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Diet and Cancer

by Bruce A. Chabner, MD, Elizabeth Chabner Thompson, MD, MPH

Many studies have tried to determine whether eating specific foods increases or decreases a person's risk of getting cancer. Unfortunately, different studies have 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 What Participants Need to Know About Clinical Trials) 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.

Although the effects of specific foods and supplements on the risk of getting cancer are unclear, there is good evidence that obesity increases the risk of a number of cancers.

Antioxidants

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.

Bioengineered 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.

Calcium

Some studies have found that higher vitamin D levels and calcium supplements may reduce the risk of precancerous polyps of the colon.

Coffee

Although some older studies appeared to show a link between coffee consumption and cancer risk, more recent studies have not shown any connection.

Saturated fats

Saturated fats may increase cancer risk. 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.

Fiber

There is little evidence that eating a diet that is high in fiber reduces cancer risk.

Fish and omega-3 fatty acids

Some recent studies suggest that omega-3 fatty acids may increase risk of prostate cancer.

Fluoride

Studies have not shown an increased risk of cancer in people who drink fluoridated water or who use toothpastes or undergo dental fluoride treatments.

Folate

In some studies, folate (folic acid) appears to help protect against colon cancer.

Food additives

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.

Garlic

Scientific studies have not shown that garlic is effective in reducing the risk of cancer.

Irradiated foods

Radiation of food, which is sometimes used to kill microorganisms in food, does not appear to increase cancer risk.

Lycopene

Some studies suggest that lycopene, which is found mainly in tomatoes, may reduce the risk of some cancers, but the evidence is very weak.

Processed meats

People who eat large amounts of processed meats may be at risk for stomach and colon and rectal cancers. Some investigators attribute this finding to nitrates, which are in luncheon meats, hams, and hot dogs. This connection is unproved. Eating meats processed by salting or smoking may increase exposure to potential cancer-causing substances.

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.

Organic food

Whether eating foods grown with organic methods reduces cancer risk is not yet known.

Pesticides

There is no evidence that pesticide residue found in small amounts on foods increases the risk of cancer.

Saccharin

Saccharin does not cause cancers in people.

Salt

Diets containing large amounts of food that has been preserved by pickling or salting may increase the risk of stomach and throat cancer. No studies have found a similar risk for a small or moderate amount of salt for flavor.

Selenium

Although earlier studies suggested some benefit, more recent studies failed to show that selenium protects against cancer.

Soy

Studies do not yet show that soy supplements reduce cancer risk. Also, some studies show that high doses of soy may increase the risk of cancers that respond to estrogen , such as some breast cancers.

Tea

Tea has not been shown to reduce cancer risk.

Vitamin D

Vitamin D may have some benefit in reducing the risk of prostate cancer and colorectal cancer.

Vitamin E

Vitamin E supplements do not appear to protect against cancer and may increase the risk of prostate cancer.