A research division of the World Health Organization announced on Monday that bacon, sausage and other processed meats cause cancer, and that red meat probably does, too.
The report by the influential group stakes out one of the most aggressive stances against meat yet taken by a major health organization, and it is expected to face stiff criticism in the United States.
The WHO conclusions are based on the work of a 22-member panel of international experts that reviewed decades of research on the link between red meat, processed meats, and cancer. The panel reviewed animal experiments, studies of human diet and health, and cell mechanisms that could lead from red meat to cancer.
But the panel's decision was not unanimous, and by raising lethal concerns about a food that anchors countless American meals, it will be controversial. The $95 billion U.S. beef industry has been preparing for months to mount a response and some scientists, including some unaffiliated with the meat industry, have questioned whether the evidence is substantial enough to draw the kinds of strong conclusions that the WHO panel did.
"We simply don't think the evidence support any causal link between any red meat and any type of cancer," said Shalene McNeill, executive director of human nutrition at the National Cattlemen's Beef Association.
The research into a possible link between eating red meat and cancer - colorectal cancer is a longstanding area of concern - has been the subject of scientific debate for decades. But by concluding that processed meats cause cancer, and that red meats "probably" cause cancer, the WHO findings go well beyond the tentative associations that other groups have reported.
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The American Cancer Society, for example, notes that many studies have found "a link" between eating red meat and heightened risks of colorectal cancer. But it stops short of telling people that the meats cause cancer. Some diets that have lots of vegetables and fruits and lesser amounts of red and processed meats have been associated with a lower risk of colorectal cancer, the American Cancer Society tells the public, but "it's not exactly clear" which factors of that diet are important.
Likewise, the Dietary Guidelines for Americans, the federal government's advice compendium, encourage the consumption of protein foods such as lean meats as part of a healthy diet. Regarding processed meats, however, the Dietary Guidelines do offer a tentative warning: "moderate evidence suggests an association between the increased intake of processed meats (e.g., franks, sausage, and bacon) and increased risk of colorectal cancer and cardiovascular disease," the guidelines say. The Dietary Guidelines stop well short of saying processed meats cause cancer, however.
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In recent years, meat consumption has been the target of multi-faceted social criticism, with debates erupting not just over its role on human health, but the impact of feedlots on the environment and on animal welfare. The public debate over the WHO's findings will likely play out in political lobbying, and in marketing messages for consumers.
But at its core, the dispute over meat and cancer revolves around science, and in particular the difficulty that arises whenever scientists try to link any food to a chronic disease.
Experiments to test whether a food causes cancer pose a massive logistical challenge - they require controlling the diets of thousands of test subjects over a course of many years. For example, one group would be assigned to eat lots of meat, and another less, or none. But for a variety of reasons involving cost and finding test subjects, such experiments are rarely done, and scientists instead often use other less direct methods, known as epidemiological or observational studies, to draw their conclusions.
"I understand that people may be skeptical about this report on meat because the experimental data is not terribly strong," said Paolo Boffetta, a professor of Tisch Cancer Institute at the Mount Sinai School of Medicine who has served on similar WHO panels. "But in this case the epidemiological evidence is very strong."
Other scientists, however, have criticized the epidemiological studies for too often reaching "false positives," that is, concluding that something causes cancer when it doesn't.
"Is everything we eat associated with cancer?" a much noted 2012 paper in the American Journal of Clinical Nutrition asked.
That paper reviewed the academic studies conducted on common cookbook ingredients. Of the 50 ingredients considered, 40 had been studied for their impact on cancer. Individually, most of those studies found that consumption of the food was correlated with cancer. When the research on any given ingredient was considered collectively, however, those effects typically shrank or disappeared.
"Many single studies highlight implausibly large effects, even though evidence is weak," the authors concluded.
While epidemioloical studies were critical in proving the dangers of cigarettes, the magnitude of the reported risks of meat is much smaller, and it is hard for scientists to rule out statistical confounding as the cause of the apparent danger.
"It might be a good idea not to be an excessive consumer of meat," said Jonathan Schoenfeld, the co-author of the American Journal of Clinical Nutrition article and an assistant professor in radiation oncology at Harvard Medical School "But the effects of eating meat may be minimal, if anything."
Moreover, critics of the decision noted that two actual experiments that tested diets with reduced meat consumption, the Polyp Prevention Trial and the Women's Health Initiative, found that subjects who lessened their meat intake did not appear to benefit by a lower cancer risk. It is possible, however, that the reductions in red meat were too small to have an effect.