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Will Processed Meat Give You Cancer?

The World Health Organization (WHO) cancer research agency IARC published a report stating that consumption of processed red meat increases risk of colorectal cancer:

“Red meat
After thoroughly reviewing the accumulated scientific literature, a Working Group of 22 experts from 10 countries convened by the IARC Monographs Programme classified the consumption of red meat as probably carcinogenic to humans (Group 2A), based on limited evidence that the consumption of red meat causes cancer in humans and strong mechanistic evidence supporting a carcinogenic effect.
This association was observed mainly for colorectal cancer, but associations were also seen for pancreatic cancer and prostate cancer.

Processed meat
Processed meat was classified as carcinogenic to humans (Group 1), based on sufficient evidence in humans that the consumption of processed meat causes colorectal cancer.”

Eating too much red meat probably increases your risk of colorectal cancer, and perhaps also pancreatic and prostate cancers. Eating processed meat is worse, as the association with colorectal cancer is more certain, and the link was determined to be causal. The IARC does not lightly make the claim of a causal link to cancer. But past documents have asserted a causal link between obesity and seven different types of cancer, so it is not unprecedented.

A NY Times article offers some uniformed and rather ridiculous commentary on the IARC report: So Will Processed Meat Give You Cancer? Here are some quotes from that commentary:

First, the author (Anahad O’Connor) says: “We do know that eating a lot of processed meat or red meat is associated with higher cancer risk; the W.H.O. report cited 800 studies documenting the association. But that’s a long way from cause and effect.”

No, it’s not a long way from cause and effect. Studies that find an association between disease and diet are always seeking to determine if the association is causal. They use statistical techniques to rule out other factors, by taking into account age, smoking, BMI, drinking, and many other factors. And when there are multiple studies, using different approaches, looking at different populations, etc., the likelihood of causality increases. In this case, 800 studies is more than sufficient to show causality — that’s why the panel of experts at IARC stated that processed meat “causes colorectal cancer”. But the evidence is less than sufficient to show causality for fresh red meat, so the report only states that red meat is “probably” carcinogenic based on “limited evidence”.

Then O’Connor says: “It may simply be the so-called healthy user bias, the idea that people who eat lots of bacon are more likely to engage in risky behaviors (like smoking or a sedentary lifestyle) that lead to cancer; and that non-meateaters exhibit other healthful virtues (like exercise or eating vegetables) that protect them.”

That is not even a possibility. Once you have hundreds of studies showing the association, each one subjected to multivariate-adjusted statistical models, the types of biases O’Connor mentions are thoroughly ruled out.

Eating too much processed red meat increases risk of colorectal cancer. The evidence is overwhelming.

Additional evidence exists that processed red meat increases risk of all-cause mortality, type 2 diabetes, cardiovascular disease mortality, all-cancer mortality, coronary heart disease, and cancers of the mouth.

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