Do red and processed meats cause cancer?

Nine years ago, WHO said yes, but insiders say the process was flawed and biased

“Cancer.” The word itself is one of the most dreaded, and almost no diagnosis is more frightening. So, when people are assured that they can prevent cancer by avoiding red and processed meats, I’d hazard a guess they do. This advice is usually dispensed for all types of cancer, even though only one kind of cancer —colorectal — has ever been classified as carcinogenic by the world’s leading agency in this field. In 2015, the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) designated processed meats (hot dogs, bacon, ham, sausage, cold cuts) as Group 1 carcinogens, a “certain” cause of cancer, and fresh red meat (beef, pork, lamb) as Group 2A, or “probable” carcinogens.

This news resounded worldwide, with hundreds of menacing headlines, including Bad Day For Bacon: Processed Meats Cause Cancer (NPR), OMG, Bacon causes cancer (NY Post), and Bacon, hot dogs and processed meats cause cancer, WHO says (PBS), and Meat as a Cause of Cancer (NYT editorial). At least one authoritative group, the American Institute for Cancer Research, the leading cancer research organization in the U.S. focused on lifestyle factors, expressed strong support for the IARC decision.

This IARC announcement was an historic event. For the first time ever, a global health organization had declared a major component of all human diets throughout history to be a likely carcinogen. Yet to support this 2015 claim, the IARC released a mere two-page summary of its findings, in Lancet Oncology.  Twenty-two international experts (the “working group”) had met for eight days in Lyon, France, to evaluate a mountain of data--more than 800 observational studies alone. Because the working group produced a mere two-page summary, their analysis could not be independently verified at the time.

More surprisingly, the Lancet’s conclusions leaned heavily on only eight papers, all from epidemiological studies which linked what people ate, or reported they ate, with cancers they developed later in life. Even though it is well-established that the vast majority of this type of nutritional study cannot prove a causal link between food consumption and disease, the agency concluded that the strength of evidence for processed meats causing cancer equaled that of tobacco and asbestos.

The figures from IARC that caught the world’s attention came from its press release accompanying the Lancet article, which asserted that eating just 1.75 ounces of processed meat daily (about the size of a matchbox) increases the risk of colorectal cancer by 18%. These claims relied on a single paper, a meta-analysis from 2011. And the only significant finding in this study was a link between cancer and fried sausage and pork meat--not beef, mutton, or lamb. That the working group employed weak findings on pork and fried sausage to generalize to all red and processed meats suggests a biased interpretation of the evidence, an important theme to which I’ll return below.