Conditions:
Cancer Prevention (Reducing the Risk)

Principal Proposed Treatments
  • Vitamin E, Selenium, Garlic, Tomatoes (Lycopene), Vitamin C, Green Tea, Soy, Isoflavones, Folate
Other Proposed Treatments
  • Calcium, Vitamin D, Fiber, Flaxseed (Lignans), Grapes (Resveratrol), Cartilage, Boron, Sulforaphane, Indole-3-Carbinol (I3C), Catechins, Quercetin, Conjugated Linoleic Acid, Fish Oil, N-Acetyl Cysteine (NAC), Turmeric, Rosemary, Betulin, Bromelain, Citrus Juices, Ellagic Acid, Genistein, Ginseng, Glycine, Grass Pollen, Inositol Hexaphosphate (Phytic Acid, IP6), Kelp, Licorice, Melatonin, MSM (Methyl Sulfonyl Methane), Milk Thistle, Nettle, OPCs (Oligomeric Proanthocyanidins), Papaw Tree Bark, Probiotics, Schisandra, Spirulina and Other Forms of Blue-Green Algae
Not Recommended Treatments
  • Beta-Carotene

Cancer is the second major cause of death (next to heart disease) in the United States. It claims the lives of more than half a million Americans each year out of the nearly 1.4 million who get the disease. The probability of getting cancer increases with age. Two-thirds of all cases are in people older than 65.¹

According to the American Cancer Society, one in two men and one in three women will face cancer during their lifetimes. However, it appears that you can significantly cut your cancer risk depending on how you choose to lead your life. That is the bright consensus of an international panel convened by the American Institute for Cancer Research and the World Cancer Research Fund.²

The panel found four key ways to reduce the odds of getting cancer: eat the right foods, exercise, watch your weight, and do not smoke. The experts reviewed diet and cancer findings from over 4,500 studies to reach this consensus.

What Causes Cancer?

Cancer is believed to begin with a mutation in a single cell. However, a cell doesn't become cancerous overnight. Several mutations in a row are necessary to create all the characteristic features of cancer. Ordinarily, cells have a self-destruct mechanism that causes them to die when their DNA is damaged. However, in developing cancer cells, something interferes with the self-destruct sequence. It may be that the cancer-causing mutations themselves turn off the countdown.

The DNA alterations that create a cancer cell give it a certain independence from the ordinary rules of cell behavior. Normal cells are highly influenced by nearby cells, with the result that they "get along" well with their neighbors. For example, the growth of a healthy cell is ruled by special growth factors given off by surrounding tissues. However, cancer cells either grow without such growth factors or simply make their own. Many types of cancer cells can also trigger the growth of new blood vessels to feed them.

Cancerous mutations appear to be caused mainly by exposure to carcinogenic substances, of which tobacco is the most common. Many carcinogens exist in the diet as well, such as salt-cured and smoked meats.

Free radicals also appear to play a major role in promoting cancer. These chemically unstable substances are produced by many factors, and are believed to affect heart disease and aging (for more information about free radicals in general, see the article on atherosclerosis). The best-documented natural treatments for preventing cancer have antioxidant properties.

Hormones can also help cancer get a start. For example, a newly formed cancer of the prostate or the breast is stimulated by the hormones that ordinarily control tissue in that part of the body. This is why estrogen-replacement therapy can increase the risk of breast cancer and why estrogen suppression is often recommended for women with a history of breast cancer.

Substances found in soy may help reduce the incidence of certain cancers by blocking the effects of estrogen and other hormones. However, this is not yet certain, and there is some conflicting evidence.³

The key to preventing cancer is to minimize your exposure to carcinogens. Quitting smoking is essential, and reducing your intake of smoked, charred, pickled, and salt-cured meat is also believed to be helpful. Eating a diet that is high in fruits, vegetables, and whole grains may also lower your chance of developing various forms of cancer, although the evidence is not entirely consistent. Vegetables in the broccoli family may be particularly helpful for breast cancer prevention, although this too, is controversial.

Because the following material is so complex, we have summarized the main points in the section titled "Putting It All Together." You can skip to it now if you want just the conclusions.

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 been performed.

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.

Vitamin E

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.^{11,12,14,263,264} The evidence regarding colon cancer is mixed.^15-22

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.⁹

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 prostate cancer.

Intervention trials of vitamin E have not found benefit for preventing lung cancer.^9,13,265

For more information, including dosage and safety issues, see the full vitamin E article.

Selenium

It has long been known that severe selenium deficiency increases the risk of cancer.²⁹ 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.³⁰ The results were so impressive that they caught the researchers by surprise. The study was actually designed to detect selenium's 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 article.

Garlic

Evidence from observational studies suggests that garlic taken in the diet as food may help prevent cancer, particularly cancer of the colon and stomach.^36–43

In one of the best of these studies, the Iowa Women's Study, women who ate significant amounts of garlic were found to be about 30% less likely to develop colon cancer.⁴⁴ Similar results were seen in other observational studies performed in China, Italy, and the United States.^45,46

We do not know for sure how garlic might work to prevent cancer. Like vitamin E, whole garlic possesses antioxidant properties.^47,48 Furthermore, various garlic extracts have also been shown to suppress the known DNA-damaging activity of several drugs and toxins.⁴⁹Finally,garlic contains high levels of selenium, which is thought to reduce the risk of cancer (see the previous discussion under the heading Selenium).⁵⁰

For more information, including dosage and safety issues, see the full garlic article.

Beta-Carotene

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.^55,56 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,⁵⁷ bladder cancer,⁵⁸ breast cancer,⁵⁹ esophageal cancer,⁶⁰ and stomach cancer.⁶¹

However, as noted above, observational studies cannot prove cause and effect. When researchers gave beta-carotene to study participants, the results have been impressively negative.

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.⁶³ 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 cancer.)

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.⁶⁴ 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.^65,66

Similarly, another study of beta-carotene supplements failed to find any effect on the risk of cancer in women.⁶⁷

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 carotenes.⁶⁹

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.

Tomatoes (Lycopene)

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.⁷⁰ 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.⁷¹ 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,⁷² as well as evidence that men with higher lycopene levels in the blood have a lower risk of prostate cancer.⁷³

Similar evidence suggests that foods containing lycopene might help prevent other forms of cancer as well, including lung, colon, and breast cancer.^74,75,262

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.

Vitamin C

Several observational studies have found a strong association between high dietary vitamin C intake and a reduced incidence of stomach cancer.^76,77,78 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.^79–84 However, dietary vitamin C intake does not appear to be associated with reduced rate of prostate cancer.⁸⁵

One study found that vitamin C supplementation at 500 mg or more daily was associated with a lower incidence of bladder cancer.⁸⁶ However, another study found no association.⁸⁷ 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.⁸⁹ Another study found similar results.⁹⁰

Green Tea

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 effects.

Laboratory and animal studies suggest that tea consumption protects against cancers of the stomach, lung, esophagus, duodenum, pancreas, liver, breast, and colon.^91,92A 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.^93,94 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.⁹⁵

One study followed 8,552 Japanese adults for 9 years.⁹⁶ 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.⁹⁷ 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.⁹⁸Interestingly, the risk of stomach cancer did not depend on the person's 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.⁹⁹ Negative results were also seen in a study conducted in Hawaii.¹⁰⁰

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.¹⁰¹ Preliminary experimental studies suggest that EGCG may help prevent skin cancer if it is applied directly to the skin.¹⁰² Green tea may also exert an estrogen-blocking effect that is helpful in preventing breast and uterine cancer,¹⁰³ and another study suggests that it might prevent the development of tumors by blocking the growth of new blood vessels.¹⁰⁴

For more information, including dosage and safety issues, see the full green tea article.

Soy

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.^105-111 However, one highly preliminary intervention trial in humans found changes suggestive of increased breast cancer risk after women took a commercial soy protein product.¹¹²

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.^266-271

For more information, including dosage and safety issues, see the full soy article.

Folate

Folate deficiency may predispose individuals toward developing cancer of the cervix,¹²⁰ colon,^121,122lung,¹²³ breast,¹²⁴ pancreas,¹²⁵ and mouth.¹²⁶ Large observational studies suggest that folate supplements may help prevent colon cancer, especially when taken for many years.^127,128,129 Since deficiency of this essential vitamin is quite common (and quite unhealthy), you really can't go wrong taking extra folate.

For more information, including dosage and safety issues, see the full folate article.

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 available.

Calcium

Some, but not all, observational and intervention studies have found evidence that calcium supplementation may reduce the risk of colon cancer.^130–139

Vitamin D

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.^140–159

Fiber

Dietary fiber has been thought to help prevent colon cancer.^160–163 However, several recent studies have found either little benefit or none at all.^164–170

Flaxseed (Lignans)

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.^171,172 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.¹⁷³Flaxseed 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.^174–181 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.¹⁸² Test tube studies found that flaxseed or one of its lignans inhibited the growth of human breast cancer cells.¹⁸³ Other studies suggest that the lignans enterolactone and enterodiol inhibited the growth of human colon tumor cells.¹⁸⁴

For more information, including dosage and safety issues, see the full articles on flaxseed and lignans.

Grapes (Resveratrol)

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.^186,187 However, little direct evidence supports the idea that resveratrol is helpful.

For more information, including dosage and safety issues, see the full resveratrol article.

Other Possible Cancer-Preventive Treatments

One large observational study suggests that higher intake of boron may reduce risk of prostate cancer.²⁰⁴

Provocative evidence suggests that a substance called sulforaphane, found in broccoli and related vegetables, may possess anticancer properties.^205,206,207 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;^208–212 however, there is some evidence that this substance might actually increase the risk of cancer in certain circumstances.^213,214

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.²¹⁵

Test tube and animal research also suggests that the supplements quercetin^216–220 and conjugated linoleic acid²²¹ might have anticancer properties.

One study provides preliminary supporting evidence for the notion that fish oil reduces the risk of prostate cancer.²²²

A preliminary study suggests that N-acetyl-cysteine (NAC) treatment may help to prevent colon cancer.²²³ However, this evidence has not been confirmed by additional studies.

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.^224–252

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.^253–260

Finally, there is some evidence that avoiding very hot beverages may reduce the risk of esophageal cancer.²⁶¹

Putting It All Together

According to one intervention trial, vitamin E might reduce the incidence of prostate cancer.

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 digestive tract.

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.

Last reviewed August 2002 by Medical Review Board

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