The WHO's International Agency for Research on Cancer (known as the IARC) is tasked with evaluating potential human carcinogens, looking at everything from certain chemicals and herbicides to cigarette smoke and wifi. Based on the best available evidence, the agency then classifies these items and behaviors as either definitely, probably, or possibly cancer-causing in humans.
On Monday, the IARC announced in the journal Lancet Oncology that processed meats — those transformed by salting, curing, or fermentation — fall into the same category as tobacco smoke, meaning "there is convincing evidence that the agent causes cancer." That’s the strongest statement the IARC can make, and they only make it when there's enough convincing data suggesting a cancer link.
To be clear, this doesn't mean that processed meat is as bad for you as smoking. What it means is that according to the agency's assessment, the links between processed meat and certain types of cancer are clear and well-established.
Specifically, the researchers found evidence that eating a 50-gram portion of processed meat daily (about one hot dog) can increase a person's risk of colorectal cancer by 18 percent.
Meanwhile, the IARC concluded that unprocessed red meat — which includes beef, veal, pork, lamb, and goat — was "probably carcinogenic." The evidence here is less definitive, but the panel found that eating about 100 grams of unprocessed red meat a day (about one hamburger) seemed to increase the risk of colorectal cancer by about 17 percent.
It's worth putting these risks in perspective. The strongest evidence that the IARC uncovered focused on one type of cancer — colorectal — and the risks related mainly to heavy meat consumption. There were also troubling associations between meat consumption and other types of cancer, including stomach and pancreatic. But again, the data was less definitive compared with the evidence for colorectal cancer.
This IARC doesn't imply that the occasional burger or pork chop will definitely increase your cancer risk. Instead, the researchers are warning that excess meat consumption can have real downsides when it comes to cancer. So if you're eating five hot dogs a day, there's a lot more to worry about than if you have one steak per month. As the panel notes, a person's cancer risk "increases with the amount of meat consumed."
Christopher Wild, director of the IARC, put it this way in a statement: "These findings further support current public health recommendations to limit intake of meat." That said, Wild added that "red meat has nutritional value." So people and public health agencies need to balance the risks and benefits when thinking about how to approach diet. Importantly, this group only looks at cancer, not all the other potential harms meat eating can bring — like environmental costs or cholesterol problems.
While the IARC noted that red meat contains many important proteins and nutrients, including vitamin B and iron, the methods by which it is prepared for human consumption appears to be problematic.
When processed meats are cured or smoked, it can create carcinogenic chemicals, including nitrates and polycyclic aromatic hydrocarbons. These preserve bacon and prosciutto, and enhance their flavors, but they also seem to increase people's cancer risk.
Meanwhile, simply cooking unprocessed red meat can also be problematic. "Cooking improves the digestibility and palatability of meat," the researchers wrote in The Lancet, "but can also produce known or suspected carcinogens, including heterocyclic aromatic amines and polycyclic aromatic hydrocarbons. High-temperature cooking by pan-frying, grilling, or barbecuing generally produces the highest amounts of these chemicals."
A lot of the evidence the IARC relied on to form its recommendation about red and processed meats came from epidemiological studies on associations between cancer and meat consumption in many different countries.
These sorts of population-wide observational studies are common in nutrition: Basically, researchers track a very large number of people, see how much meat they consume, and check their health outcomes over time. But because they're not as rigorous as experimental studies like randomized trials — in which researchers would randomly assign people to eat or abstain from certain foods — they are more likely to be biased.
People who already eat red meat, for example, may be less health-conscious than those who avoid a meat-heavy diet, and these factors can confuse observational data. But because it's extremely difficult (not to mention expensive) to assign people to certain diets for long periods of time (since you'd have to make sure the people in each group stuck to their randomly assigned diet over the course of their lifetime), researchers rely on observational data when it comes to nutrition.
The meat industry has been quick to seize on the potential uncertainty in the science and dispute the IARC's conclusions. According to the Washington Post, experts from the industry "questioned whether the evidence is substantial enough to draw the kinds of strong conclusions that the WHO panel did." The Wall Street Journal has reported that the North American Meat Institute said the IARC findings "defy both common sense and dozens of studies showing no correlation between meat and cancer and other studies showing the many health benefits of balanced diets that include meat."
The IARC defended its research by arguing that the link between meat eating and cancer appeared in many types of studies, not just observational research. "On the basis of the large amount of data and the consistent associations of colorectal cancer with consumption of processed meat across studies in different populations, which make chance, bias, and confounding unlikely as explanations," they wrote in The Lancet.