Colon and Rectal Cancers

Description

Alternative Names

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 from a number of 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.

It should be noted that it is nearly impossible to do controlled studies on dietary factors, since peoples eating habits can rarely be made consistent. Dietary studies also use a variety of different approaches to obtain results that make comparisons very difficult. To help determine their specific effects researchers are studying the phytochemicals (plant chemicals) in fruits and vegetables and also fiber (which is also found in whole grains.)

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, which have been associated with health protection, although 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 and there have been studies reporting 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. Any benefits or risks appear to depend on the type of compounds that make up fats (the fatty acids) or other nutrients or substances with fatty acids.

Saturated Fats and Trans-fatty Oils. Some studies had found an association between colon cancer and consumption of saturated fats (found primarily in animal fats). The association is not altogether clear, however, and more recent evidence has not supported a strong link. Some experts suggest that the real hazard is iron from red meat, which is often high in saturated fats and may have confused study results.

Of further interest, however, is a 2001 study that reported a possible link between colon cancer and trans fatty acids, which are partially hydrogenated oils found in stick margarine, fried foods and commercial baked goods. The association is supported by known chemical effects of these manufactured fats, and more research is warranted.

Monounsaturated Fats. Monounsaturated fats are mostly present in olive, canola, and peanut oils and in most nuts. Olive oil, for example, may protect the colon. Some evidence suggests that it reduces levels of deoxycholic acid, an acid found in bile that has tumor-promoting properties.

Polyunsaturated Fats. Polyunsaturated fats are found in both plant and fish oils, and are composed of essential fatty acids including omega-3 and omega-6 fatty acids. These fatty acids may play different roles in colon cancer.

• Omega-3 fatty acids are found in oily fish and canola oil, soybeans, flaxseed, and certain nuts and seeds. They have been associated with protection against inflammation in the intestinal tract. It should be noted, however, that not all studies show protection. For example, omega-3 fatty acids in fish are composed of docosahexaenoic (DHA) and eicosapentaneoic (EPA) acids. Some evidence suggests that EPA--although not DHA--may be protective.
• Omega-6 fatty acids, found in corn, safflower, soybean, and sunflower oil, constitute most of the oils consumed in the US. Some omega-6 fatty acids are important for health. However, a high intake has been associated with heart disease and certain cancers.

Currently it is reasonable to reduce saturated fats and transfatty acids and to favor monounsaturated oils and polyunsaturated oils, particularly containing omega-3 fatty acids.

Meat and High-Temperature Cooking

Some 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.

Some research has been focusing on acrylamide, a chemical found in high amounts in certain foods cooked at high temperatures, especially fried potatoes, and also bread products. Animal studies have suggested that acrylamide is a carcinogen. A surprising 2003 study, however, found no evidence of risk for colorectal or other cancers with high intake of foods that contain large amounts of this chemical.

Dairy Products and Calcium

Milk, Lactose, and Probiotics. In a Finish 2001 population 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 half a million people, people who consumed more milk and calcium had a lower risk of developing colorectal cancer. Milk not only contains 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.) Results are mixed on other fermented milk products, such as buttermilk and cheese, which in one study were associated with a higher risk. The reasons for this were not clear.

Calcium. Calcium, which is found in dairy products, is also associated with colon cancer protection. Most studies show a possible protective effect from either high-calcium diets or calcium supplements. The protective effect has been observed as early as one year after calcium supplementation began. A large 2002 study concluded that daily intake of about 700 mg, from food or supplements, reduces the risk of colon cancer, but intake beyond this level does not add any further protection. Calcium supplements may even offset certain effects of dietary iron, found in red meat and other foods, which may increase the risk for colon cancer. More work in this area is needed, however.

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 a number of 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 malignant. 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 the 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.