WHO report says processed meat causes cancer, confusion follows

Many news stories about a new WHO report classifying processed meat as a carcinogen and red meat as a probable carcinogen presented it as a report about risk. Here's how the journalists got confused.

Agency weighed the evidence on carcinogens, not the risk

A World Health Organization agency labelled processed meats like hot dogs as "carcinogenic to humans." (David Banks/Associated Press)

Confused about processed or red meat and cancer? If so, you could blame the reporting on that big story.

Some journalists who covered a recent report by a World Health Organization agency about processed meat causing cancer and red meat probably causing cancer found that report confusing.

So it's no surprise that, once their stories were published, more people became confused, and many wondered about the risks of eating meat.

The report, by the WHO's International Agency for Research on Cancer, looks at the evidence about whether processed meat and red meat cause cancer.

It is not a report about risk.

Processed meat 'carcinogenic to humans'

IARC evaluated all the evidence available and decided to classify consumption of processed meat as "carcinogenic to humans" and consumption of red meat as "probably carcinogenic to humans." Classification is their specialty.

IARC didn't do its own field research. It went through more than 800 epidemiological studies on cancer and red and processed meat and weighed those findings together, IARC spokeswoman Véronique Terrasse says.

While the studies were not absolute in their findings, IARC determined there is more than enough evidence to conclude eating processed meat can cause cancer.

The IARC article, which appears in The Lancet Oncology, includes a definition of processed meat but isn't clear on what products that includes. In a Q&A document, IARC does list hot dogs, ham, sausages, corned beef and beef jerky, and says they may contain poultry. That document also says, " The cancer risks associated with consumption of poultry and fish were not evaluated."

However, Kurt Straif, a report co-author and the head of the IARC unit that produces their evaluations, says that in the studies they reviewed, "the processed meat may contain poultry."

IARC doesn't consider the weight of the evidence on red meat convincing enough yet to say that it causes cancer, only that it probably does. That includes horse meat like this steak served at Toronto's La Palette restaurant. (Chris Young/Canadian Press)

Asked whether the IARC evaluation means processed poultry is carcinogenic to humans, Straif added that in the studies, "processed meat typically consist of beef and pork meat" but the more detailed analyses of different types of processed meats did not show conclusive differences.

Processed poultry accounts for 40 per cent of the processed meat sold worldwide.

IARC doesn't consider the weight of the evidence on red meat convincing enough yet to say that it causes cancer, only that it probably does. In other words, more research is needed, but until then limit your consumption.

A report about evidence, not risk

For red meat and processed meat, Terrasse says, "The classification gives you an idea about the strength of evidence that it causes cancer but it doesn't give you any clue about how much you need to be exposed to have cancer, it's not a classification about the risk."

IARC's findings got major attention, not because they were a surprise, or even new. For example, since 2002, the American Cancer Society has recommended we limit our consumption of red and processed meat, Susan Gapstur, ACS vice president of epidemiology told CBC News.

She says the IARC report, "just further supports the importance of eating and consuming a  healthful diet."

Since 2002, the American Cancer Society has recommended we limit our consumption of red and processed meat. (Tony Gutierrez/Associated Press)

In addition to cutting back on the consumption of red and processed meat, that includes "choosing healthier alternatives like poultry, fish, beans, and enriching your diet with other plant-based products," she says.

The attention comes from the weight IARC carries in the medical community and with governments. Epidemiologist Roberta Ferrence told CBC News that IARC is highly respected, "the last word on cancer" causality. She is the senior scientific advisor at the Ontario Tobacco Research Unit and a University of Toronto professor.

Press release feeds confusion

While IARC doesn't provided guidelines on risk, it does mention that a 2011 meta-analysis study found a 17 per cent increased risk of bowel cancer from eating 100 g of red meat per day and an 18 per cent increase from consuming 50 grams of processed meat per day. 

Although that isn't their estimate, Straif told CBC News that the IARC Working Group views that meta-analysis as "the best estimate of risk."

However, an IARC press release incorrectly attributes the risk estimate. It says it was the IARC experts who "concluded that each 50 gram portion of processed meat eaten daily increases the risk of colorectal cancer by 18%."

And then risk became a big part of the news reporting, and the confusion, even though IARC's work isn't about risk. A phrase search on Google for that quote from the press release comes back with over 5,000 results. (IARC's principles would allow them to "undertake to estimate dose–response relationships.")

The American Cancer Society and Cancer Research U.K., also cite that 2011 meta-analysis, so the issue is old news made new again by journalists reading that IARC press release. For the record, this is from the ACS in 2012: "Current evidence supports approximately a 15 per cent to 20 per cent increased risk of cancers of the colon and/or rectum per 100 g of red meat or 50 g of processed meat consumed per day."

Group 1 carcinogens

IARC has been criticized for including processed meats in its Group 1 of carcinogens, because it doesn't explain how such meats differ in risk from other Group 1 causes like tobacco, for example.

Worldwide, about 526,000 deaths from cardiovascular disease and 34,000 deaths from bowel cancer are attributable to a diet high in processed meat, the Institute for Health Metrics and Evaluation says. (CBC)

But IARC isn't looking at the risk, it's looking at the evidence, and saying the evidence is there for processed meats. 

Ferrence, a go-to expert on tobacco, has no problem with that. "There are lots of things that are Group 1 carcinogens that are less hazardous than tobacco. 

"It means they're known carcinogens, it doesn't say anything about the magnitude of the risk, or the magnitude of the consequences."

And she points out that the risks of smoking and the risks of a diet high in processed meats are not limited to just one disease, so a risk assessment of just bowel cancer, the focus of the meta-analysis, won't give someone a complete picture on which to make personal choices. 

Other health issues for processed meat

Researchers most frequently associate eating red and processed meat with bowel (or colorectal) cancer, but there's also been significant research linking processed meat to stomach cancer and red meat to pancreatic or prostate cancer, as well as limited research linking both meats to several other cancers.

Cancer isn't the only health issue around processed meat, or even the most serious. The Institute for Health Metrics and Evaluation's Global Burden of Disease study estimates that in 2013, the number of deaths attributable to a diet high in processed meat was 526,000 for cardiovascular disease, 84,000 for diabetes and related diseases, and 34,000 for bowel cancer. That totals 644,000 deaths worldwide.

Bowel cancer ranks as the third most-commonly diagnosed cancer in Canada (excluding non-melanoma skin cancers), according to the Canadian Cancer Society. The CCS estimates that this year, 25,100 people in Canada will be diagnosed and 9,300 Canadians will die from bowel cancer. They estimate a Canadian has a one in 15 chance of developing that cancer during their lifetime.

Besides red and processed meats, radiation, alcohol, tobacco, and obesity are also listed as risk factors for bowel cancer. (Ed Andrieski/Associated Press)

Besides red and processed meats, radiation, alcohol, tobacco, and obesity are also listed as risk factors for bowel cancer.

And the length of time for consumption also may matter, Ferrence points out, noting that the cancer risk doubles for every 10 years someone smokes. 

Ferrence does say that, "Smoking is probably a greater risk, but smoking is a greater risk than almost anything else."

For her, the take-away from the IARC report is that "people would be wise to reduce their intake or eliminate" processed meats from their diet. That's something she did long ago; she didn't wait for IARC.


  • An earlier version of this story was missing the word 'that,' and as a result a statistic could be misinterpreted. The Canadian Cancer Society estimates that in Canada, about 1 in 14 Canadian men and one in 16 women will develop colorectal cancer during their lifetime.
    Nov 03, 2015 11:30 AM ET