The WHO Says Processed Meat Is a Carcinogen. Is it?

by Katherine Pett

Just in time for Halloween, the world had another deadly villain to dress as for costume parties: Bacon.

The Internet exploded last week when the WHO’s International Agency for Research on Cancer (IARC) released a report classifying processed meat as “carcinogenic to humans” based on what they determined as “sufficient evidence that consumption of processed meat causes colorectal cancer.” Unprocessed, red meat was classified as a probable carcinogen based on “limited evidence.”

Unsurprisingly, news media and social media were quick to respond.  Reactions looked something like this:

Well, your risk of colorectal cancer is already low. Adding more processed meat takes up absolute risk by less than one percent!

Others argued about the WHO’s ability to make this determination.

This is based on observational evidence! You can’t determine cause from survey studies! Correlation does not equal causation!

Even people who didn’t quibble with the WHO’s findings had qualms about its delivery. The Atlantic went so far as to call the IARC “confusogenic,” and locked in an “ivory-tower mentality.”

The Atlantic is certainly right that the report is confusing, and had no help from much of the media, who were quick to misinterpret the story. Some headlines characterized the IARC report as a study, when it was actually a decision made on the basis of previous research.  Most, emphasized the fact that processed meat now shares the same classification as smoking, creating the implication that the WHO finds processed meat as risky as a pack-a-day habit.  It doesn’t. And predictably, every organization associated with red meat promotion loudly questioned the strength of the evidence.

As a quick recap, the IARC categorizes substances into four different groups: carcinogenic, probably carcinogenic, possibly carcinogenic, and probably not carcinogenic. These categorizations aren’t based on the risk of getting cancer from the substances examined; it is based on the total amount of evidence that something is associated with cancer.

But the ruckus raised good questions: How much evidence is there that processed meat is a carcinogen? Can the WHO really say that something causes cancer from observational studies—that is, studies based on following populations, rather than performing experiments?

I asked Professor Mark Woodin, of the Tufts School of Public Health to weigh in on what he thinks of the report.

“The term ‘cause,’ as used here by the IARC, would be better phrased as ‘increased risk.’ As in, ’high consumption of processed meat increases your risk of cancer.’ Accurately getting at cause requires experiments, such as large randomized trials. Sometimes, associations in well-run observational studies are big enough to merit consideration of the word ‘cause.’ Typically, such risk ratios are larger than 3.0 and sometimes can be much larger than that; think lung cancer and smoking,” he said.

He also pointed out that it is problematic to describe any food as a “cause” of disease, particularly when analysis is based on survey data. The WHO estimated that the increase in risk of colorectal cancer goes up by only 18% for every 50 grams increase of processed meat consumption per day. This is about 2 ounces of meat or 2 slices of bacon.

“The very small increase in the risk ratio could well be due to unmeasured confounding variables…In general, single foods have very small risk ratios, and even macronutrients, such as protein or fat, have relatively modest associations with disease. Part of the issue is accurately quantifying a person’s diet. It’s extremely difficult for people to accurately remember their long-term dietary habits.”

Perhaps tellingly, the WHO is cautioning people not to get too bent out of shape: “The latest IARC review does not ask people to stop eating processed meats, but indicates that reducing consumption of these products can reduce the risk of colorectal cancer.”

So what does this mean for you? If you are someone who might be at a greater risk of colorectal cancer, consider reducing your intake.

If not, remember that recommendations to reduce red meat and processed meat intake are consistent with the national dietary guidelines. Regardless of whether these foods increase cancer risk, processed meats are often high in sodium and saturated fat, which are two nutrients also consistently associated with increased chronic disease risk.

Katherine Pett is a second-year student in the Biochemical and Molecular Nutrition program.  Data is the new bacon.

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