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.
