Dietary Factors

Previous research suggested that diets low in fruits and vegetables and high in meats pose a risk for colon cancer, and that those rich in fruits and vegetables are protective against many cancers.

Fruits, Vegetables, and Whole Grains

There has been a prevailing belief based on several studies that high intake of fruits and vegetables can lower the risk for colorectal cancer. Studies have been mixed, however, on their benefits. A 2002 study, for example, reported that these foods do not prevent polyps from forming but may help prevent them from becoming cancerous.

Phytochemicals. Many studies have demonstrated the cancer-fighting effects of plant chemicals called phytochemicals. Fruits and vegetables that contain phytochemicals can often be identified by colors:

  • Dark green (broccoli, spinach, kale, collard greens, mustard greens). These specific vegetables contain chemicals called isothiocyanates, which have been associated with a lower risk for cancer in general.
  • Red (red pepper, tomatoes, watermelon, raspberries, pink grapefruit). Lycopene is a chemical found in these foods that may have strong cancer-protective properties. Cooking tomatoes appears to increase their benefits.
  • Yellow-orange (carrots, pumpkin, sweet potatoes, oranges, tangerines). The colors in these foods are due to carotenoids. Carotenoids have been associated with health protection, although they may not have much effect on colon cancer itself.
  • Blue-black (many berries). Dark berries appear to have potent chemicals that may be protective against cancer. In one animal study, extracts from black raspberries reduced colon cancer tumors in rats.

Organosulfurs are important food chemicals that are part of the allium family. Studies have reported health benefits from foods containing them. These compounds are found in garlic, leeks, onions, chives, scallions, and shallots. A review of 300 studies concluded that people who eat raw or cooked garlic regularly experience about two-thirds the risk of colorectal cancer as people who eat little or none. Another analysis, however, found the available evidence about garlic to be inconclusive. Garlic supplements, in any case, do not appear to be protective.

Fiber. Studies have been mixed on whether fiber (found in fruits, vegetables, and whole grains) protects the colon from cancer. For example, three major studies in 2002 and 2003 reported no difference in the development of colorectal polyps or cancer recurrence with high intake of fiber. On the other hand, other studies have been positive. In fact, 2003 results of the European Prospective Investigation into Cancer and Nutrition (EPIC) -- the largest study ever conducted on the role of diet in the development of cancer -- suggested that fiber is protective regardless of its source. However, in the study, the greatest benefits were observed for the left side of the colon and the least for the rectum. In any case, fiber, which is only found in plant products, may be beneficial for the heart and have other health advantages.

Fats and Oils

The role of fats in inflammatory bowel disease is complex and not fully known. A 2006 study from the Women’s Health Initiative found that a low-fat diet did not help reduce the risk for colorectal cancer. However, the study did not distinguish between types of fat.

  • Monounsaturated (olive, peanut, canola oils; avocados, nuts) and omega-3 polyunsaturated (fish, flaxseed oil, walnuts) fats are the healthiest types of fats.
  • Saturated fats (red meat, butter, high-fat dairy products) and trans-fats (hydrogenated fat found in snack foods, fried foods, commercial baked goods) are unhealthy types of fats.

Current dietary guidelines recommend that adults limit the total fat in their diet to between 25 – 35% of total daily calories. Saturated fat intake should be less than 7%, and trans fats less than 1%, of total daily calories. (Patients with heart disease or diabetes may need to limit unhealthy fat in their diet even further.) Most fats should come from polyunsaturated and monounsaturated fat sources.

[For more information on fats, see In-Depth Report #43: Heart healthy diet; and #42: Diabetes diet.]

Meat and High-Temperature Cooking

Evidence suggests that red meat raises the risk for colon cancer. Red meat contains dietary iron, which has been associated with a higher risk for colon cancer. In fact, early results in 2000 from the largest study on diet and cancer to date have supported previous studies linking red meat with intestinal tumors.

High-temperature cooking (grilling, broiling, or pan-frying) has been specifically associated with increased risk for colon polyps and colon cancer. Over-cooking meat increases the amount of carcinogens called heterocyclic amines, which has been associated with cancerous changes.

Dairy Products and Calcium

Milk, Lactose, and Probiotics. In a 2001 study, adults who drank the most milk had the lowest risk for colon cancer. A 2004 study published in the Journal of the National Cancer Institute supports this conclusion. In this review of 10 epidemiologic studies that included more than 500,000 people, those who consumed more milk and calcium had a lower risk of developing colorectal cancer. Milk contains not only calcium but also other compounds, such as lactose, that may help protect against colon cancer.

Yogurt specifically has been associated with a lower risk for colon cancer if it contains live active bacterial cultures, such as Lactobacillus acidophilus, that are called probiotics. These "friendly bacteria" appear to protect the colon from cancerous changes. (Acidophilus and other probiotic capsules are also available in health food stores.)

Calcium. Calcium, which is found in dairy products, is associated with colon cancer protection. Many studies have shown a possible protective effect from either high-calcium diets or calcium supplements. However, a 2006 study from the Women’s Health Initiative found that calcium and vitamin D supplements do not reduce women’s colorectal cancer risk. Experts still recommend that postmenopausal women take these supplements for bone health.

Total Calories and Sugar

Obesity has been associated with colon cancer. In some studies of people under 67 years old, the amounts of fat and protein were less important than the total number of calories consumed: the higher the energy intake, the greater the risk for developing colon cancer. In older adults, high calorie intake did not make any significant difference. Other studies have indicated that excessive sugar-intake may increase the risk for colon cancer.

Coffee and Tea

Studies conducted in several countries have found that drinking four or more cups of coffee a day is associated with a lower risk for colorectal cancer. Green tea may have beneficial properties, but more research is needed in both of these areas.

Vitamin and Mineral Supplements

Folate and B Vitamins. There is evidence that the B vitamin folate (called folic acid) is protective. Both folate and vitamin B12 convert the amino acid homocysteine to methionine, a chemical that protects certain genes that help prevent cells from becoming cancerous. Folate is found in beans, citrus fruits, and green vegetables, but benefits seem higher when taking supplements. The protective effect appears to be greatest for people who are genetically predisposed to colorectal cancer.

Antioxidant Supplements. Antioxidants are chemicals that help eliminate harmful particles called oxygen-free radicals that have been associated with cancerous changes. Some studies have associated supplements of the antioxidants selenium and vitamins A, C, D, and E with lower colon cancer risk, but most studies have found no protective effect.

Review Date: 9/1/2006
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital