People who eat a lot of red meat or processed meats may be raising their risk for colon cancer. Although this link has been shown before, a new study by American Cancer Society researchers helps explain the relationship.

"Our study was better able to separate the risk associated with meat consumption from that associated with other factors that affect colorectal cancer risk, especially obesity and physical inactivity," said co-author Michael Thun, MD, MS, chief of epidemiology and surveillance research at ACS.

The verdict: Eating large amounts of red or processed meat over a long period of time can indeed raise colorectal cancer risk. But the risks from such a diet are smaller than those from obesity and lack of exercise, both for colon cancer and for overall health.

"While these risks to overall health are not in the same league as the risk from smoking, obesity, and physical inactivity," said Thun, "these findings are important because red and processed meat are major components of the diet of many Americans, and because there is now substantial evidence that long-term high consumption increases the risk of colon cancer."

The findings, published in this week's Journal of the American Medical Association (Vol. 293, No. 2: 172-182), are based on a long-term study of nearly 149,000 adults between the ages of 50 and 74. The participants filled out a questionnaire about their eating habits in 1982, and again in 1992/1993. Thun and his colleagues looked at how many people had developed colon cancer by 2001, then analyzed the risk according to how much red meat, poultry, or fish the people had eaten.

The people who ate the most red meat in both time periods were 30%-40% more likely to develop cancer in the lower part of the colon, compared to people who ate the least. People who ate the most processed meats were 50% more likely to develop colon cancer and 20% more likely to develop rectal cancer compared to those who ate the least.

So how much meat are we talking about?

For red meat (beef, lamb, pork), the researchers defined "high" consumption as 3 or more ounces per day for men -- or about the amount of meat in a large fast-food hamburger. For women the "high" amount was 2 or more ounces per day. For processed meat (bacon, sausage, hot dogs, ham, cold cuts) "high" consumption was 1 ounce eaten 5 or 6 days per week for men, and 2 or 3 days per week for women. A slice of bologna weighs about 1 ounce; 2 slices of cooked bacon weigh a little more than half an ounce.

A person who eats red meat a couple of times a week would have been in the lowest consumption group in the study, Thun said.

Eating poultry and fish did not raise the risk of colon cancer. In fact, people who ate more poultry and fish than red meat were less likely to develop the disease.

The study could not compare meat eaters with people who ate no meat because too few participants reported eating no meat.

The study findings add weight to ACS dietary guidelines, which recommend limiting red meat in favor of other sources of protein, such as poultry, fish, or beans, Thun said.

"This is not a condemnation of red meat, but it is part of a growing body of evidence that red meat shouldn't be the mainstay of your diet," he explained.

Researchers aren't certain what it is about red meat that might influence cancer risk. The iron and fat it contains may be culprits. For processed meat, the salt, smoke residue, and nitrates and nitrites used as preservatives may play a role. Or it may have to do with the way the meat is cooked; high temperatures can create higher levels of cancer-causing substances in the meat.

For these reasons, if you do eat red meat, the ACS guidelines advise choosing smaller portions and lean cuts and baking, broiling, or poaching the meat rather than frying or charbroiling it.

The ACS study did not look at how people cooked their meat, only how much they ate.

"The overall message is that less is better," Thun said, "but it depends where you're starting from. If you're eating red meat 3 times a day, it's reasonable to scale back to once a day. If you're eating it once a day, it's reasonable to scale back to a few times per week."

Other steps you can take to lower your risk of colon cancer include not smoking, maintaining a healthy body weight, getting regular exercise, and following ACS screening guidelines, Thun said. Screening can detect colon cancer early, when it is easier to treat, and can even prevent it by finding and removing colon growths that could become cancerous.

Colon cancer is the third most common cause of cancer cases and deaths in both men and women in the US, striking more than 145,000 people and killing more than 56,000 each year.