Do red and processed meats really cause cancer? Ten years ago, the WHO and IARC led us to believe so, but experts say the decision-making process was biased and flawed.

It has now been ten years since that October of 2015, when the International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO), classified processed meats (such as hot dogs, bacon, ham, sausages, and cold cuts) as Group 1 carcinogens, meaning a “proven cause” of cancer, and fresh red meats (beef, pork, lamb) as “probably carcinogenic” (Group 2A). Much has changed since then, and today we know for certain, thanks to insider reports collected by science journalist Nina Teicholz, that the IARC’s decision-making process was biased and influenced.

The IARC announcement was undoubtedly a historic event: for the first time, a global health organisation declared a fundamental component of the human diet as potentially carcinogenic. Twenty-two international experts, the “working group”, met for eight days in Lyon, France, to assess a mountain of data from more than 800 observational studies (the lowest-quality type of scientific evidence). The outcome was a two-page document published in Lancet Oncology that offered no possibility of independent verification of the analysis. Despite this, the news made headlines around the world, sparking alarm and generating claims that are still echoed today by opponents of meat consumption.

Weak Foundations for Strong Conclusions

The Lancet conclusions were based on only eight epidemiological studies that linked people’s self-reported diets with cancers developed later on. Yet, it is well known that the vast majority of such studies cannot prove a causal relationship between diet and disease. Despite this, the agency concluded that the evidence against processed meat was as strong as that against tobacco or asbestos.

The headline-grabbing figure came from the IARC press release: eating just 50 grams of processed meat per day, an amount about the size of a matchbox, would increase the risk of colorectal cancer by 18%. This claim was based on a single study, a 2011 meta-analysis, and the only statistically significant result concerned the consumption of fried sausages and pork. The fact that the working group generalised these weak findings to all red and processed meats suggests a distorted interpretation of the evidence.

Even the famous “+18%” is questionable. It actually referred to relative risks of 1.17 for red meat and 1.18 for processed meat, where 1 indicates no relationship at all. In other words, the increase was only 0.17 or 0.18, essentially negligible. Translated into absolute risk, these numbers would raise a 50-year-old man’s lifetime risk of developing colon cancer from 4.5% to 4.68%. In practical terms, out of 10,000 fifty-year-old men, 450 would develop colon cancer regardless of meat consumption. Only 18 additional cases would occur due to eating meat, while 9,682 men would remain healthy either way. In epidemiology, such small differences are too minor to be meaningful and are therefore considered insignificant. Nevertheless, the prevailing message became: “Meat causes cancer.”

Evidence Deliberately Ignored

According to interviews Nina Teicholz conducted with seven members of the working group, the IARC explicitly excluded key studies, not minor ones. Among them were two large randomised controlled clinical trials that had reduced red meat consumption as part of dietary interventions aimed at lowering cancer risk. In other words, the only studies of the most rigorous kind, the only ones truly capable of demonstrating a causal link between diet and disease, were left out. Both trials, funded by the National Institutes of Health (NIH), were considered “powerful,” meaning large and long enough to produce reliable, generalisable data, and neither found any effect of meat on cancer risk. The first, the Polyp Prevention Trial, found no differences after eight years between those who followed a low-fat, low-meat diet and those who did not. The second, the massive Women’s Health Initiative, which examined nearly 50,000 women, found no reduction in cancer risk among those following low-meat diets.

When one IARC member suggested including these studies, the group’s chair, Bernard Stewart, rejected the idea, arguing that the trials modified multiple dietary variables, not just meat. This was true, but if reducing meat intake does not lower cancer rates, that finding represents significant evidence against the IARC’s conclusions. Ignoring such a contradiction means excluding critical data. Even more concerning, as one participant noted, many of the accepted epidemiological studies also involved hundreds of different foods, yet IARC included them without issue. It is therefore legitimate, as Teicholz emphasises, to say that IARC favoured weak data over strong evidence.

As the two-page report itself stated: “The greatest weight was given to prospective cohort studies conducted in the general population.” That is exactly the opposite of what the fundamental rules of the scientific method require. None of the studies considered included human clinical trials, and even the animal studies failed to show any clear effect of red or processed meat on tumour development.

“A Bacon-Based Diet Appears to Protect Against Carcinogenesis.”

For the mechanistic evidence, three possible mechanisms were proposed: heme iron, genotoxicity, and oxidative stress. One of the main authors, Denis Corpet, a professor at the University of Toulouse, had previously tried to demonstrate the carcinogenicity of bacon but ended up with the opposite result, publishing studies showing that mice fed bacon produced fewer carcinogenic compounds. In fact, the study concluded: “A bacon-based diet appears to protect against carcinogenesis.”

A later study from 2000 similarly stated: “The results suggest that N-nitroso compounds derived from bacon do not increase colon carcinogenesis in rats.” At that point, Corpet discovered that when rats were fed calcium-deficient diets, more cancer-related compounds were produced. In subsequent experiments, he used calcium-deficient diets, and in a 2013 study where rats were fed hot dogs, he concluded: “This is the first experimental evidence that processed meat promotes colon carcinogenesis in rats.” Some members of the working group pointed out that this effect occurred only in calcium-deficient conditions and urged him to include a note of caution. Still, Corpet strongly opposed the idea and refused to include it.

Another weakness in the mechanistic evidence was that many experiments did not isolate the effect of meat from the fats in which it was cooked. It is well known that heated vegetable oils produce toxic oxidative compounds, a fact Corpet himself acknowledged: “Oxidised fats are toxic, and those results on cooked meat could depend on the fats.” Yet the group did not pursue this line of inquiry. Were they perhaps attributing the harmful effects of seed oils to meat itself?

As for the epidemiological evidence, it was reportedly rushed through on the final afternoon of the meeting, without in-depth review. As David Klurfeld, then at the U.S. Department of Agriculture, stated: “We didn’t really have the chance to analyse them.” Even the IARC’s own Lancet report acknowledged the limitations of the epidemiological studies: “No clear association was observed in many of the high-quality studies, and the evidence in humans is limited. The presence of dietary or lifestyle confounding factors cannot be excluded.” Yet only a few lines later, the text contradicted itself, claiming: “The epidemiological evidence is substantial.” So, which is it, limited or substantial? Regardless of wording, the numbers speak for themselves: the associations between red or processed meat and colorectal cancer are inconsistent and minimal, with relative risks of 0.17 and 0.18, nowhere near the 10–30 times higher cancer risk caused by cigarette smoking.

An Independent Evaluation

Four years after the IARC’s decision, in 2019, the prestigious journal Annals of Internal Medicine published the most rigorous, extensive, and comprehensive reviews ever conducted on the topic. Unlike most members of the IARC group, these authors had never published research on meat and cancer and had no personal or professional stakes in the issue. Instead, they were experts in interpreting scientific evidence using an internationally recognised method: GRADE (Grading of Recommendations, Assessment, Development, and Evaluation). Their conclusion was clear: the evidence linking red meat to cancer is of “low” or “very low” quality.

Bernard Stewart replied to an email stating that he was unaware of any criticism of the IARC’s work, but he did not respond to the GRADE analysis. Another group member, David Klurfeld, commented: “I was a veteran in nutrition and well aware of the anti-meat bias in the field, but I still thought that when experts looked at the totality of the evidence, they would change their minds. I was naïve.” At this point, the question is legitimate: Was the IARC review truly impartial?

During the meeting, some participants began to suspect that the outcome had been predetermined. This seems plausible, considering that 17 of the 22 members had already published numerous studies attempting to prove that meat causes cancer, even when their own results were inconclusive, revealing an evident bias against meat. For example, Alicja Wolk of the Karolinska Institute in Sweden had published 38 studies on the subject, never finding significant associations, yet she continued pursuing the same line of research. Why such persistence? At what point does a researcher have an ethical obligation to admit that a hypothesis is probably false?

Several other members had similar publication histories, appearing ever hopeful that “the next study” would yield the desired outcome. From this perspective, the Lyon meeting seems to have been an opportunity for the majority of the group to formalise a conclusion that would finally confirm their preexisting beliefs. Many of the studies reviewed were, in fact, their own. As one participant noted: “They weren’t independent reviewers. They were reviewing their own data!“

How, then, can an evaluation of one’s own data be considered objective? Furthermore, it appears that three or four of the sixteen IARC members were committed vegetarians, suggesting that the IARC team might have held personal biases against meat that could have influenced their deliberations. The IARC claims that its working groups reach conclusions “by consensus,” yet the red and processed meat group did not work to achieve unanimity, and no such consensus was ever reached. Moreover, according to an IARC spokesperson, no further contributions from the group members were requested during the drafting of the final monograph.

Does Everything Cause Cancer According to the IARC?

Since 1965, the IARC has analysed 1,045 natural, biological, and chemical agents, and has declared 52% of them carcinogenic. A 2013 study found that 80% of the ingredients in a single cookbook had been the subject of cancer risk studies, most of which had weak or inconclusive evidence. The IARC has even classified shift work and very hot beverages as cancer-causing. In 2019, it eliminated the category “probably not carcinogenic,” meaning that the best classification any substance or food can now hope for is “not classifiable.” In practice, for the IARC, everything is a potential risk.

Even Bernard Stewart himself expressed doubts about the IARC’s processes in a 2016 Lancet editorial titled “How Do We Judge What Causes Cancer? The Meat Controversy.” He and another member wrote that “Eating red meat does not necessarily lead to cancer,” since cancer results from a “complex interplay of agents and biological responses.” They emphasised that the concept of “no safe dose,” valid for cigarettes, is “meaningless” when applied to meat. However, they added, the fault does not lie with the IARC itself. It is policymakers who should clearly distinguish between carcinogens with no safe dose and those that are only potentially carcinogenic. This gap between evidence reviews and policy implementation is troubling. Few politicians understand this distinction, and the IARC certainly does not clarify it in its classifications.

For years, some researchers have tried to pin the blame for cancer on red and processed meat, but the already weak evidence has become increasingly inconsistent. Meanwhile, the notion that “red meat causes cancer” has been cemented in public opinion, solidifying this perception as “settled science.” While it is undeniable that radioactivity, smoking, and industrial chemicals cause cancer, red meat, an essential human food since the dawn of time, is not among them. The IARC’s process illustrates how, through funding, bias, and institutional power, a failed hypothesis can be kept alive for an entire decade.

A heartfelt thanks goes to science journalist Nina Teicholz for her difficult yet invaluable work in exposing these crucial behind-the-scenes dynamics and bringing uncomfortable truths to light.