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NEWS & VIEWS |
High consumption of red or processed meats has been linked to colorectal cancer in numerous studies. Recent work by ACS epidemiologists helps put the relationship into perspective.
"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 coauthor Michael Thun, MD, MS, Vice President for Epidemiology and Surveillance Research at the 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 [from meat consumption] 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 JAMA (2005;293:172–182), are based on a long-term study of nearly 149,000 men and women aged 50 to 74 in the Cancer Prevention Study (CPS) II Nutrition Cohort, a subset of the CPS II Mortality Cohort. 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, processed meat, poultry, or fish the people had eaten.
People who ate the most red meat in both time points (1982 and 1992/1993) were 30% to 40% more likely to develop cancers of the lower colon or rectum than those who ate the least red meat. Those who ate the most processed meats were 50% more likely to develop cancers of the lower colon (descending and sigmoid) compared with those who ate the least. These analyses compared risk among people in the top third (highest tertile) of meat consumption with those in the bottom third (lowest tertile).
For red meat (beef, lamb, pork), the cutoff for the highest tertile was three or more ounces per day for men—approximately the amount of meat in a large fast-food hamburger. For women, the top tertile started at two or more ounces per day. For processed meat (bacon, sausage, hot dogs, ham, cold cuts), the highest tertile started at one ounce eaten five or six days per week for men, and two or three days per week for women. A slice of bologna weighs about one ounce; two slices of cooked bacon weigh a little more than half an ounce.
Eating poultry and fish did not raise the risk of colon cancer. To the contrary, people in the highest tertile of poultry and fish consumption were 30% less likely to develop descending and sigmoid colon cancers than were people in the lowest tertile. The ratio of red meat to poultry and fish consumption was also predictive; compared with those in the lowest tertile, people in the highest tertile were about 50% more likely to be diagnosed with cancers of the descending and sigmoid colon.
The study findings add weight to current ACS dietary guidelines, which recommend limiting red meat in favor of other sources of protein like 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.
This study did not address what it is about red meat that might influence cancer risk. The iron and fat it contains may be culprits. Or it may have to do with the way the meat is prepared; cooking at high temperatures can create carcinogenic heterocyclic amines. For processed meat, the salt, smoke residue, and nitrates and nitrites used as preservatives may play a role.
For these reasons, the ACS guidelines advise people who do eat red meat to choose smaller portions and lean cuts, and bake, broil, or poach the meat rather than frying or charbroiling it.
"The overall message is that less is better," Thun said, "but it depends where you’re starting from. If you’re eating red meat three 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."
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