Also Known As: Retinol (A)
Vitamin A: What does it do?
is a fat-soluble vitamin with four major functions in the body: (1) It helps
cells reproduce normally—a process called differentiation (cells that have not properly
differentiated are more likely to undergo pre-cancerous changes). (2) It is required for
vision; vitamin A maintains healthy cells in various structures of the eye and is required for
the transduction of light into nerve signals in the retina. (3) It is required for normal
growth and development of the embryo and fetus, influencing genes that determine the
sequential development of organs in embryonic development. (4) It may be required for normal
reproductive function, with influences on the function and development of sperm, ovaries and
placenta.
For some people, water-soluble forms of vitamin A supplements appear to be better absorbed
than fat-soluble vitamin A.
Who is likely to be deficient of Vitamin A?
People who limit their consumption of
liver, dairy foods, and beta-carotene-containing vegetables can develop a vitamin A deficiency.
Extremely low birth weight babies (2.2 pounds or less) are at high risk of being born with a
deficiency, and vitamin A shots given to these infants have been reported in double-blind
research to reduce the risk of lung disease.1 The earliest deficiency sign is poor
night vision. Deficiency symptoms can also
include dry skin, increased risk of
infections, and metaplasia (a precancerous condition). Severe deficiencies causing
blindness are extremely rare in Western societies.
Less severe deficiencies are more likely to occur with a variety of conditions causing malabsorption. A high incidence of vitamin A
deficiency in people infected with HIV has
also been reported. People with hypothyroidism
have an impaired ability to convert
beta-carotene to vitamin A.2 3 For this reason, some doctors suggest
taking supplemental vitamin A (perhaps 5,000–10,000 IU per day) if they are not
consuming adequate amounts in their diet.
Very old people with type 2 diabetes have
shown a significant age-related decline in blood levels of vitamin A, irrespective of their
dietary intake.4
How much Vitamin A is usually taken?
For most people, up to 25,000 IU (7,500 mcg) of vitamin A per day is considered safe.
However, people over age 65 and those with liver disease should probably not supplement with
more than 15,000 IU per day, unless supervised by a doctor. In women who could become pregnant, the maximum safe intake is being
re-evaluated. However, less than 10,000 IU (3,000 mcg) per day is generally accepted as safe.
There is concern that larger intakes could cause
birth defects. Whether the average person would benefit from vitamin A supplementation
remains unclear.
Are there any side effects or interactions with Vitamin A?
Since a 1995 report from the New England Journal of Medicine,5 women
who are or could become pregnant have been
told by doctors to take less than 10,000 IU (3,000 mcg) per day of vitamin A to avoid the risk
of birth defect. A recent report studied
several hundred women exposed to 10,000–300,000 IU (median exposure of 50,000 IU) per
day.6 Three major malformations occurred in this study, but all could have happened
in the absence of vitamin A supplementation. Surprisingly, no congenital malformations
happened in any of the 120 infants exposed to maternal intakes of vitamin A that exceeded
50,000 IU per day. In fact, the high-exposure group had a 50% decreased risk for malformations
compared with infants not exposed to vitamin A. The authors noted that some previous studies
found no link between vitamin A and birth defects, and argued the studies that did find such a
link suffered from various weaknesses. A closer look at the recent study reveals a 32% higher
than expected risk of birth defects in infants exposed to 10,000–40,000 IU of vitamin A
per day, but paradoxically a 37% decreased risk for those exposed to even higher levels. This
suggests that both “higher” and “lower” risks may have been due to
chance.
Excessive dietary intake of vitamin A has been associated with birth defects in humans in
fewer than 20 reported cases over the past 30 years.7 8 Presently, the
level at which vitamin A supplementation may cause birth defects is not known, though combined
human and animal data suggest that 30,000 IU per day should be considered safe.9
Women who are or who could become pregnant should consult with a doctor before supplementing
with more than 10,000 IU per day.
supplements can both help and hurt children. Many people have heard that vitamin
A supplements support immune function and
prevent infections. This is true under some
circumstances. However, vitamin A can also increase the risk of infections, according
to the findings of a double-blind trial.10 In a study of African children between
six months and five years old, a 44% reduction in the risk of severe diarrhea was seen in those children given four
100,000–200,000 IU applications of vitamin A (the lower amount for those less than a
year old) during an eight-month period. On further investigation, the researchers discovered
that the reduction in diarrhea occurred only in children who were very malnourished. For
children who were not starving, vitamin A supplementation actually increased the risk
of diarrhea compared with the placebo group. The vitamin A-supplemented children also had a
67% increased risk of coughing and rapid breathing, signs of further lung infection,
although this problem did not appear in children infected with AIDS. These findings should be
of concern to American parents, whose children are not usually infected with AIDS or severely
malnourished. Such relatively healthy children fared poorly in the African trial in terms of
both the risk of diarrhea and the risk of continued lung problems. provided no
benefit to the well-nourished kids. Therefore, it makes sense to not give vitamin A
supplements to children unless there is a special reason to do so, such as the presence of a
condition causing malabsorption (e.g., celiac disease).
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The information presented in Healthnotes is for informational
purposes only. It is based on scientific studies (human, animal, or in
vitro), clinical experience, or traditional usage as cited in each article.
The results reported may not necessarily occur in all individuals. For many of
the conditions discussed, treatment with prescription or over the counter
medication is also available. Consult your doctor, practitioner, and/or
pharmacist for any health problem and before using any supplements or before
making any changes in prescribed medications.
References:
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