Cancer Prevention and Diet
When it comes to cancer there are no guarantees. However, there are choices you can make to significantly reduce your risks.
Cancer refers to a large number of diseases categorized by unregulated replication of cells. This may result in cancers that may affect many different body systems, commonly the colon, breast, prostate, and lung. Other less common diseases include cancers of the brain, bone, blood (leukemia), bladder, skin (melanoma), pancreas, uterus, esophagus, head, and neck.
The contents of this article are limited to information about diet and to a discussion of cancer prevention--not treatment. Prevention of cancer in a person who has never had cancer is called "primary" prevention. Primary prevention is the focus of this article. This article includes a discussion of studies that have assessed whether certain dietary ingredients may be beneficial in connection with the reduction of risk of developing cancer.
This information is provided solely to aid consumers in discussing supplements with their healthcare providers. It is not advised nor is this information intended to advocate, promote, or encourage self-use of this information for cancer risk reduction. Some studies suggest an association between high blood or dietary levels of a particular dietary ingredient with a reduced risk of developing cancer. Even if such an association were established, this does not mean that dietary supplements containing large amounts of the dietary ingredient will necessarily have a cancer risk reduction effect.
Prevention of a recurrence in a cancer patient who is in remission is called "secondary" prevention. Whether the information in this article would be helpful to people interested in secondary prevention is, for the most part, unknown. For people who are undergoing cancer treatment, there is preliminary evidence that a healthy diet carries benefits even after diagnosis, though dietary changes should be reviewed with the treating doctor.
Cancer is the second leading cause of death in Americans. Information on the prevention of breast, colon, lung, and prostate cancers is not provided in this article. To find out more about these specific forms of cancer, read the Breast Cancer Prevention, Colon Cancer Prevention, Lung Cancer Prevention, and Prostate Cancer Prevention articles.
The following dietary changes have been studied in connection with cancer.
Alcohol and Cancer
Alcohol consumption significantly increases the risk of cancers of the mouth (oral/oropharyngeal cancer), throat (esophageal cancer), and voice box (laryngeal cancer), particularly in conjunction with cigarette smoking.1, 2, 3 Most studies documenting these associations also report that former drinkers have significantly lower risks for these cancers compared with current drinkers. Strong correlations between alcohol consumption and the risk of having liver cancer have also been reported.4, 5
Little is known about the effect of alcohol intake on the risk of female cancers other than breast cancer. Of the few published studies, findings have been inconsistent.6, 7, 8, 9
Whole grains (such as rye, brown rice, and whole wheat) contain high amounts of insoluble fiber--the type of fiber some scientists believe may help protect against a variety of cancers. In an analysis of the data from many studies, people who eat relatively high amounts of whole grains were reported to have low risks of lymphomas and cancers of the pancreas, stomach, colon, rectum, breast, uterus, mouth, throat, liver, and thyroid.10 Most research focusing on the relationship between cancer and fiber has focused on breast and colon cancers.
Consuming a diet high in insoluble fiber is best achieved by switching from white rice to brown rice and from bakery goods made with white flour or mixed flours to 100% whole wheat bread, whole rye crackers, and whole grain pancake mixes. Refined white flour is generally listed on food packaging labels as "flour,""enriched flour,""unbleached flour,""durum wheat,""semolina," or "white flour." Breads containing only whole wheat are often labeled "100% whole wheat."
The following two possibilities are both strongly supported by research findings:
Compared with meat eaters, most,11 but not all,12 studies have found that vegetarians are less likely to be diagnosed with cancer. Vegetarians have also been shown to have stronger immune function, possibly explaining why vegetarians may be partially protected against cancer.13 Female vegetarians have been reported to have lower estrogen levels compared with meat-eating women, possibly explaining a lower incidence of uterine and breast cancers.14 A reduced risk for various cancers is only partly,15 not totally,16 explained by differences in body weight, smoking habits, and other lifestyle issues.
Fruits and Vegetables
Consumption of fruits and vegetables is widely accepted as lowering the risk of most common cancers.17 Many doctors recommend that people wishing to reduce their risk of cancer eat several pieces of fruit and several portions of vegetables every day. Optimal intakes remain unknown.
Most doctors also recommend that people should not consider supplements as substitutes for the real thing. Some of the anticancer substances found in produce have probably not yet been discovered, while others are not yet available in supplement form. More important, some research, particularly regarding synthetic beta-carotene, does not support the idea that taking supplements has the same protective value against cancer as does consumption of fruits and vegetables.
Flavonoids are found in virtually all herbs and plant foods. Consumption of flavonoid-rich onions and apples contain large amounts of one flavonoid called quercetin. Consumption of flavonoids in general, or quercetin-containing foods in particular,18 has been associated with protection against cancer in some,19 but not all,20 preliminary studies.
Tomatoes contain lycopene--an antioxidant similar in structure to beta-carotene. Most lycopene in our diet comes from tomatoes, though traces of lycopene exist in other foods. Lycopene inhibits the proliferation of cancer cells in test tube research.21
A review of published research found that higher intake of tomatoes or higher blood levels of lycopene correlated with protection from cancer in 57 of 72 studies. Findings in 35 of these studies were statistically significant.22 Evidence of a protective effect for tomato consumption was strongest for cancers of the prostate, lung, and stomach, but some evidence of a protective effect also appeared for cancers of the pancreas, colon, rectum, esophagus (throat), mouth, breast, and cervix.
Cabbage, Brussels sprouts, broccoli, and cauliflower belong to the Brassica family of vegetables, also known as "cruciferous" vegetables. In test tube and animal studies, these foods have been associated with anticancer activity,23 possibly due to several substances found in these foods, such as indole-3-carbinol,24 glucaric acid (calcium D-glucarate),25 and sulforaphane.26 In a preliminary human study, people who ate cruciferous vegetables were reported to have a lower-than-average risk for bladder cancer.27
Meat (How it Is Cooked) and Childhood Cancers
In one report, high consumption of hot dogs was associated with an almost tenfold increase in the risk of childhood leukemia when compared with low consumption.28 In another report, maternal consumption of hot dogs and childhood consumption of hamburgers or hot dogs at least once per week were associated with a doubling of the risk of cancers in children.29 A review of nine studies found an association between consumption by pregnant women of cured meat and the risk of brain cancer in their offspring.30 These associations do not yet constitute proof that eating hot dogs or hamburgers causes cancer in children, and evidence linking cured meat consumption to childhood cancers remains somewhat inconsistent.31
In the report studying the effects of eating hot dogs and hamburgers, the association between meat eating and leukemia was weakest among children who took vitamin supplements. Processed meats, such as hot dogs, contain nitrates and nitrites--precursors to carcinogens. Antioxidants found in multivitamins keep nitrates and nitrites from converting into those carcinogens. Therefore, the association between vitamin consumption in children and protection against childhood cancers remains plausible, though unproven.
Fish eaters have been reported to have low risks of cancers of the mouth, throat, stomach, colon, rectum, pancreas,32 lung,33 breast,34 and prostate.35 The omega-3 fatty acids found in fish are thought by some researchers to be the components of fish responsible for protection against cancer.36
Years ago, researchers reported the greater the consumption of coffee in a country, the higher the risk of pancreatic cancer in that country.37 An analysis of data from studies published between 1981 and 1993 did find some association between high consumption of coffee and an increased risk of pancreatic cancer.38 Surprisingly, however, the same report found that people drinking only one or two cups of coffee per day had, on average, a lower risk of pancreatic cancer compared with people who never drink coffee.
Most,39, 40, 41 but not all,42 published reports have shown coffee drinkers are at increased risk of bladder cancer, though in one case the relationship was found only in men.43 In another study, the association was found only with caffeinated coffee.44 A review of 35 trials found a small (7%) increased risk of bladder cancer in coffee drinkers compared with people not drinking coffee--a difference not statistically significant.45
Scientists have known for many years that severe restriction of calories dramatically reduces the risk of cancer in laboratory animals.46 Scientists speculate that caloric content of the human diet may also affect cancer rates,47 though much less is known about the effect, if any, of moderate caloric restrictions in humans. In one report, adults with cancer were more likely to have consumed more calories during childhood compared with healthy adults.48 In other reports, attempts to find associations between reduced intake of calories and cancer have produced mixed results.49, 50, 51
Only severe restriction in caloric intake provides significant protection in animal studies. As most people are unlikely to severely restrict calories, the association between caloric restriction and protection from cancer may ultimately prove to only be of academic interest.
In studying data from country to country, incidence of ovarian cancer correlates with dietary fat intake.52 According to preliminary research, consumption of saturated fat, dietary cholesterol (as found in eggs),53 and animal fat in general54 correlates with the risk of ovarian cancer.
Preliminary studies suggest dietary fat may correlate with the risk of uterine cancer.55 Some of the excess risk appears to result from increased body weight that results from a high-fat diet.56
Many years ago, researchers reported that animals on a high-fat diet formed skin cancers more rapidly than did other animals.57 Although some preliminary human research has found no relationship between dietary fat intake and the risk of skin cancer,58 precancerous lesions of the skin have been prevented in people put on a low-fat diet.59
A chain of carbon atoms in which several are not attached to the maximum possible amount of hydrogen is called "polyunsaturated"--in other words, unsaturated with hydrogen in several places. When nutrition researchers talk about polyunsaturated fatty acids, they are often referring primarily to linoleic acid--a fatty acid found in nuts and seeds and most vegetable oils.
In animal research, the consumption of polyunsaturated fatty acids increases the risk of some cancers.60 However, in humans, most,61, 62, 63 though not all,64 reports do not find an association between polyunsaturates and cancer risks.
A preliminary study has reported an association between an increasing intake of sugar or sugar-containing foods and an increased risk of gallbladder cancer.65 Whether this association exists because sugar directly promotes cancer or because sugar consumption is only a marker for some other dietary or lifestyle factor remains unknown.
In preliminary research, increasing intake of salt correlates with increased risk of stomach cancer.66, 67 Associations between foods preserved with salt and the risk of cancers of the head and neck have also been reported.68
Animal studies suggest that the antioxidant or immune-enhancing effect of whey protein may produce anti-cancer effects.69, 70, 71 Preliminary human case reports suggest that 30 grams per day of whey protein may improve responses to anti-cancer medications, but more research is needed.72