Eating processed meat such as sausage and bacon can cause cancer in humans, the World Health Organization's cancer agency says.
Monday's official designation from the France-based International Agency for Research on Cancer (IARC), part of the WHO, places processed meat in the Group 1 list as "carcinogenic to humans." The agency said it based the classification on sufficient evidence for colorectal cancer.
Tobacco, asbestos and diesel fumes are also on the Group 1 list.
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Processed meat refers to salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation. Most processed meats contain pork or beef. Examples include sausage, canned meat, beef jerky and anything smoked.
"Our expert working group estimated that the consumption of processed meat increases the risk of colorectal cancer about 18 per cent per 50 gram portion eaten daily," said Dana Loomis, deputy head of the monographs section at the International Agency for Research on Cancer in Lyon, France. "In relative terms, that's about one-tenth of the cancer risk associated with cigarette smoking."
To put the risk into perspective, Cancer Research UK notes out of every 1,000 people in the U.K., about 61 will develop bowel cancer at some point in their lives. Among 1,000 people who eat a lot of processed meat, you'd expect 66 to develop bowel cancer at some point in their lives — 10 more than the group that eats the least processed meat.
Since red meat isn't established as a cause of cancer, the figure is less certain, Loomis said. A serving of meat is about 50 to 75 grams, about the size of a human palm.
Consumption of processed meat is also associated with stomach cancer, according to the report.
Doctors also warned that a diet high in red meat is linked to cancers, including those of the colon and pancreas.
Red meat includes mammalian muscle meat such as beef, veal, pork, lamb, mutton, horse, and goat meat, under the definitions of the report. Scientists classified it as a "probable" carcinogen based on its association with colorectal cancer as well as cancers of the pancreas and prostate.
The agency's carcinogen categories reflect the quality of scientific evidence, not the level of risk.
The group of 22 scientists from WHO's cancer agency also noted red meat contains important nutrients such as iron and zinc.
How meat is cooked makes a difference. The report's experts said grilling, pan-frying or other high-temperature ways of cooking red meat produce the highest amounts of chemicals suspected to cause cancer.
The cancer agency noted research by the Global Burden of Disease Project suggests 34,000 cancer deaths per year worldwide are linked to diets heavy in processed meat. In comparison, one million deaths a year are linked to smoking, 600,000 a year to alcohol consumption and 200,000 a year to air pollution.
Ian Johnson, a nutrition researcher with the Institute of Food Research who is unconnected to the IARC findings, said meat consumption is one of many factors contributing to high rates of bowel cancer in the U.S., western Europe and Australia.
Sian Beven of the Canadian Cancer Society called the information from the new report important in helping individuals to choose their diet.
"There are a number of different things that contribute to cancer risk, and in this case, in respect to red meat, we continue to encourage people to limit their consumption of red meat to about three servings per week and really to be careful and to avoid processed meat, really save that for particular occasions if you're to consume it," Bevan said.
Researchers are still trying to understand the limited evidence for how red meat is related to cancer risk, she said.
It isn't known whether the cancer risk is lower for newer processed meat alternatives such as turkey bacon or as result of livestock practices such as grass-fed beef, Loomis said.
The North American Meat Institute said in a statement that "cancer is a complex disease not caused by single foods."
Dave Meli, an executive head butcher in Toronto, welcomed the findings. "I would rather eat a very small quantity of the best-tasting animal than eat large quantities of the worst-tasting animal," he said.