Principal Proposed Treatments for Cancer Prevention (Reducing the Risk):
It is rather difficult to prove that taking a certain supplement will reduce
the chance of developing cancer. You really need enormous long-term,
double-blind studies in which some people are given the supplement while others
are given placebo. However, relatively few studies of this type have
For most supplements, the evidence that they help prevent cancer comes from
observational studies, which are much less reliable. Observational studies have
found that people who happen to take in high levels of certain vitamins in their
diets develop a lower incidence of specific cancers. However, in such studies it
is very difficult to rule out other factors that may play a role. For example,
individuals who take vitamins may also exercise more, or take better care of
themselves in other ways. Such \\\"confounding factors\\\" make the results of
observational studies less reliable.
Although this may sound like a theoretical issue, it has very practical
consequences. For example, based primarily on observational studies, hormone
replacement therapy was promoted as a heart-protective treatment for
post-menopausal women. However, when placebo-controlled studies were performed,
hormone replacement therapy proved to increase the risk of heart disease.
It is now thought that apparent benefits of hormone replacement therapy were due
to the fact that woman who used it belonged to a higher socioeconomic class than
those who did not use it. (For a variety of reasons, some of which are not
known, higher income is associated with improved health.)
Only two supplements have any evidence from double-blind trials to support their
potential usefulness for cancer prevention: vitamin E and selenium.
For all other supplements, supporting evidence is limited to observational
studies, as well as preliminary evidence from animal and test tube studies.
The results of observational trials have been mixed, but on balance, they
suggest that high intake of vitamin E is associated with reduced risk of many
forms of cancer, including stomach, mouth, throat, laryngeal, lung, liver and
prostate cancer. The evidence regarding colon cancer is mixed.
However, as noted above, the results of observational studies are unreliable as
guidelines to treatment. The results of double-blind, placebo-controlled studies
are far more persuasive in drawing conclusions about cause and effect.
One such trial provides evidence that taking vitamin E supplements can help
prevent prostate cancer. In this double-blind study of 29,133 smokers, those who
were given 50 mg of synthetic vitamin E (dl-alpha-tocopherol) daily for 5 to 8
years showed a 32% reduction in the incidence of prostate cancer and a 41% drop
in prostate cancer deaths.
Surprisingly, results were seen soon after the beginning of supplementation.
This was unexpected because prostate cancer grows very slowly. A cancer that
shows up today actually started to develop many years ago. The fact that vitamin
E almost immediately lowered the incidence of prostate cancer suggests that it
may somehow block one of the last the steps in the development of detectable
Intervention trials of vitamin E have not found benefit for preventing lung
For more information, including dosage and safety issues, see the full vitamin E
It has long been known that severe selenium deficiency increases the risk of
cancer. However, by itself, this does not prove that taking selenium supplements
will make a difference if you are not deficient in it.
One double-blind study did find that selenium supplements can dramatically
reduce the incidence of cancer. The results were so impressive that they caught
the researchers by surprise. The study was actually designed to detect
seleniums effects on skin cancer. It followed 1,312 individuals, half of whom
were given 200 mcg of selenium daily. People participating in the study were not
deficient in selenium, although their selenium levels fell toward the bottom of
the normal range. The participants were treated for an average of 2.8 years and
were followed for about 6 years. Although no significant effect on skin cancer
was found, the researchers were startled when the results showed that people
taking selenium had a 50% reduction in overall cancer deaths and significant
decreases in cancer of the lung (40%), colon (50%), and prostate (66%). The
findings were so remarkable that the researchers felt obliged to break the blind
and allow all the participants to take selenium.
While this evidence is very promising, it has one major flaw. The laws of
statistics tell us that when researchers start to deviate from the question
their research was designed to answer, the results may not be trustworthy. For
this reason, further research needs to be done to confirm that selenium actually
can help prevent these types of cancer.
For more information, including dosage and safety issues, see the full selenium
Evidence from observational studies suggests that garlic taken in the diet as
food may help prevent cancer, particularly cancer of the colon and stomach.
In one of the best of these studies, the Iowa Womens Study, women who ate
significant amounts of garlic were found to be about 30% less likely to develop
colon cancer. Similar results were seen in other observational studies performed
in China, Italy, and the United States.
We do not know for sure how garlic might work to prevent cancer. Like vitamin E,
whole garlic possesses antioxidant properties. Furthermore, various garlic
extracts have also been shown to suppress the known DNA-damaging activity of
several drugs and toxins.Finally,garlic contains high levels of selenium, which
is thought to reduce the risk of cancer (see the previous discussion under the
For more information, including dosage and safety issues, see the full garlic
The story of beta-carotene and cancer is full of contradictions. It starts in
the early 1980s, when the cumulative results of many studies suggested that
people who eat a lot of fruits and vegetables are significantly less likely to
get cancer. A close look at the data pointed to carotenes as the active
ingredients in fruits and vegetables. It appeared that a high intake of dietary
carotene might significantly reduce the risk of lung cancer, bladder cancer,
breast cancer, esophageal cancer, and stomach cancer.
However, as noted above, observational studies cannot prove cause and effect.
When researchers gave beta-carotene to study participants, the results have been
Most studies enrolled people in high-risk groups such as smokers, because it is
easier to see results when you look at people who are more likely to develop
cancer to begin with.
The anticancer bubble burst for beta-carotene in 1994 with the results of the
Alpha-Tocopherol, Beta-Carotene (ATBC) study. These results showed that
beta-carotene supplements did not prevent lung cancer, but actually increased
the risk of getting it by 18%. This trial had followed 29,133 male smokers in
Finland who took supplements of about 50 IU of vitamin E (alpha-tocopherol), 20
mg of beta-carotene (more than ten times the amount necessary to provide the
daily requirement of vitamin A), both, or placebo daily for 5 to 8 years. (In
contrast, vitamin E was found to reduce the risk of cancer, especially prostate
In January 1996, researchers monitoring the Beta-Carotene and Retinol Efficacy
Trial (CARET) confirmed the prior bad news with more of their own: The
beta-carotene group had 46% more cases of lung cancer deaths. This study
involved smokers, former smokers, and workers exposed to asbestos. Alarmed, the
National Cancer Institute ended the $42 million CARET trial 21 months before it
was planned to end.
At about the same time, the 12-year Physicians Health Study of 22,000 male
physicians was finding that 50 mg of beta-carotene (about 25 times the amount
necessary to provide the daily requirement of vitamin A) taken every other day
had no effect—good or bad—on the risk of cancer or heart disease. In this study,
11% of the participants were smokers and 39% were ex-smokers.
Similarly, another study of beta-carotene supplements failed to find any effect
on the risk of cancer in women.
What is the explanation for this discrepancy?
One possibility is that beta-carotene alone is not effective. The other
carotenes found in fruits and vegetables may be more important for preventing
cancer than beta-carotene. One researcher has suggested that taking
beta-carotene supplements actually depletes the body of other beneficial
It is also possible that intake of carotenes as such are unrelated to cancer,
and that some unrelated factor common to individuals with a high carotene diet
is the cause of the benefits seen in observational trials.
Lycopene, a carotenoid like beta-carotene, is found in high levels in
tomatoes and pink grapefruit. Lycopene appears to exhibit about twice the
antioxidant activity of beta-carotene and may be more helpful for preventing
cancer than the better-known vitamin.
In one observational study, elderly Americans consuming a diet high in tomatoes
showed a 50% reduced incidence of cancer. Men and women who ate at least seven
servings of tomatoes weekly developed less stomach and colorectal cancers
compared to those who ate only two servings weekly.
In another study, 47,894 men were followed for 4 years in an observational study
looking for influences on prostate cancer. Their diets were evaluated on the
basis of how often they ate fruits, vegetables, and foods containing fruits and
vegetables. High levels of tomatoes, tomato sauce, and pizza in the diet were
strongly connected to reduced incidence of prostate cancer. After an evaluation
of known nutritional factors in these foods as compared to other foods, lycopene
appeared to be the common denominator. Additional impetus has been given to this
idea by the discovery of lycopene in reasonably high levels in the human
prostate, as well as evidence that men with higher lycopene levels in the blood
have a lower risk of
Similar evidence suggests that foods containing lycopene might help prevent
other forms of cancer as well, including lung, colon, and breast cancer.
Cooked tomatoes appear to be more bioavailable (more readily used by the body)
than raw tomatoes, especially when the tomatoes are cooked in oil. Tomato juice
does not seem to be helpful.
Several observational studies have found a strong association between high
dietary vitamin C intake and a reduced incidence of stomach cancer. It has been
proposed that vitamin C may prevent the formation of carcinogenic substances
known as N-nitroso compounds in the stomach.
Observational studies have also linked higher vitamin C in the diet with reduced
risk of colon, esophageal, laryngeal, bladder, cervical, rectal, breast, and
perhaps lung cancer. However, dietary vitamin C intake does not appear to be
associated with reduced rate of prostate cancer.
One study found that vitamin C supplementation at 500 mg or more daily was
associated with a lower incidence of bladder cancer. However, another study
found no association. Similarly, in another observational study, 500 mg or more
of vitamin C daily over a period of 6 years was not associated with reduced
incidence of breast cancer. Another study found similar results.
Both green tea and black tea come from the tea plant called Camellia sinensis,
which has been cultivated in China for centuries. The key difference between the
two is in preparation. For black tea, the leaves are allowed to oxidize, a
process believed to lessen the potency of therapeutic compounds known as
polyphenols. Green tea is made by lightly steaming the freshly cut leaf, a
process that prevents oxidation and possibly preserves more of the therapeutic
Laboratory and animal studies suggest that tea consumption protects against
cancers of the stomach, lung, esophagus, duodenum, pancreas, liver, breast, and
colon .A 1994 study of skin cancer in mice found that both black and green teas,
even decaffeinated versions, inhibited skin cancer in mice exposed to
ultraviolet light and other carcinogens. After 31 weeks, mice given the teas
brewed at the same concentration humans drink had 72 to 93% fewer skin tumors
than mice given only water.
However, results from observational studies in humans have not been so
clear-cut—some have found evidence of a protective effect, and others have not.
One study followed 8,552 Japanese adults for 9 years. Women who drank more than
10 cups daily had a delay in the onset of cancer and also a 43% lower total rate
of cancer occurrence. Males had a 32% lower cancer incidence, but this finding
was not statistically significant.
A study in Shanghai, China, found that those who drank green tea had
significantly less risk of developing cancers of the rectum and pancreas than
those who did not. No significant association with colon cancer incidence was
found. A total of 3,818 residents aged 30 to 74 were included in the population
study. For men, those who drank the most tea had a 28% lower incidence of rectal
cancer and a 37% lower incidence of pancreatic cancer compared to those who did
not drink tea regularly. For women, the respective differences in cancer
frequency were even greater: 43% and 47%.
Another study in Shanghai found similar associations for stomach cancer. Green
tea drinkers were 29% less likely to get stomach cancer than nondrinkers, with
those drinking the most tea having the least risk. Interestingly, the risk of
stomach cancer did not depend on the persons age at which he or she started
drinking green tea. Researchers suggested that green tea may disrupt the cancer
process at both the intermediate and the late stage.
However in an observational study of 26,311 Japanese individuals, no reduction
in stomach cancer rates were seen. Negative results were also seen in a study
conducted in Hawaii.
The active ingredients in green tea (presuming it does offer some benefit) are
believed to be polyphenols, especially one known as epigallocatechin gallate (EGCG).
Like vitamin C, polyphenols may block the formation of nitrosamines and other
cancer-causing compounds and may trap or detoxify carcinogens. Preliminary
experimental studies suggest that EGCG may help prevent skin cancer if it is
applied directly to the skin. Green tea may also exert an estrogen-blocking
effect that is helpful in preventing breast and uterine cancer, and another
study suggests that it might prevent the development of tumors by blocking the
growth of new blood vessels.
For more information, including dosage and safety issues, see the full green tea
In many animal studies, soybeans, soy protein, or other soy extracts
decreased cancer risk, and observational studies in people have found suggestive
associations between higher soy consumption and lower incidence of
hormone-related cancers such as prostate, breast, and uterine cancer. However,
one highly preliminary intervention trial in humans found changes suggestive of
increased breast cancer risk after women took a commercial soy protein product.
Soybeans provide estrogen-like compounds known as isoflavones, especially
genistein and daidzein. These substances bind to the same sites in the body as
estrogen, occupying these sites and keeping natural estrogen away. Estrogen
stimulates certain forms of cancer, but soy isoflavones exert a milder
estrogen-like effect that may not stimulate cancer as much as natural estrogen.
This could help protect against cancer. Soy may additionally reduce levels of
the body’s own estrogen, which would also have a protective effect.
For more information, including dosage and safety issues, see the full soy
Folate deficiency may predispose individuals toward developing cancer of the
cervix, colon, lung, breast, pancreas, and mouth. Large observational studies
suggest that folate supplements may help prevent colon cancer, especially when
taken for many years. Since deficiency of this essential vitamin is quite common
(and quite unhealthy), you really cant go wrong taking extra folate.
For more information, including dosage and safety issues, see the full folate
Other Proposed Treatments for Cancer Prevention (Reducing the Risk):
The substances mentioned in this section have less evidence behind them than
the antioxidants discussed previously. However, this is a rapidly growing field.
By the time you read this section, new information will undoubtedly be
Some, but not all, observational and intervention studies have found evidence
that calcium supplementation may reduce the risk of colon cancer.
Some studies have connected higher vitamin D levels with a lower incidence of
cancer of the breast, colon, pancreas, and prostate, as well as melanoma, but
overall the research has yielded mixed results.
Dietary fiber has been thought to help prevent colon cancer. However, several
recent studies have found either little benefit or none at all.
Substances known as lignans are found in several foods and may produce
anticancer benefits. They are converted in the digestive tract to estrogen-like
substances known as enterolactone and enterodiol. Like soy isoflavones (see the
previous discussion under the heading Soy), these substances prevent estrogen
from attaching to cells and may thereby block its cancer-promoting effects.
Lignans are found most abundantly in flaxseed (the whole seed), a high-fiber
grain that has been cultivated since ancient Egyptian times. However, contrary
to some reports, flaxseed oil contains no lignans. Flaxseed oil is a rich source
of the omega-3 fatty acid alpha-linolenic acid.
Although flaxseed or flaxseed oil is sometimes recommended as prevention or
treatment for cancer, the evidence is still extremely preliminary. Weak evidence
also suggests that the alpha-linolenic acid in flaxseed oil may act against
breast cancer. Low levels of alpha-linolenic acid in breast fatty tissues were
associated with an increase in cancer and its spread (metastasis) to other areas
of the body. Test tube studies found that flaxseed or one of its lignans
inhibited the growth of human breast cancer cells. Other studies suggest that
the lignans enterolactone and enterodiol inhibited the growth of human colon
For more information, including dosage and safety issues, see the full articles
on flaxseed and lignans.
Resveratrol is a phytochemical found in at least 72 different plants,
including mulberries and peanuts. Grapes are its richest source. Red wine, which
is made from grapes, contains a lot of resveratrol, which may account for some
of the beneficial effects attributed to wine in some studies.
Resveratrol is an antioxidant with intriguing anticancer effects as determined
in test tube studies. However, little direct evidence supports the idea that
resveratrol is helpful.
For more information, including dosage and safety issues, see the full
Other Possible Cancer-Preventive Treatments
One large observational study suggests that higher intake of boron may reduce
risk of prostate cancer.
Provocative evidence suggests that a substance called sulforaphane, found in
broccoli and related vegetables, may possess anticancer properties. Recently,
broccoli sprouts have been touted for cancer prevention on the basis of their
high content of sulforaphane. However, this recommendation is still highly
speculative. Another constituent of broccoli-family vegetables,
indole-3-carbinol, has also shown promise as a cancer-preventative agent;
however, there is some evidence that this substance might actually increase the
risk of cancer in certain circumstances.
Catechins are an active compound of many plant foods. Some evidence suggests
that catechins from fruit may reduce risk of cancers high up in the digestive
tract, while catechins from tea may reduce risk of rectal cancer.
Test tube and animal research also suggests that the supplements quercetin and
conjugated linoleic acid might have anticancer properties.
One study provides preliminary supporting evidence for the notion that fish oil
reduces the risk of prostate cancer.
A preliminary study suggests that N-acetyl-cysteine (NAC) treatment may help to
prevent colon cancer. However, this evidence has not been confirmed by
Very preliminary evidence also suggests some cancer-preventive benefits for the
spices turmeric and rosemary as well as for betulin (from white birch tree),
bromelain, citrus juices, ellagic acid (from grapes, raspberries, strawberries,
apples, walnuts, and pecans), genistein, ginseng, glycine, grass pollen,
inositol hexaphosphate (phytic acid, IP6), kelp, licorice, melatonin, MSM, milk
thistle, nettle, OPCs (oligomeric proanthocyanidins), papaw tree bark,
probiotics or \\\"friendly\\\" bacteria such as acidophilus, schisandra, and spirulina
or other types of blue-green algae.
Mixed evidence from observational trials suggests that high consumption of
fruits and vegetables might reduce the risk of digestive tract cancers, ovarian
cancer, and overall cancer death.
Finally, there is some evidence that avoiding very hot beverages may reduce the
risk of esophageal cancer.
Putting It All Together:
According to one intervention trial, vitamin E might reduce the incidence of
An intervention trial found, by accident, that selenium supplementation might be
helpful for preventing lung, colon, and prostate cancers. Because this study was
not designed to look for evidence regarding prevention of these forms of cancer,
the results may not be reliable.
Remaining evidence regarding possible cancer prevention comes almost exclusively
from observational studies, which are less reliable than intervention studies.
Garlic might help prevent stomach and colon cancer and perhaps other cancers.
Purified beta-carotene has not been shown to prevent cancer (and it may even
increase the risk), but carotenes in the diet appear to protect against lung
cancer as well as cancer of the bladder, breast, stomach, and the upper
Folate supplements may help prevent colon cancer and possibly other types of
cancer as well.
Tomato consumption may reduce the occurrence of prostate cancer as well as,
possibly, stomach, lung, breast, and colon cancer, perhaps due to their content
of the natural carotene lycopene. Cooked tomatoes appear to be more bioavailable
(more readily used by the body) than raw tomatoes, especially when the tomatoes
are cooked in oil.
Green tea may help prevent cancer of the colon, skin, stomach, small intestine,
pancreas, lungs, breast, and uterus.
Soy may help prevent hormone-sensitive cancers, such as those of the breast,
prostate, uterus, and colon.
Please refer to the articles on these substances to learn about safety issues
associated with some of them.