• Comment: My love of meat just hit a snag
• Why we love bacon
• 'Every bacon sandwich knocks half an hour off your life'
• Worried about all that meat? Best vegetarian recipes

A research division of the World Health Organisation has announced 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 organisation, and it is expected to face stiff criticism.

The WHO findings were drafted by a panel of 22 international experts who 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 meals, it will be controversial.

In reaching its conclusion, the panel cited studies suggesting that eating an additional 100 grams of red meat per day raises the risk of colourectal cancer by 17 per cent; eating an extra 50 grams of processed meat daily raises the risk by 18 per cent, according to the research cited. It quoted figures suggesting that 34,000 cancer deaths a year worldwide were attributable to diets high in processed meats.

The Australian Dietary Guidelines recommend limiting red meat consumption to about 450g per week. Processed meat is considered a "discretionary food choice", which also includes items such as cakes, ice cream and burgers that are not essential to a balanced diet but add enjoyment to eating in "occasional small amounts".

"For an individual, the risk of developing colourectal cancer because of their consumption of processed meat remains small, but this risk increases with the amount of meat consumed," says Kurt Straif, an official with the World Health Organisation's International Agency for Research on Cancer, which produced the report. "In view of the large number of people who consume processed meat, the global impact on cancer incidence is of public health importance."

The research into a possible link between eating red meat and cancer - colourectal 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.

The American Cancer Society, for example, notes that many studies have found "a link" between eating red meat and heightened risks of colourectal 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 colourectal cancer, the American Cancer Society tells the public, but "it's not exactly clear" which factors of that diet are important.

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 probably play out with political lobbying and in marketing messages for consumers.

A US industry group, the North American Meat Institute, called the WHO report "dramatic and alarmist overreach", and it mocked the panel's previous work for approving a substance found in yoga pants and classifying coffee, sunlight and wine as potential cancer hazards.

The WHO panel "says you can enjoy your yoga class, but don't breathe air (Class I carcinogen), sit near a sun-filled window (Class I), apply aloe vera (Class 2B) if you get a sunburn, drink wine or coffee (Class I and Class 2B), or eat grilled food (Class 2A)," said Betsy Booren, vice-president of scientific affairs for the group.

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 sceptical 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 criticised 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 epidemiological 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.

Washington Post