Who may need extra vitamin A to prevent a deficiency?
Children with low body stores of vitamin A who have measles may require vitamin A supplementation, as discussed above. Also, individuals with chronic fat malabsorption also poorly absorb vitamin A. Fat malabsorption can occur with cystic fibrosis,
sprue, pancreatic disorders, and after stomach surgery. Healthy adults usually
have a one-year 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, can result in deficiency and may
require vitamin A supplementation (8). Children may only have enough stores
of vitamin A to last several weeks, so children with fat malabsorption may require
immediate vitamin A supplementation (8).
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 many types of cancer (1, 28). There is evidence that higher
intake of green and yellow vegetables or food sources of beta carotene and/or
vitamin A decreased the risk of lung cancer (29). A number of studies have tested
the role of beta-carotene supplements in cancer prevention(30). Unfortunately,
recent intervention studies have not supported a protective role for beta carotene
in cancer prevention. 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 (32). The IOM
states that "B-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 B-carotene as a provitamin A source for the prevention of
vitamin A deficiency in populations with inadequate vitamin A nutriture" (13).
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. Toxicity can result in dry, itchy skin, headache,
fatigue, hair loss, loss of appetite, vomiting, and liver damage. When toxic
symptoms arise suddenly, which can happen after consuming very large amounts
of vitamin A over a short period of time, signs of toxicity include dizziness,
blurred vision, and muscular uncoordination (6-8, 33, 34).
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. A generally recognized safe
upper limit of intake for vitamin A from diet and supplements is 1,600 to 2,000
RE (8,000 to 10,000 IU) per day (33, 35). The Institute of Medicine is currently
reviewing the scientific literature on vitamin A. They are considering revising
the RDAs and establishing an Upper Limit (UL) of safe intake for vitamin A.
Vitamin A toxicity also can cause severe birth defects.
Women of child-bearing age are advised to limit their total daily intake of
vitamin A (retinol) from foods and supplements combined to no more than 1,600
RE (8,000 IU) per day.
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 (36). 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 (37). 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 the
active form of 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 (38).
Recent clinical trials that suggested a greater incidence
of lung cancer and total mortality (death) in current smokers who supplemented
their diet with 20 mg of beta-carotene per day have raised concern about the
safety of beta-carotene supplements. 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 (39).
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
(40). 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 (13).
The Institute of Medicine did not set a Tolerable Upper
Intake Level (UL), the highest level of daily nutrient intake that is likely
to pose no risk of adverse health effects, for B-carotene or carotenoids. Instead,
they concluded that B-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
(13).
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