 mcg per day (37).
This issue was also examined by
researchers with the Nurses Health Study, who looked at the association
between vitamin A intake and hip fractures in over 72,000 postmenopausal
women.
In this study, women who consumed the most
vitamin A in foods and supplements (greater than or equal to 3000 mcg
per day as retinol equivalents, which is over three times the recommended
intake for adult men and women) had a significantly increased risk of
experiencing a hip fracture as compared to those consuming the least amount.
(less than 1250 mcg per day of retinol equivalents). The effect was lessened
by use of estrogens but still raises questions about the effects of a
high intake of vitamin A. In particular this raises questions about the
effect of preformed vitamin A or retinol because retinol intake greater
than 2000 mcg per day was associated with an increased risk of hip fracture
as compared to a retinol intake less than 500 mcg (38).
A recent longitudinal study in
more than 2,000 Swedish men was the first to measure blood levels of retinol
to assess the risk of fractures in men.
The investigators found that the risk of
fractures was greatest in men with the highest serum retinol levels (greater
than 75.62 mcg per d/l). The risk of fracture was further increased in
men with the highest serum retinol levels. Men with retinol in the 99th
percentile (greater than 103.12 mcg per d/l) had an overall risk of fracture
that exceeded the risk among men with lower levels of serum retinol by
a factor of seven. High vitamin A intake does not necessarily equate to
high serum retinol; serum retinol is regulated by factors besides vitamin
A intake, including age, gender, hormones and genetics. Serum beta carotene,
however, was not associated with the risk of fracture. The researchers’
findings, which are consistent with the results of studies in animals,
as well as in vitro (laboratory studies) and epidemiologic dietary studies,
suggest that intakes above the Upper Limit or approximately two times
that of the RDA, may pose subtle risks to bone health that require further
investigation. Vitamin D, which may contribute to osteoporosis, was not
measured. Additional clinical studies evaluating vitamin D and calcium
as well as retinol for risk of fracture are warranted (39).
On the other hand, the Centers for Disease Control reviewed data
from the Third National Health and Nutrition Examination survey (NHANES
III), 1988-94, to determine whether there was any association between
bone mineral density and fasting blood levels of retinyl esters, a form
of vitamin A (40).
Blood levels of retinyl esters in 5,800
participants were in the normal range and researchers did not find any
significant associations between bone mineral density and blood levels
of retinyl esters. Additional research is needed to clarify the association
between high levels of vitamin A intake and osteoporosis.
There is no evidence of an association between beta-carotene
intake, especially from fruits and vegetables (many of which are naturally
high in beta-carotene), and increased risk of osteoporosis.
Current evidence points to a possible association
with vitamin A as retinol only.
If you have specific questions regarding
your intake of vitamin A and risk of osteoporosis, it is recommended that
you discuss this information with your physician or other trained health
care practitioner to determine what’s best for your personal health.
What
is the health risk of too much vitamin A?
Hypervitaminosis A refers to high storage levels of vitamin A in the body
that can lead to toxic symptoms. There are three major adverse effects
of hypervitaminosis A:
- birth defects,
- liver abnormalities,
- reduced bone mineral density that may result
in osteoporosis (1)
Toxic symptoms can also arise after
consuming very large amounts of preformed vitamin A over a short period
of time.
Signs of acute toxicity include nausea
and vomiting, headache, dizziness, blurred vision, and muscular uncoordination
(1, 7-9, 41, 42).
Although hypervitaminosis A can
occur when very large amounts of liver are regularly consumed, most cases
of vitamin A toxicity result from an excess intake of vitamin A in supplements.
The Institute of Medicine has established
Daily Tolerable Upper Levels (UL) of intake for vitamin A from supplements
that apply to healthy populations (1). The UL was established to help
prevent the risk of vitamin A toxicity. The risk of adverse health effects
increases at intakes greater than the UL. The UL does not apply to malnourished
individuals receiving vitamin A either periodically or through fortification
programs as a means of preventing deficiency. It also does not apply to
individuals being treated with vitamin A by medical doctors for diseases
such as retinitis pigmentosa.
Table
3: Tolerable Upper Intake Levels (UL) for preformed vitamin A in micrograms
(ug) and International Units (IU)
for infants, children, and adults (1)
Retinoids are compounds that are
chemically similar to vitamin A.
Over the past 15 years, synthetic retinoids
have been prescribed for acne, psoriasis, and other skin disorders (43).
Isotretinoin (Roaccutane® or Accutane®) is considered an effective
anti-acne therapy.
At very high doses, however, it can be
toxic, which is why this medication is usually saved for the most severe
forms of acne (44-46). The most serious consequence of this medication
is birth defects. It is extremely important for
sexually active females who may become pregnant and who take these medications
to use an effective method of birth control. Women of childbearing age
who take these medications are advised to undergo monthly pregnancy tests
to make sure they are not pregnant.
What
is the health risk of too many carotenoids?
Nutrient toxicity traditionally refers to adverse health effects from
a high intake of a particular vitamin or mineral. For example, large amounts
of active, or preformed, vitamin A (naturally found in animal foods such
as liver but also available in dietary supplements) can cause birth defects.
Provitamin A carotenoids such
as beta-carotene are generally considered safe because they are not traditionally
associated with specific adverse health effects.
The conversion of provitamin A carotenoids
to vitamin A decreases when body stores are full, which naturally limits
further increases in storage levels. A high intake of provitamin A carotenoids
can turn the skin yellow, but this is not considered dangerous to health.
Recent clinical trials that associated
beta-carotene supplements with a greater incidence of lung cancer and
death in current smokers raised concern about the effects of beta-carotene
supplements on long-term health. However, conflicting studies make it
difficult to interpret the health risk.
For example, the Physicians’ Health
Study compared the effects of taking 50 mg beta-carotene every other day
to a placebo (sugar pill) in over 22,000 male physicians and found no
adverse health effects (47). Also, a trial that tested the ability of
four different nutrient combinations to inhibit the development of esophageal
and gastric cancers in 30,000 men and women in China suggested that after
5 years those participants who took a combination of beta-carotene, selenium
and vitamin E had a 13% reduction in cancer deaths (48).
One point to consider is that there may
be a relationship between alcohol and beta-carotene because “only
those men who consumed more than 11 g per day of alcohol (approximately
one drink per day) showed an adverse response to B-carotene supplementation”
in the lung cancer trial (1).
The Institute of Medicine did not set
a Tolerable Upper Intake Level (UL) for carotene or carotenoids. Instead,
they concluded that beta-carotene supplements are not advisable for the
general population. As stated earlier, however, they may be appropriate
as a provitamin A source or for the prevention of vitamin A deficiency
in specific populations (1).
Selected
Food Sources of Vitamin A
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” (49). The
following tables list a variety of dietary sources of vitamin A and provitamin
A carotenoids.
As the tables show, liver, eggs
and whole milk are good animal sources of vitamin A. Many orange fruits
and green vegetables are good sources of provitamin A carotenoids. Including
these foods in your daily diet will help you meet your daily need for
vitamin A.
In addition, food manufacturers fortify
a wide range of products with vitamin A. Breakfast cereals, pastries,
breads, crackers, cereal grain bars and other foods may be fortified with
10% to 15% of the Daily Value (DV) for vitamin A. If you want more information
about building a healthful diet, refer to the Dietary
Guidelines for Americans (49) and
the Food
Guide Pyramid (50).
Table
4: Selected Animal Sources of Vitamin A (18)
Animal sources of vitamin
A provide the best aborbed form of this vitamin
