Foods make up the main parts of the diet, such as meat, dairy, grains, fruits and vegetables, and legumes (beans). The nutrients in those foods that the human body turns into energy are called macronutrients. They include fat, carbohydrate, and protein. (Alcohol is also considered a macronutrient. Learn more about Alcohol and Cancer).

Finding the association with cancer

Many foods and nutrients have long been studied for cancer prevention, but finding a specific link between a food or nutrient and cancer is difficult. One challenge is that foods contain many components, both nutrients and non-nutrients, that may (or may not) contribute to cancer prevention. Second, most people eat a variety of foods and drinks, creating interactions that are challenging to study. Sometimes, nutrients have different effects on the body, depending on the amount of the nutrient and how much of it your body already has. For instance, consider how an athlete in training has different nutritional needs than someone who is not physically active. And, some research shows that food preparation may influence the risk or benefits of a food. For example, eating fish that has been preserved by salting or fermenting may increase risk for some types of cancer, but eating fish that hasn't been preserved seems to protect against several other types of cancer. All of these variables make studying food and cancer prevention complicated.

Currently, this is what is known about selected foods and macronutrients and cancer prevention:

Dietary fat. Dietary fat has often been linked to breast and colon cancers. A panel assembled by the American Institute for Cancer Research (AICR) in 1997 concluded that total fat and saturated fat may increase the risk of cancer. The largest study looking at the effects of dietary fat and cancer was the Women's Health Initiative (WHI). The WHI compared approximately 20,000 postmenopausal women who were assigned to a low-fat diet (20% of calories could be from fat) and 30,000 women who continued eating diets where up to 38% of the calories were from fat. After eight years of follow up, there was no difference in the number of new cases of breast cancer between the groups. As data from this study continue to be analyzed, it seems that a low-fat diet may slightly lower risk, but a true difference hasn't been determined yet. New cases of colon cancer were also measured, and there was no difference between the number of new cases of colon cancer in women who ate the lower-fat diet and the women who did not. In addition, smaller studies suggest that a low-fat diet may lower the risk of skin cancer returning, but the explanation for this is not well understood.

Dietary fiber. Fiber is a descriptive term for the indigestible compounds from plants. It comes from the outer layer of grains and is in fruits, vegetables, legumes, and nuts and includes substances such as cellulose, pectin, and lignin. Fiber helps add bulk to stool and helps move food more quickly through the digestive system. Studies show that high-fiber diets are linked to low cancer rates. However, it is difficult to know whether it is the fiber or the total effect of eating fruits, vegetables, and whole grains. Also, people who eat high-fiber diets tend to practice other healthy behaviors that lower cancer risk, such as doing regular physical activity.

Multiple studies have been done testing whether dietary fiber prevents colon cancer. At this time, high-fiber diets and/or supplements have no proven benefit against colon adenomas (polyps), but they may have some benefit against colon cancer. The Institute of Medicine (IOM) Food and Nutrition Board recommends that women and men consume 25 and 38 grams of fiber each day, respectively.

The AICR panel concluded dietary fiber may lower overall cancer risk, but that there is not enough evidence that cereals (grains, such as barley and oats) and resistant starches (a term for a small amount of starch that is not converted into sugar but passes into the large intestine) reduce cancer risk.

Protein and Dairy Foods

In addition to fat and fiber, protein and dairy products are important parts of the diet. The relationship between these foods and cancer is mixed.

• Meat, fish, shellfish, and eggs are the major sources of animal protein in most diets. Of those, red meat and processed meat are often studied as risk factors for cancer. Most of the studies suggest that people who eat more red meat have higher risk for developing colon cancer than those who eat less red meat, although the method of preparation (rather than simply the amount of meat) also appears to influence risk. When looking at all of the studies about red meat and cancer, the AICR panel found that red meat might be associated with a higher risk of cancer.
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• Dairy foods are a varied food group. Dairy foods are usually a good source of calcium, a mineral that is generally associated with a lower risk of disease, but many of these foods also contain saturated fat, which is associated with a higher risk of disease. Reduced-fat or nonfat dairy food choices are the best choices for including dairy in your diet. The bacterial ingredients of active yogurt products may help your digestive tract. The relationship between dairy foods and cancer is being investigated. Large analyses of multiple studies of dairy foods and prostate cancer have shown conflicting results: sometimes dairy increases the risk of prostate cancer and sometimes it has no effect. Dairy foods and calcium may protect against colon polyps and colon cancer. There has been no association found between breast cancer or ovarian cancer and dairy products.

In conclusion, it appears that eating fruits and vegetables may be helpful for preventing cancer, but the role of dietary fat and fiber is not as clear. It is likely that the related factors of physical activity and weight control play a role in cancer prevention along with diet. Read the general recommendations for diet and nutrition to learn more about improving overall health through diet.

More Information

Prevention: Vitamins and Minerals

Prevention: Plant-Based Foods

Prevention: Diet and Nutrition Resources

Additional Resources

American Institute for Cancer Research (www.aicr.org)