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You are what you eat.

It's a bit of folk wisdom that contains more than a kernel of truth. Diet has a powerful influence on many diseases, including America's number two killer, cancer. But because cancer is so complex, with many genetic and environmental factors affecting risk, the link between your menu and your risk has been hard to decipher. In the case of red meat and colon cancer, however, new research provides a plausible explanation for a long-suspected association.

Establishing a link

Although the results vary, studies from around the world have suggested that a high consumption of meat is linked to an increased risk of colon cancer. In some studies, fresh meat appears culpable; in others, it's processed, cured, or salted meat — but in all cases the worry is confined to red meat, not chicken.

The best evidence comes from a pair of large 2005 studies, one from Europe, the other from the United States. The European research tracked 478,000 men and women who were free of cancer when the study began. During nearly five years of follow-up, 1,329 people were diagnosed with colon cancer. The people who ate the most red meat (about 5 ounces a day or more) were about a third more likely to develop colon cancer than those who ate the least red meat (less than an ounce a day on average). Their consumption of chicken did not influence risk one way or the other, but a high consumption of fish appeared to reduce the risk of colon cancer by about a third. The effects of red meat and fish held up after the results were adjusted for other potential colon cancer risk factors, including body weight, caloric consumption, alcohol consumption, smoking, physical exercise, dietary fiber, and vitamins.

The U.S. study, sponsored by the American Cancer Society, added important information about the effects of long-term meat consumption. The subjects were 148,610 people between the ages of 50 and 74. Each person reported on his or her dietary patterns and health habits when the study began in 1982 and again, 10 to 11 years later. A high consumption of red and processed meats at both dates was linked with a substantial increase in the risk of cancer in the lower colon and rectum. Conversely, the long-term consumption of large amounts of fish and poultry appeared protective.

These two studies are impressive, and they don't stand alone. A meta-analysis of 29 studies of meat consumption and colon cancer concluded that a high consumption of red meat increases risk by 28%, and a high consumption of processed meat increases risk by 20%.

Red meat, prostate cancer, and other malignancies According to estimates, more than 79,000 American men were diagnosed with colorectal cancer in 2007, but prostate cancer was diagnosed in nearly three times more men. Some studies have reported a moderate to strong association between red meat consumption and the risk of prostate cancer; others have not found a link. Two studies provide perspective. One reported that a high consumption of red meat, especially cooked, processed meat, was associated with a doubling of prostate cancer risk in African American, but not white, men. The other found that very well done meat appeared to increase risk by 40%, but meat cooked at lower temperatures did not boost the risk. So for the prostate as well as the colon, it may be wise to cut down on red meat, especially processed, grilled, and barbecued meats. Other studies have linked red meat to stomach cancer, bladder cancer, and breast cancer. In contrast, diets low in meat have been associated with increased life expectancies. It's enough to make you think twice before you slap a few hot dogs and burgers on the grill next summer.

Seeing red

Scientists have offered a number of explanations for the link between red meat and colon cancer. One theory blames heterocyclic amines (HCAs), chemicals produced when meat is cooked at high temperatures. HCAs may play a role, but since high levels can also be present in cooked chicken, they are unlikely to be the whole explanation. Preservatives have also been implicated in the case of processed meats; nitrates are a particular worry, since the body converts them to nitrosamines, which are carcinogenic. But since fresh meat is also linked to colon cancer, preservatives can't be the whole answer.

Scientists from England have offered a new explanation. Their investigation recruited healthy volunteers who agreed to stay in a metabolic research unit where their diet could be carefully controlled and all of their fecal waste could be collected and analyzed. The volunteers ate one of three test diets for a period of 15 to 21 days. The first diet contained about 14 ounces of red meat a day, always prepared to minimize HCA formation. The second diet was strictly vegetarian, and the third contained large amounts of both red meat and dietary fiber.

Stool specimens from the 21 volunteers who consumed the high-meat diet contained high levels of N-nitroso compounds (NOCs), which are potentially cancer-causing chemicals. The 12 volunteers who ate vegetarian food excreted low levels of NOCs, and the 13 who ate meat and high-fiber diets produced intermediate amounts.

These results are interesting enough on their own, but Dr. Michelle Lewin and her colleagues went one step further. They were able to retrieve cells from the lining of the colon that are shed into the stool with every bowel movement as a normal event. The cells from people eating the high-meat diet contained a large number of cells that had NOC-induced DNA changes; the stools of vegetarians had the lowest number of cells with damaged genetic material, and the people who ate high-meat, high-fiber diets produced intermediate numbers of damaged cells.

Really red? Did you ever wonder what puts the red into red meat? The short answer is myoglobin, a protein that looks red when it binds with oxygen. But after a few days in the refrigerator, myoglobin gives up its oxygen and the meat turns brown. To keep meat looking rosy, manufacturers may pump in some carbon monoxide, which sticks to myoglobin like glue, keeping it red for weeks. Carbon monoxide is also used to keep tuna looking fresh, and a variety of additives are used to improve the appearance of other foods. The moral: Don't judge a food by its color.

Where's the beef?

The study from England showed that large amounts of red meat can produce genetic damage to colon cells in just a few weeks. It's an important finding, but it does not prove that red meat causes cancer. None of the cells were malignant, and the body has a series of mechanisms to repair damaged DNA. In most cases, the repairs are successful, but when they fail, cells can undergo malignant transformation.

Still, the research fits with earlier epidemiologic data raising a red flag about red meat. Instead of counting on your body to repair your damaged DNA, do everything you can to prevent damage in the first place.

In the case of colon cancer, there is quite a lot you can do. Keep your caloric intake reasonable and exercise regularly. Both of these choices appear to lower substantially the risk of colon cancer, and together they prevent obesity, a major cancer risk factor. Avoid tobacco in all its forms, and if you choose to drink, limit yourself to an average of no more than two drinks a day. Eat foods that have been associated with protection from colon cancer: a dietary pattern that includes good amounts of calcium from dairy products (low- or nonfat, please); vitamin D; fruits; vegetables; whole grains; and fish appear best. And yes, that dietary pattern eschews large amounts of red meat and other animal fats. Low-dose aspirin may also reduce risk. But even with all this, be sure to get the colon cancer screening tests that are appropriate for your age, family history, and risk factors.

Many American men were raised on a diet of meat and potatoes. Neither makes the grade as a health food, but a 2004 survey by the American Institute for Cancer Research found that 72% of adults center their meals around meat and other animal products.

You don't have to give up red meat to be healthy, but the evidence suggests that you'd be wise to limit your consumption. Learn to think of vegetables and grains as the main dish and meat as the side dish. Two 4-ounce portions a week should be safe; even then, choose lean cuts, trim away excess fat, and avoid charring your meat on a grill. Limit processed, cured, and salted meats as much as possible. Substitute fish and chicken or turkey (without the skin) for red meat as your main protein source, and experiment with beans as a source of protein, fiber, and vitamins. It may seem like a radical new diet, but if you change gradually and encourage your family to join you, you'll find that a healthful diet comes naturally and is tasty and enjoyable. Your colon will thank you — and so will your heart.

Disclaimer:

As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.