Because people are interested in the relationship that specific foods, nutrients, or lifestyle factors have to specific cancers, research on health behaviors and cancer risk is often widely publicized. No one study, however, provides the last word on any subject, and single news reports may overemphasize what appear to be contradictory or conflicting results. In brief news stories, reporters cannot always put new research findings in their proper context. Therefore, it is rarely, if ever, advisable to change diet or activity levels based on a single study or news report. The following questions and answers address common concerns about diet and physical activity in relation to cancer.
Alcohol
Does alcohol increase cancer risk?
Yes. Alcohol increases the risk of cancers of the mouth, pharynx (throat), larynx (voice box), esophagus, liver, and breast, and probably of the colon and rectum. People who drink alcohol should limit their intake to no more than 2 drinks per day for men and 1 drink per day for women. A drink is defined as 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of 80-proof distilled spirits. The combination of alcohol and tobacco increases the risk of some cancers far more than the effect of either drinking or smoking. Regular consumption of even a few drinks per week is associated with an increased risk of breast cancer in women, especially in women who do not get enough folate. Women at high risk of breast cancer may want to consider not drinking any alcohol.
Antioxidants
What are antioxidants, and what do they have to do with cancer?
The body appears to use certain nutrients in vegetables and fruits to protect against damage to tissues that occurs constantly as a result of normal metabolism (oxidation). Because such damage is linked with increased cancer risk, the so-called antioxidant nutrients are thought to protect against cancer. Antioxidants include vitamin C, vitamin E, carotenoids, and many other phytochemicals (chemicals from plants). Studies suggest that people who eat more vegetables and fruits, which are rich sources of antioxidants, may have a lower risk for some types of cancer. Clinical studies of antioxidant supplements are currently under way but have not yet shown a reduction in cancer risk from vitamin or mineral supplements (also see entries for: beta-carotene, lycopene, vitamin E, supplements). To reduce cancer risk, the best advice at present is to consume antioxidants through food sources, rather than supplements.
Aspartame
Does aspartame cause cancer?
Aspartame is a low-calorie artificial sweetener that is about 200 times sweeter than sugar. Current evidence does not show any link between aspartame ingestion and increased cancer risk. People with the genetic disorder known as phenylketonuria should avoid aspartame in their diets.
Beta-carotene
Does beta-carotene reduce cancer risk?
Because beta-carotene, an antioxidant chemically related to vitamin A, is found in vegetables and fruits, and because eating vegetables and fruits is linked with a reduced risk of cancer, it seemed plausible that taking high doses of beta-carotene supplements might reduce cancer risk. But the results of 3 major clinical trials show this is not the case. In 2 studies in which people were given high doses of beta-carotene supplements in an attempt to prevent lung cancer and other cancers, the supplements were found to increase the risk of lung cancer in cigarette smokers, and a third found neither benefit nor harm from them. Therefore, consuming vegetables and fruits that contain beta-carotene may be helpful, but high-dose beta-carotene supplements should be avoided.
Bioengineered Foods
What are bioengineered foods, and are they safe?
Bioengineered foods are made by adding genes from other plants or organisms to increase a plant’s resistance to pests, retard spoilage, or improve transportability, flavor, nutrient composition, or other desired qualities. In theory, these added genes might create substances that could cause harmful reactions among sensitized or allergic individuals. But there is currently no evidence that the substances found in bioengineered foods now on the market are harmful or that they would either increase or decrease cancer risk because of the added genes.
Calcium
Is calcium related to cancer?
Several studies have suggested that foods high in calcium might help reduce the risk for colorectal cancer, and that supplementing the diet with calcium modestly reduces the formation of colorectal adenomas (polyps). But there is also evidence that a high-calcium intake, primarily through supplements, is linked with increased risk for prostate cancer, especially for prostate cancers that are more aggressive. In light of this, both men and women should strive to consume recommended levels of calcium, primarily through food sources. Recommended intake levels of calcium are 1,000 mg/day for people ages 19 to 50 years and 1,200 mg/day for people older than 50. Dairy products are excellent sources of calcium, as are some leafy vegetables and greens. People who get much of their calcium from dairy products should select low-fat or non-fat choices to reduce their intake of saturated fat.
Cholesterol
Does cholesterol in the diet increase cancer risk?
Cholesterol in the diet comes only from foods with animal sources -- meat, dairy products, eggs, and animal fats such as butter or lard. Although some of these foods (for example, processed and red meats) are linked with higher risk of certain cancers, at this time there is little evidence that this increased risk is specifically related to cholesterol. Lowering blood cholesterol reduces heart disease risk, but there is no evidence that lowering blood cholesterol has an effect on cancer risk.
Coffee
Does drinking coffee cause cancer?
Caffeine may heighten symptoms of fibrocystic breast lumps (a type of benign breast disease) in some women, but there is no evidence that it increases the risk of breast cancer or other types of cancer. The link between coffee and cancer of the pancreas, widely publicized in the past, has not been confirmed by recent studies. There does not appear to be any connection between coffee drinking and cancer risk.
Fat
Will eating less fat lower cancer risk?
There is little evidence that the total amount of fat consumed affects cancer risk. But diets high in fat tend to be high in calories and may contribute to obesity, which in turn is linked with an increased risk of several types of cancer. There is evidence that certain types of fats, such as saturated fats, may increase cancer risk. There is little evidence that other types of fat (omega-3 fatty acids, found mainly in fish), monounsaturated fatty acids (found in olive and canola oils), or other polyunsaturated fats reduce cancer risk.
Fiber
What is dietary fiber, and can it prevent cancer?
Dietary fiber includes a wide variety of plant carbohydrates that are not digestible by humans. Specific categories of fiber are "soluble" (like oat bran) or "insoluble" (like wheat bran and cellulose). Soluble fiber helps to reduce blood cholesterol, thereby lowering the risk of coronary heart disease. Good sources of fiber are beans, vegetables, whole grains, and fruits. Links between fiber and cancer risk are weak, but eating these foods is still recommended because they contain other nutrients that may help reduce cancer risk and because of their other health benefits.
Fish
Does eating fish protect against cancer?
Fish is a rich source of omega-3 fatty acids. Studies in animals have found that these fatty acids suppress cancer formation or slow cancer progression, but there is limited evidence of a possible benefit in humans.
While eating fish rich in omega-3 fatty acids is linked with a reduced risk of heart disease, some types of fish (large predatory fish such as swordfish, tilefish, shark, and king mackerel) may contain high levels of mercury, polychlorinated biphenyls (PCBs), dioxins, and other environmental pollutants. Women who are pregnant, breast-feeding, or planning to become pregnant, and young children should not eat these fish. People should vary the types of fish they eat to reduce the likelihood of exposure to toxins.
Research has not yet shown whether taking omega-3 or fish oil supplements produces the same possible benefits as eating fish. .
Fluorides
Do fluorides cause cancer?
Extensive research has examined the effects of fluorides given as dental treatments or added to toothpaste, public water supplies, or foods on cancer risk. Fluorides have not been found to increase cancer risk.
Folate
What is folate, and can it prevent cancer?
Folate is a B vitamin found in many vegetables, beans, fruits, whole grains, and fortified breakfast cereals. Since 1998, all grain products in the United States have been fortified with folate. Too little folate may increase the risk of cancers of the colon, rectum, and breast, especially in people who drink alcoholic beverages. Current evidence suggests that to reduce cancer risk, folate is best obtained by eating vegetables, fruits, and enriched grain products.
Food Additives
Do food additives cause cancer?
Many substances are added to foods to preserve them and to enhance color, flavor, and texture. New additives must be cleared by the Food and Drug Administration (FDA) before entering the food supply. Rigorous testing in animals to look for any effects on cancer is done as part of this process. Additives are usually present in very small quantities in food, and no convincing evidence has shown that any additive at these levels causes human cancers.
Garlic
Can garlic prevent cancer?
The health benefits of the allium compounds contained in garlic and other vegetables in the onion family have been publicized widely. Garlic is currently under study for its ability to reduce cancer risk. There is not enough evidence at this time to support a specific role for this vegetable in cancer prevention.
Genetics
If our genes determine cancer risk, how can diet help prevent cancer?
Damage to the genes that control cell growth can be either inherited or acquired during life. Certain types of mutations or genetic damage can increase the risk of cancer. Nutrients in the diet can protect DNA from being damaged. Physical activity, weight control, and diet might delay or prevent the development of cancer in people with an increased genetic risk for cancer. The many interactions between diet and genetic factors are an important and complex topic, and a great deal of research is under way in this area.
Irradiated Foods
Do irradiated foods cause cancer?
No. Radiation is increasingly used to kill harmful organisms on foods in order to extend their "shelf life." Radiation does not remain in the foods after treatment, and consuming irradiated foods does not appear to increase cancer risk.
Lycopene
Will lycopene reduce cancer risk?
Lycopene is the red-orange carotene pigment found mainly in tomatoes and tomato-based foods and to a lesser extent in pink grapefruit and watermelon. Several studies have reported that consuming tomato products reduces the risk of some cancers, but whether lycopene is the nutrient responsible is uncertain. Even if lycopene in foods is linked with lower risk for cancer, it can't be concluded that high doses taken as supplements would be either more effective or safe.
Meat: Cooking and Preserving
Should I avoid processed meats?
Some studies have linked eating large amounts of processed meat to increased risk of colorectal and stomach cancers. This connection may or may not be due to nitrites, which are added to many luncheon meats, hams, and hot dogs to maintain color and to prevent contamination with bacteria. Eating processed meats and meats preserved by methods involving smoke or salt increases exposure to potential cancer-causing agents and should be reduced as much as possible.
How does cooking meat affect cancer risk?
Adequate cooking is necessary to kill harmful germs within meat. But some research suggests that frying, broiling, or grilling meats at very high temperatures creates chemicals that might increase cancer risk. Although these chemicals can damage DNA and cause cancer in animals, it is not clear how much they (as opposed to other components in meat) may contribute to the increased colorectal cancer risk seen in people who eat large amounts of meat in some studies. Techniques such as braising, steaming, poaching, stewing, and microwaving meats produce fewer of these chemicals.
Obesity
Does being overweight increase cancer risk?
Yes. Being overweight or obese is linked with an increased risk of cancers of the breast (among postmenopausal women), colon, endometrium, esophagus, kidney, and possibly other sites as well. Although research on whether losing weight reduces cancer risk is limited, some research suggests that weight loss does reduce the risk of breast cancer. Because of other proven health benefits, people who are overweight are encouraged to lose weight. Avoiding excessive weight gain in adulthood is important not only to reduce cancer risk but also to reduce the risk of other chronic diseases.
Olive Oil
Does olive oil affect cancer risk?
Consumption of olive oil is linked with a reduced risk of heart disease, but is most likely neutral with respect to cancer risk. Although olive oil is a healthy alternative to butter and margarine, it is a significant source of calories and should be used in moderation.
Organic Foods
Are foods labeled "organic" more effective in lowering cancer risk?
The term organic is popularly used to designate plant foods grown without pesticides and genetic modifications. At this time, no research exists to demonstrate whether such foods are more effective in reducing cancer risk than are similar foods produced by other farming methods.
Pesticides and Herbicides
Do pesticides in foods cause cancer?
Pesticides and herbicides can be toxic when used improperly in industrial, agricultural, or other occupational settings. Although vegetables and fruits sometimes contain low levels of these chemicals, overwhelming scientific evidence supports the overall health benefits and cancer-protective effects of eating vegetables and fruits. At present there is no evidence that residues of pesticides and herbicides at the low doses found in foods increase the risk of cancer, but produce should be washed thoroughly before eating.
Physical Activity
Will increasing physical activity lower cancer risk?
Yes. People who engage in moderate to vigorous levels of physical activity are at a lower risk of developing colon and breast cancer than those who do not. This risk reduction is independent of the impact of activity on weight. Data for a direct effect on the risk of developing other cancers is more limited. Even so, obesity and being overweight have been linked to many types of cancer, and physical activity is a key factor in maintaining or achieving a healthy body weight. In addition, physical activity has beneficial effects against heart disease and diabetes.
Phytochemicals
What are phytochemicals, and do they reduce cancer risk?
The term phytochemicals refers to a wide variety of compounds made by plants. Some of these compounds protect plants against insects or perform other important functions. Some have either antioxidant or hormone-like actions both in plants and in people who eat them. Because consuming vegetables and fruits reduces cancer risk, researchers are looking for specific components responsible for the beneficial effects. At this time, no evidence has shown that phytochemicals taken as supplements are as beneficial as the vegetables, fruits, beans, and grains from which they are extracted.
Saccharin
Does saccharin cause cancer?
No. In rats, high doses of the artificial sweetener saccharin can cause bladder stones to form that can lead to bladder cancer. But saccharin consumption does not cause the formation of bladder stones in humans. Saccharin has been removed from the list of established human carcinogens by the US National Toxicology Program.
Salt
Do high levels of salt in the diet increase cancer risk?
Studies in other countries link diets containing large amounts of foods preserved by salting and pickling with an increased risk of stomach, nasopharyngeal, and throat cancer. No evidence suggests that moderate levels of salt used in cooking or in flavoring foods affect cancer risk.
Selenium
What is selenium, and can it reduce cancer risk?
Selenium is a mineral that contributes to the body's antioxidant defense mechanisms. Animal studies suggest that selenium protects against cancer, and one study has shown that selenium supplements might reduce the risk of lung, colon, and prostate cancer. But repeated and well-controlled studies are needed to confirm whether selenium is helpful in preventing these cancers. High-dose selenium supplements are not recommended, as there is only a narrow margin between safe and toxic doses. The maximum dose in a supplement should not exceed 200 micrograms per day.
Soy Products
Can soy-based foods reduce cancer risk?
Soy-derived foods are an excellent source of protein and a good alternative to meat. Soy contains several phytochemicals, some of which have weak estrogen activity and appear to protect against hormone-dependent cancers in animal studies. At this time there is little data showing that soy supplements can help reduce cancer risk. High doses of soy could possibly increase the risk of estrogen-responsive cancers, such as breast or endometrial cancer.
Women with breast cancer should consume only moderate amounts of soy foods as part of a healthy, plant-based diet. They should not ingest very high levels of soy in their diet or take concentrated sources of soy such as soy-containing pills or powders, or supplements containing high amounts of isoflavones.
Sugar
Does sugar increase cancer risk?
Sugar increases caloric intake without providing any of the nutrients that reduce cancer risk. By promoting obesity and elevating insulin levels, high sugar intake may indirectly increase cancer risk. White (refined) sugar is no different from brown (unrefined) sugar or honey with regard to these effects on body weight or insulin. Limiting foods such as cakes, candy, cookies, sweetened cereals, and high-sugar beverages such as soda can help reduce sugar intake.
Supplements
Can nutritional supplements lower cancer risk?
There is strong evidence that a diet rich in fruits, vegetables, and other plant-based foods may reduce the risk of cancer. But there is no proof at this time that supplements can reduce cancer risk. Some high-dose supplements may actually increase cancer risk.
Can I get the nutritional equivalent of vegetables and fruits in a pill?
No. Many healthful compounds are found in vegetables and fruits, and these compounds most likely work in combination to exert their beneficial effect. There are also likely to be important, but as of yet unidentified, components of whole foods that are not included in supplements. The small amount of dried powder in the pills that are represented as being equivalent to vegetables and fruits often contains only a small fraction of the levels contained in the whole foods.
Food is the best source of vitamins and minerals. Supplements, however, may be helpful for some people, such as pregnant women, women of childbearing age, and people with restricted dietary intakes. If a supplement is taken, the best choice is a balanced multivitamin/mineral supplement containing no more than 100% of the "Daily Value" of most nutrients.
Tea
Can drinking tea reduce cancer risk?
Some researchers have suggested that tea might protect against cancer because of its antioxidant content. In animal studies, some teas (including green tea) have been shown to reduce cancer risk, but findings from human population studies are mixed. At this time, tea has not been proven to reduce cancer risk in humans.
Trans-Fats
Do trans-saturated fats increase cancer risk?
Trans-saturated fats are made during the manufacture of hydrogenated oils such as margarines or shortenings to make them solid at room temperature. Recent evidence shows that trans-fats raise blood cholesterol levels. Their relationship to cancer risk has not been determined, but people are advised to eat as few trans-fats as possible.
Vegetables and Fruits
Will eating vegetables and fruits lower cancer risk?
In the majority of population studies, greater consumption of vegetables and fruits has been linked to a lower risk of lung, oral, esophageal, stomach, and colon cancer. Because we don't know which of the many compounds in these foods are most helpful, the best advice is to eat 5 or more servings of an assortment of colorful vegetables and fruits each day.
What are cruciferous vegetables, and are they important in cancer prevention?
Cruciferous vegetables belong to the cabbage family and include broccoli, cauliflower, Brussels sprouts, and kale. These vegetables contain certain chemicals thought to reduce the risk for colorectal cancer. The best evidence suggests that consuming a wide variety of vegetables, including cruciferous and other vegetables, reduces cancer risk.
Is there a difference in nutritional values among fresh, frozen, and canned vegetables and fruits?
Yes, but they can all be good choices. Fresh foods are usually considered to have the most nutritional value. But frozen foods can often be more nutritious than fresh foods because they are often picked ripe and quickly frozen (whereas fresh foods may lose some of their nutrients in the time between harvesting and consumption). Canning is more likely to reduce the heat-sensitive and water-soluble nutrients because of the high heat used when processing. Be aware that some fruits are packed in heavy syrup, and some canned vegetables are high in sodium (salt). Choose vegetables and fruits in a variety of forms.
Does cooking affect the nutritional value of vegetables?
Boiling vegetables, especially for long periods, can leach out their content of water-soluble (B and C) vitamins. Microwaving and steaming are the best ways to preserve the nutritional content of vegetables.
Should I be juicing my vegetables and fruits?
Juicing can add variety to the diet and can be a good way to consume vegetables and fruits, especially if chewing or swallowing is a problem. Juicing also improves the body's absorption of some of the nutrients in vegetables and fruits. But juices may be less filling than whole vegetables and fruits and may contain less fiber. Fruit juice in particular can contribute quite a few calories if large amounts are consumed. Commercially juiced products should be 100% vegetable or fruit juices and should be pasteurized to remove harmful germs.
Vegetarian Diets
Do vegetarian diets reduce cancer risk?
Vegetarian diets include many health-promoting features. They tend to be low in saturated fats and high in fiber, vitamins, and phytochemicals. It is not possible to conclude at this time, however, that a vegetarian diet has any special benefits for the prevention of cancer. Diets including lean meats in small to moderate amounts can also be healthful. Strict vegetarian diets that avoid all animal products, including milk and eggs, should be supplemented with vitamin B12, zinc, and iron (especially for children and premenopausal women).
Vitamin A
Does vitamin A lower cancer risk?
Vitamin A (retinol) is obtained from foods in 2 ways: pre-formed from animal food sources and derived from beta-carotene in plant-based foods. Vitamin A is needed to maintain healthy tissues. Vitamin A supplements, whether in the form of beta-carotene or retinol, have not been shown to lower cancer risk, and high-dose supplements may, in fact, increase the risk for lung cancer in current and former smokers.
Vitamin C
Does vitamin C lower cancer risk?
Vitamin C is found in many vegetables and fruits, especially oranges, grapefruits, and peppers. Many studies have linked consumption of foods rich in vitamin C to a reduced risk for cancer. But the few studies in which vitamin C has been given as a supplement have not shown a reduced risk for cancer.
Vitamin D
Does vitamin D lower cancer risk?
There is a growing body of evidence from population studies (not yet tested in clinical trials) that vitamin D may have helpful effects on some types of cancer, including cancers of the colon, prostate, and breast. Vitamin D is obtained through skin exposure to ultraviolet (UV) radiation and through diet, particularly products fortified with vitamin D such as milk and cereals, and supplements. But many Americans do not consume sufficient amounts of vitamin D.
The current national recommended levels of intake of vitamin D (200 to 600 IU per day) may not be enough to meet needs, especially among those with little sun exposure, the elderly, people with dark skin, and exclusively breastfed babies. More research is needed to define the best levels of intake and blood levels of vitamin D for cancer risk reduction, but recommended intake is likely to fall between 200 and 2000 IU, depending on age and other factors. To reduce the health risks linked with UV radiation exposure while getting the most potential benefit from vitamin D, a balanced diet, supplementation, and limiting sun exposure to small amounts are the preferred methods of obtaining vitamin D.
Vitamin E
Does vitamin E lower cancer risk?
Alpha-tocopherol is the most active form of vitamin E and is a powerful antioxidant. In one study, male smokers who took alpha-tocopherol had a lower risk of prostate cancer compared with those who took a placebo. But several other studies have not found the same link. While studies now under way will help clarify this, the promise of alpha-tocopherol for reducing cancer risk appears to be dimming.
Water and Other Fluids
How much water and other fluids should I drink?
Drinking water and other liquids may reduce the risk of bladder cancer, as water dilutes the concentration of cancer-causing agents in the urine and shortens the time in which they are in contact with the bladder lining. Drinking at least 8 cups of liquid a day is usually recommended, and some studies show that even more may be helpful.
