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  Common Questions About Diet and Cancer  
  Common Questions About Diet and Cancer
Because people are interested in the relationship of specific foods or nutrients to specific cancers, research on dietary factors 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 context. Therefore, it is rarely, if ever, advisable to change diet or activity levels based on a single study or news report, especially if the data are reported as 鈥減reliminary.鈥 The following questions and answers address common concerns about diet and physical activity in relation to cancer.


Does alcohol increase cancer risk?

Alcohol increases the risk of cancers of the mouth, pharynx, larynx, esophagus, liver, and breast. 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 cancer 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. Women at high risk of breast cancer may want to consider not drinking any alcohol.


What are antioxidants, and what do they have to do with cancer?

Certain nutrients in vegetables and fruits appear to protect the body against the damage to tissues that occurs constantly as a result of normal metabolism. Because such damage is associated with increased cancer risk, these so-called antioxidant nutrients are thought to protect against cancer. Antioxidants include vitamin C, vitamin E, selenium, carotenoids, and many other phytochemicals (chemicals from plants). Studies suggest that people who eat more vegetables and fruits, which are rich food sources of these antioxidants, have a lower risk for some types of cancer. Clinical studies of antioxidant supplements are currently underway but have not yet shown a reduction in cancer risk from vitamin supplements (also see beta-carotene, lycopene, selenium, vitamin C, vitamin E, supplements). To reduce cancer risk, the best advice presently is to consume antioxidants through food sources, rather than supplements.


Does aspartame cause cancer?

Aspartame is a low-calorie artificial sweetener that is about 200 times sweeter than sugar. Currently, 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.


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 associated with a reduced risk of cancer, it seemed plausible that taking high doses of beta-carotene supplements might reduce cancer risk. In three major clinical trials, people were given high doses of synthetic beta-carotene in an attempt to prevent lung cancer and other cancers. Two studies found beta-carotene supplements to be associated with a higher risk of lung cancer in cigarette smokers,and a third found neither benefit nor harm from beta-carotene supplements. Therefore, consuming vegetables and fruits that contain beta-carotene may be helpful, but high-dose beta-carotene supplements may actually be harmful, especially for cigarette smokers.

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鈥檚 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. However, 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.


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). However, there is also evidence that a high-calcium intake, primarily through supplements, is associated 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 1000 mg/day for people ages 19 to 50 years and 1200 mg/day for people over 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 fat-free choices to reduce their intake of saturated fat. Those wishing to take calcium supplements should consult their health care professional.

Does cholesterol in the diet increase cancer risk?

Cholesterol in the diet comes only from foods from animal sources鈥攎eat, dairy, eggs, and animal fats. At present, there is little evidence that cholesterol itself causes the increased risk of certain cancers associated with eating foods from animal sources. Lowering blood cholesterol reduces the risk of cardiovascular disease, but there is no evidence that lowering blood cholesterol has an effect on cancer risk.


Does drinking coffee cause cancer?

Caffeine may heighten symptoms of fibrocystic breast lumps (a type of benign breast disease) in some women, but no evidence has shown that it increases the risk of breast cancer or other types of cancer. The association between coffee and cancer of the pancreas, widely publicized in the past, has not been confirmed by other studies.


Will eating less fat lower cancer risk?

Diets high in fat are also high in calories and contribute to obesity, which in turn is associated with an increased risk of cancer at several sites. Although all types of fats have similar numbers of calories, there are certain types, such as saturated fats, that may have a greater effect on increasing cancer risk. Fats containing omega-3 fatty acids (see fish oils) may reduce cancer risk.


What is dietary fiber, and can it prevent cancer?

Dietary fiber includes a wide variety of plant carbohydrates that are not digested 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. Associations between fiber and cancer risk are inconclusive, but eating these foods is still recommended because they contain other substances that can help prevent cancer and because they have other health benefits.

Fish Oils

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, and this may also be a possible benefit in humans. Research has not yet demonstrated possible benefit by taking omega-3 or fish oil supplements.


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. Fluorides do not increase cancer risk.

Folic Acid

What is folic acid, and can it prevent cancer?

Folic acid (closely related to folate or folacin) is a B vitamin found in many vegetables, beans, fruits, whole grains, and fortified breakfast cereals. Since 1998, all grain products in the US have been fortified with folic acid. Too little folic acid may increase the risk of cancers of the colon, rectum, and breast. Current evidence suggests that to reduce cancer risk, folic acid is best obtained by eating fruits, vegetables, 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. 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.


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, mostly because garlic has very small effects on blood cholesterol levels. Garlic is currently under study for its ability to reduce cancer risk. There is insufficient evidence to support a specific role for this vegetable in cancer prevention.


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 underway in this area.

Irradiated Foods

Why are foods irradiated, and do irradiated foods cause cancer?

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.


Will lycopene reduce cancer risk?

Lycopene is the red-orange carotenoid (antioxidant) found at high levels in tomatoes and tomato-based foods. Several studies have reported that consuming tomato products reduces the risk of some cancers, but whether lycopene is the nutrient responsible is uncertain. Absorption of lycopene is increased when lycopene-rich vegetables are cooked and are consumed together with fats, although only very small amounts of fat are needed for absorption. There is no evidence that high doses taken as supplements are safe or effective in cancer prevention.

Meats -- Cooking and Preserving

Should I avoid preserved meats?

Some studies have linked eating large amounts of preserved meat to increased risk of colorectal and stomach cancers. This association may or may not be due to nitrites, which are added to many lunch meats, hams,and hot dogs to maintain color and to prevent contamination with bacteria. Nitrites can be converted in the stomach to carcinogenic nitrosamines, which may increase the risk of stomach cancer. Diets high in vegetables and fruits that contain vitamin C and phytochemicals retard the conversion of nitrites to nitrosamines. Eating meats preserved by methods involving smoke or salt should be reduced to lessen exposure to potential cancer-causing agents.

How does cooking meat affect cancer risk?

Adequate cooking is necessary to kill harmful microorganisms in meat. However, some research suggests that frying, broiling, or grilling meats at very high temperatures creates chemicals that might increase cancer risk. Although these chemicals cause cancer in animal experiments, it is uncertain whether they actually cause cancer in people. Techniques such as braising, steaming, poaching, stewing, and microwaving meats produce fewer of these chemicals.


Does being overweight increase cancer risk?

Yes. Being overweight or obese is associated with an increased risk for developing cancers of the breast (among postmenopausal women), colon, endometrium, gallbladder, esophagus, pancreas, kidney, and possibly other sites as well.

Olive Oil

Does olive oil affect cancer risk?

Consumption of olive oil is not associated with any increased or decreased risk of cancer. Olive oil is a significant source of calories and should be used in moderation.

Organic Foods

Are foods labeled
鈥渙rganic鈥 best in lowering cancer risk?

The term 鈥渙rganic鈥 is popularly used to designate plant foods grown without pesticides and genetic modifications. At present, 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.

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. Obesity and being overweight have been linked to many types of cancer, and physical activity is a key factor in maintaining or achieving a healthful body weight. In addition, physical activity has beneficial effects against cardiovascular disease and diabetes.


What are phytochemicals, and do they reduce cancer risk?

Phytochemicals refer to a wide variety of compounds produced by plants. Some of these compounds protect plants against insects or perform other essential 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 fruits, vegetables, beans, and grains from which they are extracted.


Does saccharin cause cancer?

In rats, high doses of the artificial sweetener saccharin can cause bladder stones to form that can lead to bladder cancer. However, saccharin consumption does not cause the formation of bladder stones in humans. If saccharin does increase cancer risk in humans, it does so at doses many times greater than amounts typically consumed. Large population studies have not reported increases in bladder cancers among people using saccharin, and the US National Toxicology Program has removed it from the list of established human carcinogens.


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 salt used in cooking or in flavoring foods affects cancer risk.


What is selenium, and can it reduce cancer risk?

Selenium is a mineral, as well as an antioxidant, that is needed by the body as part of its 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. However, 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 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. Most of the human studies compare Asian and Western populations, however, whose differences in cancer risk may be due to factors other than soy consumption. No solid evidence has shown that soy supplements reduce cancer risk, although human studies are inconclusive at this time. Another theory suggests that high doses of soy might increase the risk of estrogen-responsive cancers, such as breast or endometrial cancer in certain women. Breast cancer survivors should consume only moderate amounts of soy foods as part of a healthful, plant-based diet.


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 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.


Can nutritional supplements lower cancer risk?

A diet rich in fruits, vegetables, and other plant foods may reduce the risk of cancer, but no evidence at this time suggests that supplements can reduce cancer risk. Many healthful compounds are found in vegetables and fruits, and these compounds most likely add to their beneficial effect. In many cases, it is not known which compounds or combination of compounds is most effective in reducing cancer risk. There are likely to be important, but as yet unidentified, components of whole food that are not included in supplements. The few clinical trials testing whether supplements can reduce cancer risk in humans have yielded disappointing results. Vitamin and mineral supplements have been shown to reduce the risk of stomach cancer in studies of malnourished people in China and South America, but other studies using high doses of single nutrients have shown no benefit and even harm (see beta-carotene). Food is the best source of vitamins and minerals. Supplements, however, may be beneficial 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 鈥淒aily Value鈥 of most nutrients, because high doses of some nutrients can have harmful effects.

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. In many cases, it is not known which compounds or combination of compounds is most effective in reducing cancer risk. There are likely to be important, but as of yet unidentified, components of whole foods that are not included in supplements. In addition, 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.


Can drinking tea reduce cancer risk?

Some researchers have suggested that tea might protect against cancer because of its content of antioxidants (see antioxidants). In animal studies, some teas (including green tea) have been shown to reduce cancer risk, but findings from population studies are mixed. Presently, tea has not been proven to reduce cancer risk in humans.


Do trans-saturated fats increase cancer risk?

Trans-saturated fats are produced 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. However, their relationship to cancer risk has not been determined.

Vegetables and Fruits

Will eating vegetables and fruits lower cancer risk?

In the majority of population studies, greater consumption of vegetables, fruits, or both together has been associated with a lower risk of lung, oral, esophageal and colon cancer. The best advice is to eat five or more servings of vegetables and fruit each day.

What are cruciferous vegetables, and are they important in cancer?

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 (also see phytochemicals).

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. However, frozen foods can be more nutritious than fresh foods because they are often picked ripe and quickly frozen. Canning is more likely to reduce the heat-sensitive and water-soluble nutrients because of the high heat necessary in processing. Be aware that some fruits are packed in heavy syrup, and some canned vegetables are high in sodium. 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 may be 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 absorption of some of the nutrients in vegetables and fruits. However, juices may be less filling than whole vegetables and fruits and may contain less fiber; also, fruit juice, in particular, can contribute lots of calories. Commercially juiced products should be 100% fruit or vegetable juices and should be pasteurized to eliminate harmful microorganisms.

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 (e.g., flavonoids). It is not possible to conclude at this time, however, that a vegetarian diet has any special benefits for the prevention of cancer. Vegetarian diets differ in composition, although all avoid red meat. A vegetarian diet can be quite healthful if it is carefully planned and provides adequate calories. Diets including lean meats in small to moderate amounts can be just as healthful. The greater the restriction of food groups in a particular diet, the more possibility there is of dietary deficiencies. Strict vegetarian diets that avoid all animal products, including milk and eggs, should be supplemented with vitamin B, 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 two ways: preformed from animal food sources and derived from beta-carotene in plant foods. Vitamin A is needed to maintain healthful 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.

Vitamin C

Does vitamin C lower cancer risk?

Vitamin C is found in many vegetables and fruits. Many studies have linked consumption of foods rich in vitamin C to a reduced risk for cancer. The few studies in which vitamin C has been given as a supplement, however, have not shown a reduced risk for cancer.

Vitamin E

Does vitamin E lower cancer risk?

In one clinical trial, men who took vitamin E (50 mg/day) had a lower risk of prostate cancer compared with men who took a placebo, but other studies are needed to confirm this observation.

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 carcinogens 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 beneficial.
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