Vitamin A
- Cell differentiation, bone growth, immunity, tooth
development, reproduction, healthy skin, hair,
mucous membranes and vision. A fat-soluble vitamin
needed for the normal functioning of the eyes. It
also helps the body resist infection, keeps skin
cells moist, and is important in the functioning of
the nervous, reproductive and respiratory systems.
The best natural sources are liver—especially fish
liver and fish-liver oils—whole milk, cheese and
eggs. Vitamin A does not exist in plants. However,
leafy green and yellow vegetables and yellow fruits
contain a pigment called carotene, which the liver
can convert into Vitamin A. Among the best sources
of carotene are dandelion and turnip greens,
carrots, collards, cantaloupe, sweet potatoes and
apricots.
Beta carotene is one
of a group of substances called carotenoids
which are converted to vitamin A in the intestinal
wall and the liver, as the body requires. About 30
of the more than 600 carotenoids which have been
identified can be converted to vitamin A in the
human body. Beta carotene is the best known of the
carotenoids as it has high pro-vitamin A activity
and is abundant in many foods. Other carotenoids
include lutein, zeaxanthin, beta-cryptoxanthin,
lycopene and alpha carotene.
What it does in
the body
The beneficial effects of beta carotene are partly
due to its conversion to vitamin A but it also has
potent activity of its own. Beta carotene is an antioxidant.
One molecule of beta carotene can wipe out up to
1000 free radicals and may also prevent them from
forming. This antioxidant ability may underlie the
protective effect of beta carotene against disorders
such as heart disease and cancer.
IMMUNE
SYSTEM
Beta carotene has been shown to stimulate and
enhance many immune system processes. It increases
the numbers of immune cells such as B and T lymphocytes
and natural killer cells. T cells play a very
important role in determining immune status and are
produced by the thymus
gland, which is particularly sensitive to free
radical and oxidative damage. Beta carotene may
also protect macrophages,
cells which engulf and destroy foreign substances,
facilitate communication between immune cells and
make the stimulatory action of interferon
on the immune system more powerful.
CANCER
Research suggests that high levels of beta carotene
can protect against certain types of cancer. Many
population studies have shown that cancer victims
often have lower dietary and/or blood beta carotene
levels than healthy individuals. The evidence from
these studies is strongest for lung cancer and is
reasonably consistent for stomach cancer. Some
research also suggests an increased risk of breast,
prostate, colorectal, ovarian and cervical cancers
with low beta carotene levels. It has also been
found that women with low beta carotene levels in
their cervical tissues may be at increased risk of
cervical cancer even though their blood levels are
normal. Increasing intake of beta carotene may help
to overcome this "tissue specific"
deficiency.
The Western Electric
Company study, which looked at the intakes of
several nutrients among over 1500 men in the 1950s,
recently reported an association between dietary
beta carotene intake and decreased cancer risk.
Those men with higher beta carotene intakes had
lower rates of death from cancer and cardiovascular
disease.
People who get a lot
of vitamin A from plants, in the form of carotenoids
may be at less risk of developing cancer than people
who obtain vitamin A from animal food sources, which
suggests a protective role for beta carotene outside
its pro-vitamin A activity. Drinking alcohol and
smoking have been shown to decrease beta carotene
levels. Beta carotene may protect against damage to
cell membranes and DNA
thus preventing abnormal cell formation and may also
slow or halt the growth of tumors by enhancing
communication between cells.
HEART
DISEASE AND STROKE High dietary beta
carotene intake may be associated with a lower risk
of cardiovascular disease. As an antioxidant,
beta carotene has been shown to inhibit oxidative
damage to cholesterol. Several studies suggest that
beta carotene may protect against heart disease.
These include the Nurses Health Study which was
established in 1976. This study involves over
120,000 female US nurses and has found a lower risk
of cardiovascular disease in those with higher beta
carotene intakes.
Another US study
involving 1299 elderly people found that those in
the highest beta carotene intake group had almost
half the risk of death from cardiovascular disease
of those in the lowest intake group.
The Health
Professionals Follow-up Study looked at the risk of
cardiovascular disease in over 51 000 men aged
between 40 and 75. Results showed that the risk of
disease for those in the highest beta carotene
intake group was 30 per cent less than for those in
the lowest intake group.
WOUND HEALING
As beta carotene has antioxidant and
anti-inflammatory properties it may promote wound
healing.
EYES
Free radical damage is implicated in the formation
of cataracts and as an antioxidant beta carotene may
exert protective effects by reducing oxidative
damage. It may also act as a filter and protect
against light-induced damage to the fiber portion of
the lens. Beta carotene may also protect against
macular degeneration, a disease affecting the retina
to which older people are particularly susceptible.
Absorption
Carotenes need bile acids for absorption. Beta
carotene is absorbed into the wall of the small
intestine where some conversion to vitamin A takes
place. Only 40 to 60 per cent of beta carotene is
absorbed. Low stomach acid decreases the absorption
of beta carotene. Beta carotene may be stored in the
lung, liver, kidneys, skin and fat.
Deficiency
Diets low in beta carotene may reduce the
effectiveness of the immune system and lead to an
increased risk of cancer and heart disease.
Sources
Good sources of beta carotene include carrots, sweet
potatoes, pumpkin and other orange winter squashes,
cantaloupe, pink grapefruit, spinach, apricots,
broccoli, and most dark green leafy vegetables. The
more intense the green, yellow or orange color the
more beta carotene the vegetable or fruit contains.
Beta carotene is not destroyed by cooking which, in
fact, may make it easier to absorb.
Recommended
dietary allowances
There is no RDA for beta carotene. An intake of 6 mg
beta carotene is needed in order to meet the vitamin
A RDA of 1000 mcg RE. 1 RE is equivalent to 6 mcg
beta carotene. Some experts recommend a daily intake
of 10 to 30 mg. The RDA for vitamin A for women who
are breast-feeding increases from 800 mcg RE to 1300
mcg RE. This can be met by increasing the intake of
beta carotene rich foods.
Supplements
Beta carotene supplements are available in various
forms, including synthetic forms and those extracted
from algae and palm oil. Some studies suggest that
those extracted from palm oil are absorbed more
efficiently. Natural beta carotene may have greater
beneficial effects than synthetic forms.
Toxic effects
Unlike vitamin A, beta carotene is not toxic
in large amounts although it may turn the skin
yellow. There is the possibility of menstrual
problems with long term excessive intake.
Therapeutic uses
Beta carotene supplements have been used in cancer
and cardiovascular disease prevention trials
including the Finnish Alpha Tocopherol Beta Carotene
Cancer (ATBC) Prevention Study, the US Carotene and
Retinol Efficacy Trial (CARET) and the US Physicians
Health Study. These studies have recently reported
results which have received wide publicity.
ATBC STUDY
The ATBC Prevention group studied 29 000 men who
smoked and drank alcohol. The results showed an 18
per cent increase in lung cancer deaths and an 11
per cent increase in ischemic heart disease deaths
in men who took daily supplements of 20 mg beta
carotene.
CARET STUDY
In January 1996 the CARET study was stopped 21
months early. This study was examining the role of
beta carotene (30 mg daily) and retinol (7500 mcg RE
daily) supplementation in the prevention of cancer
and heart disease in over 18 000 smokers and
asbestos exposed individuals. The trial was stopped
halted when the results showed a 28 per cent
increased risk of lung cancer, a 26 per cent
increase in the risk of death from cardiovascular
disease and a 17 per cent increase in overall deaths
in the group receiving the supplements.
PHYSICIANS HEALTH
STUDY
This study examined the effect on over 22 000 male
doctors of 50 mg beta carotene taken every other day
for 12 years. The results suggest that beta carotene
has no effect, either positive or negative, on the
risk of cardiovascular disease or cancer. Analysis
of a subgroup of 333 men in the study with a prior
history of heart disease suggested that beta
carotene supplements reduced the risk of heart
attacks and death by a small amount. There are a
number of possible explanations for the adverse
effects of beta carotene supplements found in these
studies and for the failure of supplements to show
the protective effects suggested by epidemiological
studies.
Beta carotene is
susceptible to oxidative
damage from alcohol and the gases in cigarette smoke
which may lead to the formation of harmful
by-products. Beta carotene may be dependent on
protection from other antioxidants, such as vitamins
C and E to exert protective effects. An individual's
total dietary intake of antioxidants may therefore
need to be considered when assessing protection by
beta carotene.
Further analyses of
results from the ATBC trial suggest that smoking and
alcohol consumption may contribute to the adverse
effects of beta carotene. The adverse effects
appeared stronger in men who drank alcohol and in
those who smoked 20 cigarettes a day than in those
who smoked less. This is confirmed by the CARET
results which showed greater risk in current smokers
than former smokers and in those who drank alcohol.
Beta carotene exists
in over 270 possible forms and some research
suggests that the specific form chosen for use in
these clinical trials was not the most active agent
and that a mixture of various forms of beta
carotene, such as that which occurs naturally, has
the most beneficial effect. It is also possible that
the large dose of one particular form of beta
carotene competed with other, possibly more
beneficial forms at vital sites in the body.
The results of these
trials point to the importance of considering total
diet and a balanced mixture of nutrients when
studying protection against cancer risk. High blood
levels of carotene seem to predict lower risk and
these high blood levels of beta carotene may be
accompanied by high levels of other carotenoids
which may also play a vital part in cancer
protection. Both the ATBC and CARET studies found
that those with higher blood beta carotene levels on
entering the trials had a lower risk of lung cancer.
Double
blind placebo
controlled studies may be more useful for evaluating
a specific drug for one condition in one population
group and less suitable for investigating
multifactorial agents in complex, mixed population
studies. These studies do not invalidate hundreds of
other studies showing that diets high in fruits and
vegetables protect against a variety of diseases.
OTHER USES
Dutch researchers looking at the effect of foods
rich in beta carotene on memory impairment and
mental function have found protective effects. The
researchers at the University of Rotterdam studied
5182 people aged 55 to 95 from 1990 to 1993. They
found that those with intakes of less than 0.9
milligrams of beta carotene per day were almost
twice as likely to have impaired memory,
disorientation and problem solving difficulty as
those with intakes of 2.1 milligrams of beta
carotene.
In addition to
exerting protective effects against a wide range of
diseases, beta carotene may slow the rate of aging
in the skin and other organs by protecting against free
radical damage caused by smoking, pollution,
ultraviolet light and other chemicals. Beta carotene
is used to decrease light sensitivity reactions in
sufferers of the disease erythropoietic
protoporphyria.
Beta carotene
supplements have been shown to have beneficial
effects in cystic fibrosis by decreasing harmful lipid
peroxidation.
Fibrocystic breast disease, a painful cystic
swelling of the breast which affects 20 to 40 per
cent of premenopausal women may be helped by vitamin
A and beta carotene.
Interactions
The conversion of beta carotene to vitamin A depends
on vitamin C, zinc and thyroid hormones.
Diabetics and people with hypothyroidism or liver
disease have trouble converting beta carotene to
vitamin A and should not rely solely on beta
carotene to meet their vitamin A requirements. Large
doses of beta carotene may increase the requirements
for vitamin E. The function of beta carotene is
enhanced by the levels of the other antioxidants,
vitamin C, vitamin E and selenium.
Cautions
Recent research suggests that large doses of beta
carotene may increase the risk of cancer in those
who drink alcohol and smoke heavily. Vitamin C
supplements may be useful in protecting against the
damaging effects of large doses of beta carotene.
