|
[The
cancer process | Symptoms of cancer
| Causes of cancer | Cancer
treatment]
[Cancer prevention | Diet
and cancer prevention | Foods to avoid
| Foods to include]
[Vitamins, minerals & cancer
| Antioxidant supplements | Other
nutrients | References]
Antioxidant
Supplements
[Vitamin A | Carotenes
| Vitamin C | Vitamin
E | Selenium | Suggested
doses | Vitamin D]
[Folic acid |
Vitamin K | Calcium | Copper
| Flouride | Iodine
| Iron | Manganese]
[Molybdenum | Zinc
| Vanadium]
The supplements most
widely used to protect against cancer are the antioxidants. There have
been several studies showing that high doses of antioxidants are beneficial,
particularly in those who are deficient. A five-year study of almost 30,000
adults in Linxian, China found a 13 per cent reduction in cancer death
rates in those given vitamin E, selenium and beta carotene supplements.36
Several studies suggest that the most beneficial effects are seen when
antioxidants are given in combination rather than alone. Antioxidants
protect each other from damage and interact in many body functions.
Vitamin
A
[Lung cancer | Leukoplakia]
Lung
Cancer
Vitamin A supplements have been used to prevent cancer recurrence in
smokers who had undergone surgery for lung cancer. In a 1993 study,
researchers gave daily doses of 90 000 mcg RE (300,000 IU) to 307 patients
took for one year. After a follow-up period of 46 months, the number
of patients with either recurrence or new tumors was 56 (37 per cent)
in the vitamin A group and 75 (48 per cent) in the control group. Eighteen
patients in the treated group developed a second primary tumor, and
29 patients in the control group developed 33 second primary tumors.37
Other studies support
the use of vitamin A as a cancer preventive in those at high risk of
disease. In a 1998 study done in Western Australia, 1024 blue asbestos
workers known to be at high risk of diseases such as mesothelioma and
lung cancer, were enrolled in a cancer prevention program using vitamin
A. Half the subjects given 30 mg per day of beta carotene and the other
half 7,500 mcg RE (25,000 IU). The workers were followed up for a five-year
period. Four cases of lung cancer and three cases of mesothelioma were
observed in those in the vitamin A group, and six cases of lung cancer
and 12 cases of mesothelioma in the beta carotene group. In the retinol
group, there was also a significantly lower rate of death from all causes.38
When the researchers compared these results with those workers who had
not taken part in the study, they found that those taking part in the
study had significantly lower death rates than non-participants.39
Leukoplakia
Vitamin A has also been shown to exert protective effects against leukoplakia,
a pre-cancerous change in mucous membranes. It often occurs in the mouth
and throat and is related to smoking. In a study done in 1997, researchers
tested the effects of the retinyl palmitate form of vitamin A on leukoplakia
of the larynx. The treatment period was five weeks and the doses used
ranged from 90 000 mcg RE per day (300,000 IU) to 270,000 mcg (1,500,000
IU) per day. Complete remission was observed in 15 out of 20 patients
and partial response was seen in the remaining five patients.40
Carotenes
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. In 1996, these
studies reported results which received wide publicity. Results from the
ATBC study showed an 18 per cent increase in lung cancer deaths in men
who took daily supplements of 20 mg of beta carotene.41
The CARET study, which
involved 18,000 smokers and people who had been exposed to asbestos was
stopped 21 months early due to 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.42 Results from the 12-year
Physicians Health Study suggest that beta carotene supplements have no
effect on the risk of cancer.43 Further
analyses of these results support the suggestion that beta carotene is
susceptible to oxidative damage by alcohol and the gases in cigarette
smoke.44 Other antioxidants, such
as vitamins C and E may help to exert protective effects against this
damage. In smokers, dietary vitamin C supplementation should accompany
beta carotene supplementation.45
(See page 55 for more information.)
Vitamin
C
[Stomach cancer
| Colon cancer | Vitamin
C and cancer treatment]
Vitamin C supplements
may have a part to play in cancer prevention.
Stomach
Cancer
Vitamin C supplements may be useful in helping to prevent stomach cancer.
In a 1996 study, researchers gave 32 patients 500 mg of vitamin C twice
daily for two weeks. Levels in gastric juices and gut tissues were increased,
raising the possibility of increased protection against free radicals.46
A 1997 Japanese
study suggests that vitamin C may inhibit the growth of Helicobacter
pylori, a stomach bacterium that increases the risk of ulcers and stomach
cancer. Vitamin C-rich diets have been found to decrease the risk of
stomach cancer. This has been attributed to the antioxidant ability
of vitamin C. However, vitamin E, which is also an antioxidant, does
not inhibit the growth of Helicobacter pylori. This suggests that vitamin
C may exert its protective effects through a biochemical mechanism.
This research suggests the possibility of a safe, side effect-free alternative
to antibiotics for the treatment of ulcers.47
Colon
Cancer
Vitamin C supplements may also help to prevent colon cancer. In a 1992
study, twenty patients with colorectal cancer were given vitamins A,
C, and E for six months and 21 patients with adenomas received placebo.
The results showed that supplementation with vitamins A, C, and E was
effective in reducing pre-cancerous abnormalities.48
Vitamin C supplements may also be beneficial in the treatment of prostate
cancer.49
Vitamin
C and Cancer Treatment
Controversy surrounds the use of vitamin C in the treatment
of cancer. The Nobel Prize winner, Linus Pauling and his colleagues
have used vitamin C to improve survival times in cancer patients but
these results have not been repeated in other studies. Vitamin C may
also benefit cancer patients who are undergoing radiation treatment
by enabling them to withstand greater doses of radiation with fewer
side effects.50
Vitamin
E
[Prostate cancer]
Vitamin E supplements,
especially when combined with selenium, have shown beneficial effects
in the prevention of certain types of cancer, including breast cancer.
Analysis of results from a 1996 US National Institute on Aging study showed
a 22 per cent decrease in the risk of death from cancer in those taking
vitamin E supplements.51
Prostate
cancer
According to more results from the ATBC study published in 1998 in the
Journal of the National Cancer Institute, vitamin E reduces the risk
of prostate cancer among smokers. Researchers studied the effects of
50 mg (75 IU) in Finnish men and the results showed a 32 per cent decrease
in the incidence of prostate cancer and a 41 per cent decrease in prostate
cancer deaths among the men taking vitamin E, compared with those who
took no vitamin E.52
Selenium
Recent large scale studies in Linxian, China found reduced
risk of cancer when selenium supplements were given to those living in
selenium-deficient areas.53 Other
studies have shown that selenium supplements protect against some types
of cancer such as rectal, ovarian, colon, lung and cervical cancers. However,
there are also studies, including the Harvard Nurses Health Study which
do not show a protective role for selenium against cancers at any major
site. Laboratory studies have shown that selenium can slow tumor cell
growth.
Results of a 1996
study showed that selenium supplements were associated with a 50 per cent
reduction in deaths from cancer. Researchers at the Arizona Cancer Center
set out to test the effectiveness of selenium supplements on the prevention
of skin cancer in over 1300 patients. Participants received a placebo
or 200 mcg selenium per day over a period of 4.5 years and a total follow-up
of 6.4 years. While the results did not show any reduction in skin cancer
risk, the selenium group had a 37 per cent reduction in cancer incidence
and a 50 per cent reduction in cancer mortality. The effects appeared
strongest for prostate (63 per cent lower risk), colorectal (58 per cent
lower risk) and lung (53 per cent lower risk) cancers.54
Suggested
Doses of Supplements
The doses of antioxidants used in cancer prevention trials
are, in many cases, higher than those which could be obtained from the
diet. Many nutrition experts recommend taking high daily doses of supplements
to help prevent cancer. This may be particularly important in anyone who
has a family history of disease or is often exposed to risk factors.
Suggested doses are:
- Beta carotene 10
to 30 mg
- Vitamin E 200
to 800 IU (134 to 536 mg alpha TE)
- Vitamin C 1000-2000
mg Selenium 200 mcg
Vitamin
D
[Colorectal cancer
| Prostate cancer]
Vitamin D is involved
in normal cell growth and maturation, and so may play a part in cancer
prevention. Laboratory experiments show that vitamin D can inhibit the
growth of human prostate cancer55
and breast cancer cells.56 Lung
cancer and pancreatic cancer57
cells may also be susceptible to the effects of vitamin D. Sunlight also
seems to be protective against several types of cancer including ovarian,58
breast and prostate cancers; and this effect may be mediated by vitamin
D levels. Synthetic vitamin D-type compounds are being investigated for
their potential as anticancer drugs.
Colorectal
Cancer
In a 1996 study, researchers conducted a population-based case-control
study to examine the relationship between vitamin D intake and disease
among 352 people with colon cancer, 217 people with rectal cancer and
512 healthy people in Stockholm, Sweden. The researchers used questionnaires
to assess the vitamin D intake for the preceding five years. The results
showed that those with the highest vitamin D intakes were around half
as likely to get cancers of the colon or rectum than those with the
lowest intakes.59
Results from the
Harvard Nurses Health Study published in 1996 suggest a link between
vitamin D and colorectal cancer. The study involved 89,448 female nurses
and covered the time period from 1980 to 1992 during which 501 cases
of colorectal cancer were documented. The results showed a link between
intake of total vitamin D and risk of colorectal cancer.60
Prostate
Cancer
In a study published in 1996, researchers in a Boston hospital collected
blood plasma samples from 14,916 participants in the Physicians' Health
Study and measured vitamin D levels. Their analysis included 232 cases
diagnosed up to 1992 and 414 age-matched control participants. Their
results showed a slightly reduced risk of prostate cancer in those with
high vitamin D levels.61
Genes affecting
the way a man's body utilizes vitamin D could affect his risk of prostate
cancer. A 1996 National Institute of Environmental Health Sciences study
found that men with a particular type of vitamin D receptor gene are
less likely than others to develop the type of prostate cancer that
requires surgery. Researchers looked at the receptor genes in 108 cancer
patients and 170 men without cancer. The results showed that 22 per
cent of cancer patients had a particular gene while only eight per cent
of the cancer-free men did. These findings support the theory that vitamin
D plays an important role in prostate cancer development.62
Folic
Acid
[Colorectal cancer
| Cervical dysplasia | Supplements]
Those with diets low
in folic acid may have a higher risk of cancer than those who eat large
amounts, particularly cancers of the cervix, lung and colon. Folic acid
is vital for the maintenance of the genetic code and regulation of cell
division in both healthy and tumor tissues. Folic acid deficiency leads
to changes similar to those seen in cancer and may affect the repair of
DNA and increase chromosome fragility. It may also diminish the ability
of the immune system to fight cancer cells and viruses. Deficiency has
been shown to affect a gene involved in suppressing tumor formation.63
Colorectal
Cancer
Results from the Alpha-Tocopherol Beta carotene Study published in 1996
suggest a relationship between folate status and colorectal cancer.
The study involved male smokers aged from 50 to 69. The researchers
measured folate levels in 144 cases of colorectal cancer and 276 healthy
people. Those with higher dietary folate intakes had a reduced risk
of colon cancer. Men with a high alcohol, low folate, low protein diet
were at higher risk for colon cancer than men who consumed a low alcohol,
high folate, high protein diet.64
Cervical
Dysplasia
Low blood levels of folic acid may increase the risk of cervical dysplasia
(pre-cancerous changes in the cells lining the cervix), possibly by
enhancing the effect of other risk factors. Researchers from the University
of Alabama investigated the links between folate deficiency and cervical
dysplasia in 294 women with the disorder and 170 healthy women. They
also assessed the impact of factors such as smoking, oral contraceptive
use, human papillomavirus (HPV) infection, and number of sexual partners.
The results showed that at low folate levels the risk of dysplasia caused
by HPV infection was increased.65
Supplements
Folic acid supplements can help to reduce the risk of cancerous changes
in several areas such as the cervix, lung and gastrointestinal tract.
In a 1997 study, researchers at the Cleveland Clinic investigated the
links between folate supplements and cancerous changes in 98 patients
with ulcerative colitis. Patients taking the supplements had a 30 per
cent lower risk of developing cancerous changes in the bowel. The lower
the folate levels the more advanced the degree of cancerous changes
in the cells.66 In a 1997 Italian
study researchers also studied the effects of folate supplements on
pre-cancerous cell changes in ulcerative colitis. The results showed
that folate reduced these changes.67
Folic acid may also help to prevent the pre-cancerous changes in lung
tissue caused by smoking.68
Folic acid supplementation
may protect abnormal cells from becoming cancerous and may reverse cervical
dysplasia in some cases. A 1996 study done at the University of Alabama
suggests that supplements may be useful in preventing the initial changes
but do not appear to affect the progress of established disease.69
Some researchers have found a higher risk of abnormalities in cervical
tissue in women using oral contraceptives and suggest that folic acid
supplements are beneficial in preventing cervical dysplasia in these
women.70
Vitamin
K
Vitamin K injections are often given to babies after birth
to reduce the risk of internal bleeding. In the early 1990s, researchers
reported a possible increase in the risk of childhood cancers in those
who were given these injections. However, the results of studies are inconclusive.
This link was examined in four studies published in the British Medical
Journal in 1998. The results of two of the four studies suggest that there
is no association between vitamin K injections and cancer; one could not
exclude the possibility; and the fourth suggested a possible increase
in the risk of leukemia.
Calcium
High intakes of calcium-containing foods are linked to
a lower risk of developing colon cancer, although the latest research
suggests that the protective effect is not very marked. Calcium may exert
its protective effects by binding to cancer-causing fats and bile acids
in the intestine and normalizing the growth of cells in the intestinal
wall. Low calcium intake may
also increase the risk of breast and cervical cancers.
Results from the Health
Professionals Study, involving almost 48,000 men aged from 40 to 75 also
suggest that a higher intake of calcium from foods and supplements slightly
reduces the risk of colon cancer.71
Data from the Nurses Health Study which involved over 89,000 nurses also
showed a small reduced risk.72
Results from the Iowa Women's Health Study published in 1998 suggest that
calcium can decrease the risk of rectal cancer. Researchers analyzed information
from 34,702 postmenopausal women who responded to a mailed survey in 1986.
After nine years of follow-up, 144 rectal cancer cases were identified.
The results showed that high total calcium intake reduced the risk of
rectal cancer.73 Other results
from this study show a reduced risk of colon cancer in women with high
intakes of calcium and vitamin D.
Copper
Copper may act to prevent cancer. Animal studies have shown
that copper has a protective role and this may be due to its antioxidant
properties as part of copper-zinc superoxide dismutase.
Flouride
Some evidence suggests that water fluoridation may be linked
to some types of cancer, although this is controversial. A study published
in 1996 reported on the relationship between fluoride concentration in
drinking water and deaths from uterine cancer in Okinawa, Japan. Fluoride
was added to the water supplies in the region in the period from 1945
to 1972. The results showed significant links between the time of water
fluoridation and deaths from uterine cancer.74
Iodine
Hypothyroidism and iodine deficiency are associated with
a higher incidence of breast cancer.
Iron
In some population studies, high iron levels have been
associated with an increased risk of throat and gastrointestinal cancers
while others have not shown links.75
Results from a study assessing the links between body iron stores and
cancer in 3,287 men and 5,269 women participating in the first National
Health and Nutrition Examination Survey found an increased risk with high
iron levels.76 Some experts believe
that the findings of increased risk are due to causes such as defects
in iron metabolism, rather than diet alone. Some studies have shown that
iron can inhibit tumor development while others have shown that it might
enhance it. Iron may increase the risk of cancer through its effect on
free radical formation.
Manganese
A form of the antioxidant enzyme, superoxide dismutase,
contains manganese. Proper function of this enzyme helps protect against
free radical damage which can cause cancer.
Molybdenum
Population studies show that people living in areas where
the soil is molybdenum- deficient have been found to have an increased
risk of stomach and esophageal cancers.3
This may be because molybdenum-deficient plants are unable to metabolize
carcinogenic compounds known as nitrosamines, which are present in high
levels in food.
Zinc
Zinc supplements have been used to improve taste perception
in people taking medications which reduce taste sensation, and in cancer
patients undergoing radiation therapy.77
This can be valuable in helping to maintain normal weight and nutrient
intake during treatment.
Vanadium
Some evidence suggests that vanadium may limit the initiation
and frequency of tumors in animals. Its role in humans is unclear.
Other
Nutrients
[Essential fatty acids
| Omega-3 fatty acid supplements | Gamma-linolenic
acid]
Essential
Fatty Acids
The levels and types of fat in the diet seem to influence cancer risk,
and disease progression. High intakes of omega-6 polyunsaturated fatty
acids seem to increase the risk of cancers while high intakes of omega-3
fatty acids may provide protection. Animal studies have demonstrated
that polyunsaturated omega-6 fatty acids stimulate carcinogenesis and
tumor growth and metastasis, whereas long-chain omega-3 fatty acids
inhibit these processes. Reducing total fat intake and increasing the
ratio of omega-3 to omega-6 fatty acids in the diet may be particularly
useful for groups at a relatively high risk for breast or prostate cancer,
and may also be useful after surgery to help prevent disease recurrence.78
Dietary intake of
essential fatty acids may play a role in prostate cancer cell proliferation.
Epidemiological studies have demonstrated that men whose dietary intake
is high in omega-6 fatty acids have a higher incidence of clinical prostate
cancer.79 Diets high in omega-3
fatty acids may have protective effects. Other research suggests that
omega-3 fatty acids inhibit breast cancer and that the degree of this
inhibition depends on background levels of omega-6 fatty acids. Results
from the European Community Multicenter Study on Antioxidants, Myocardial
Infarction, and Cancer (EURAMIC) study published in 1998, suggest that
an increase in the ratio of omega-3 fatty acids to total omega-6 fatty
acids in fat tissue decreases the risk of breast cancer. In this study,
total levels of omega-3 or omega-6 fat were not consistently associated
with breast cancer.80
Population and laboratory
studies suggest that omega-3 fatty acids may help to prevent and inhibit
colon cancer. In a study published in 1995, death rates for colorectal
cancer in 24 European countries were correlated with current fish and
fish oil consumption, and with consumption ten and 23 years previously.
In men, there was a reduced risk of death from colorectal cancer and
current intake of fish, a weaker link with fish consumption ten years
earlier, and none with consumption 23 years earlier. The researchers
concluded that fish consumption is associated with protection against
the later stages of colorectal cancer, but not with the early initiation
stages.81
Omega-3
Fatty Acid Supplements
Omega-3 fatty acids may be beneficial in preventing as treating cancer
as they seem to exert tumor-suppressive effects.82
In a study published in 1997, Norwegian researchers studied the relationship
between incidence of lung cancer and intake of dietary fats, high fat
foods, fish, and fish products in 25,956 men and 25,496 women aged from
16 to 56. During the follow-up period, 153 cases of lung cancer were
identified. The results showed that those who took cod liver oil supplements
had around half the risk of those who did not.83
Essential fatty
acids may also boost immune function which may help in cancer prevention
and treatment. Studies on the immune T cells in cancer patients taking
fish oil capsules suggest that omega-3 fatty acids bring about beneficial
changes. In a Greek study published in 1998, researchers investigated
the effect of dietary omega-3 polyunsaturated fatty acids and vitamin
E on the immune status and survival in both well-nourished and malnourished
cancer patients. The study involved 60 patients with solid tumors who
were randomized to receive dietary supplementation with either fish
oil (18 g of omega-3 fatty acids) or placebo daily. The authors measured
various indicators of immune function. The results showed that omega-3
fatty acids had a significant immune- enhancing effect and seemed to
prolong the survival of malnourished patients.84
Gamma-linolenic
Acid
Gamma-linolenic acid has been shown to be effective in killing cancer
cells and is well-established as a topical treatment for some types
of cancer, including bladder cancer.85
It has also been shown to kill various other types of cancer cells.86
There are many other
compounds under investigation for their anticancer potential, including
soybeans, tea and garlic. Bioflavonoids, colored pigments from fruit
and vegetables, may also have anticancer properties. Cruciferous vegetables
such as broccoli, cauliflower and cabbage also contain anticancer compounds.
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