There are plenty of health-related reasons to avoid that second hotdog or rethink your decision to double the bacon — and cancer is one of them, the World Health Organization said on Monday.
The WHO’s International Agency for Research on Cancer (IARC) has announced it is classifying processed meats as a carcinogenic — meaning there is now “sufficient” evidence to conclude that the consumption of everything from sausages to corned beef can cause cancer, particularly colorectal cancer.
The IARC also found that red meat can “probably” cause cancer, although the evidence it found to support this link is less robust. These conclusions — published in the medical journal Lancet Oncology — were arrived at by 22 global experts after reviewing more than 800 studies.
The IARC’s announcement is the strongest statement yet by a public health body linking meat and cancer. It has also provoked anxiety amongst carnivores and backlash from the meat industry, which blasted the IARC findings as “dramatic and alarmist.”
On the face of it, the announcement seems frightening. The IARC has designated processed meats a “Group 1” cancer hazard, which places them in the same category as everything from asbestos to tobacco.
But the “Group 1” category only means there is sufficient evidence that these things can lead to cancer; it does not mean everything in this category is equally dangerous or equally likely to cause cancer.
Experts also stress that the cancer risk of consuming a smoked meat sandwich still pales in comparison to that of smoking a cigarette. At this point, the public health message isn’t to quit meat, which can be a good source of nutrients like protein and iron, just to eat it in moderation.
“I don’t think people need to all of a sudden become vegetarian or avoid meat altogether,” said Rayjean Hung, a cancer epidemiologist with Mount Sinai’s Lunenfeld-Tanenbaum Research Institute, who has previously worked with the IARC. “You need to think about what is a balanced meal, what is moderation.”
The evidence linking cancer and meat consumption has been accumulating for some time, according to Hung.
Even before the IARC’s announcement, the Canadian Cancer Society was recommending that people restrict their red meat consumption to three servings per week. They also recommended avoiding processed meat altogether (or, perhaps more realistically, reserving it for “special occasions” such as holiday dinners and sporting events).
But the IARC’s analysis is the most comprehensive to date, examining 800 papers and considering the evidence from a variety of angles, including human risk, animal risk and toxicology, said Katie Wright, a senior manager of research communications with the Canadian Cancer Society’s Ontario division.
While there isn’t enough data to conclude what constitutes a “safe” level of meat, the IARC did find that greater consumption translated to greater risk: for every 50 grams of processed meat per day (the equivalent of one or two bacon strips), the cancer risk grows by 18 per cent. For red meat, every 100 grams per day increased risk by 17 per cent.
While these numbers sound dramatic, Wright cautioned that they also need to be taken in context. Compare them with tobacco, which increases cancer risk by 20 or 30 fold. In the United Kingdom, tobacco is also the cause of 86 per cent of lung cancers; red meat and processed meat only accounts for 21 per cent of all bowel cancers (Canadian statistics for the percentage of cancers caused by meat consumption are currently unavailable, Wright says).
And while colorectal cancer is a serious disease — there will be 25,100 cases in Canada this year — people who do get cancer from red meat or processed meat are “eating red meats or processed meats every day, for a long period of time,” Wright said.
“One third of cancers can be prevented by eating well and living a healthy, active lifestyle,” Wright said. “If you’re really looking at doing something to improve your health, quitting smoking — or not starting to smoke — is still the No. 1 thing you can do.
How does meat consumption rank on the spectrum of healthy and unhealthy foods and behaviours?
Everybody needs to eat. But nobody needs to smoke, and the World Health Organization has identified tobacco use as the single greatest avoidable risk factor for cancer mortality worldwide. In a given year, tobacco use accounts for nearly a quarter of all cancer deaths and kills six million people worldwide — including more than 600,000 as a result of second-hand smoke. Smokers shave 10 years off their life expectancy compared to non-smokers, and quitting before the age of 40 reduces the risk of dying from a smoking-related disease by 90 per cent, according to the U.S. Centers for Disease Control and Prevention. “Let's say this: there are some known benefits of eating meat, but there is no known benefit of smoking,” says Rayjean Hung, a cancer epidemiologist with Mount Sinai’s Lunenfeld-Tanenbaum Research Institute.
Many studies — but not all — have linked moderate alcohol consumption to health benefits, including a lower risk of heart disease, stroke and diabetes. Canadian guidelines define “moderate” as no more than two drinks a day and 10 per week for women, or three a day and 15 per week for men. But overdrinking carries profound health risks, and the Canadian Centre on Substance Abuse calculates that 4,600 lives would be saved every year if Canadians stuck to the guidelines. Having 3.5 drinks a day can double or triple the risk of mouth and esophageal cancers and increase colon and breast cancer risk by 1.5 times, the CCSA says, to say nothing of accidents and other alcohol-related trauma. Teetotallers may have slightly worse health outcomes than moderate drinkers (and some evidence disputes this), but heavy drinkers have the worst health risks of all, and the WHO pegs 3.3 million deaths annually to harmful alcohol use.
As part of their justification for listing processed meats as carcinogens and red meats as possible carcinogens, the World Health Organization cited a 2011 meta-analysis that found a 17 per cent increased risk of colorectal per 100 gram serving of red meat per day, and an 18 per cent increase per 50 gram serving of processed meat per day. David Spiegelhalter, Cambridge University’s Winton Professor of the Public Understanding of Risk, explains that if 100 people ate 50 grams of processed meat every single day, the expected cases of bowel cancer would rise from six to seven. The risk is small, but other sources of protein are associated with health benefits: one 2004 meta-analysis found that every 20-gram increase in fish intake was linked to a 7-per-cent decrease in mortality from coronary heart disease.
The WHO names physical inactivity as the fourth leading risk factor for global mortality, naming it as the cause of between one-fourth and one-fifth of all cases of breast and colon cancers, diabetes and heart disease. Exercise — one subset of physical activity, which also includes movement on the job or in transit — has been linked to so many positive physical and mental health outcomes that some doctors consider it medicine. Adults should get 150 minutes of moderate to vigorous aerobic activity a week, Canadian guidelines say, but more sweat provides greater benefits. Adding two strength training sessions a week also helps.
Often: Fruits and vegetables
At this point, everybody knows they should eat their fruits and veggies, but the scientific evidence keeps rolling in anyway — and some of it is staggering. In one British study that followed over 65,000 people, those who ate seven or more portions of fruit and vegetables daily were 42-per-cent less likely to die of any cause than those who ate none, and reduced their cancer risk by 25 per cent. But even those who only consumed one to four portions of fruits and vegetables daily saw their all-cause mortality risk drop by 14 per cent. Other large epidemiological studies have found much smaller reductions in cancer risk associated with fruit and vegetable consumption, but significant reductions in heart disease risk.”
Evidence linking processed meats to cancer is strong, and evidence linking red meats to cancer is mounting. But the reason why they might boost cancer risk is not clear. Scientists are investigating all of these cancer-causing mechanisms in meat, and more.
In the last decade, the microbiome — the community of microbes that co-exists in our bodies, mainly in the intestines — has become a focus of intense scrutiny for its potentially overlooked influence on human health. A Nature Communications paper published in January had 20 rural South Africans undergo a two-week “diet swap” with 20 African-Americans, who have much higher rates of colon cancer. After giving up their usual meat-heavy, fibre-poor diets, the Americans saw shifts in their microbiotic makeup toward one that produced more digestive byproducts associated with cancer prevention, like butyrate, which is released when the body metabolizes fibre.
In eight out of 10 cases of bowel cancer, the patient carries a faulty copy of a gene called APC. Carriers develop far more intestinal polyps, which can lead to cancer, than non-carriers. In a U.K. study of mice, animals that carried the faulty gene and were fed a diet rich in iron — found in high levels in red meat — were far more likely to develop cancer than those fed a low-iron diet. The bad-gene bad-diet combination appeared to switch on proteins that led to iron accumulation in bowel cells, which causes the cells to grow out of control. More genes have been identified that are linked to bowel cancer, and scientists are still investigating how they interact with dietary and other environmental factors.
A group of chemicals called N-nitroso compounds have been linked to colorectal, stomach, esophogeal and other cancers in humans. The pathways are murky, but scientists are examining the link between nitrites and nitrates — both found in processed meats — and N-nitroso compounds in those foods, as well as other chemicals in red meats that trigger the body to create N-nitroso during digestion. Cooking meats at high temperatures may also produce certain chemical compounds that increase cancer risk, such as polycyclic aromatic hydrocarbons; these are found in much higher amounts in tobacco smoke and at industrial sites, however.
Clarification – October 27, 2015: This article was edited from a previous version that did not make clear that the cancer risk relates to daily consumption of processed meats and red meat.