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.
