Many studies have tried to determine whether specific foods increase or decrease a person's risk of getting cancer. Unfortunately, different studies have had conflicting results, so it is hard to know what effect foods or dietary supplements have on cancer risk. A common problem is that when studies find that people who eat more of a certain food seem to have lower rates of a certain cancer, it can be difficult to tell whether those people also were different in terms of other risk factors (such as where they live, how much they smoke and drink, and so forth). Often, when doctors do a controlled trial (see also The Science of Medicine What Participants Need to Know About Clinical Trials People expect doctors to use treatments that work well and to stop using those that do not. However, it is often difficult for doctors and other scientists to tell which treatments work. Making... read more ) and randomly give some people a seemingly helpful food or supplement, the studies do not show a beneficial effect. Some foods and supplements have been studied more than others, and many studies are ongoing. The most convincing evidence is from studies that show diets low in fiber and high in processed meats increase cancer risk. Obesity, regardless of the type of diet, increases the risk of many cancers.
Alcohol increases the risk of cancers of the mouth, throat, esophagus, liver, breast, and the colon and rectum. People who smoke as well as drink have a much higher risk of these cancers.
Antioxidants, such as vitamins C and E and beta-carotene (vitamin A), are part of a well-balanced diet. However, studies have not shown that taking supplements containing these antioxidants decreases the risk of cancer. There is some evidence that taking high doses of beta-carotene or vitamin E supplements may increase the risk of certain types of cancer.
Although some early studies show an increased risk of bladder cancer, brain cancer, and lymphomas with certain sweeteners, these studies were done in animals. No studies in humans show an increased risk of cancer with the use of these sweeteners.
Bioengineered foods (genetically modified [GMO] foods)
Genes from different plants or from certain microorganisms are added to the genes of some plants to increase the plants’ hardiness or resistance to pests or to improve them in some other way. No current evidence demonstrates that bioengineered foods have any effect on cancer risk.
Some studies have found that higher vitamin D levels and calcium supplements may reduce the risk of precancerous polyps of the colon. However, other studies suggest that a high calcium intake increases the risk of prostate cancer.
Although some older studies appeared to show a link between coffee consumption and cancer risk, more recent studies have not shown any connection.
Some studies suggest that eating a diet that is high in fiber reduces the risk of cancer, especially colorectal cancer.
Fish and omega-3 fatty acids
Some recent studies in animals suggest that omega-3 fatty acids may stop cancers from growing or slow their growth. However, these findings have not been replicated in humans.
Studies have not shown an increased risk of cancer in people who drink fluoridated water or who use toothpastes or undergo dental fluoride treatments.
Some evidence indicates a higher cancer risk in people with folate (folic acid) deficiency, but whether the deficiency is the cause of cancer is unknown. In contrast, other less conclusive evidence suggests that excess folate may increase cancer risk. A person eating a normal diet requires no additional folate.
Food additives must be approved by the Food and Drug Administration before they are included in foods, so new additives undergo extensive testing. So far, no evidence shows that the levels of additives found in food products increase the risk of cancer.
Scientific studies have not shown that garlic is effective in reducing the risk of cancer.
Radiation of food, which is sometimes used to kill microorganisms in food, does not increase cancer risk.
Some studies suggest that lycopene, which is found mainly in tomatoes, may reduce the risk of some cancers, but the evidence is very weak.
Meats cooked at high temperatures
Eating meat cooked at high temperatures, for example by grilling or broiling, may introduce cancer-causing chemicals and increase cancer risk.
There is no evidence that organically grown foods reduce cancer risk more than the same foods grown by other methods.
Obese people have higher risks of diverse cancers.
There is no evidence that pesticide residue found in small amounts on foods increases the risk of cancer.
People who eat large amounts of processed meats may be at risk for stomach and colon and rectal cancers. Some evidence suggests that this is caused by nitrates in luncheon meats, hams, and hot dogs.
Some studies have found higher rates of some types of cancers in countries where fat intake is higher. However, no studies have found that decreasing fat intake decreases the risk of cancer. Of more importance, however, is that foods that contain high levels of saturated fats also contain many calories and may contribute to obesity, which is a risk factor for cancer and other health problems.
There is no convincing evidence that selenium reduces cancer risk.
There is no convincing evidence that spices such as tumeric, capsaicin (red pepper), cumin, or curry decrease cancer risk.
There is no convincing evidence that regular or green tea decreases cancer risk.
Vitamin D when taken with omega-3 fatty acid may decrease risk of death from cancer but does not decrease risk of developing cancer. Any potential benefit is greater in Blacks.
There is no convincing evidence that vitamin E supplements decrease cancer risk, and some evidence suggests an increased risk of prostate and other cancers.
Additional information about Diet and Cancer
The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
American Cancer Society: Stay Healthy: The American Cancer Society provides tips for people to make healthy choices and reduce the risk of cancer
National Cancer Institute: Cancer Causes and Prevention: The National Cancer Institute provides information on nutrients that may be associated with increases or decreases in the risk of cancer