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The WHO's new warnings about bacon and cancer, explained

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  1. Consumption of processed meat — including hot dogs, bacon, and lunch meats — can definitely increase the risk of cancer, according to a new statement from a World Health Organization research agency.
  2. The group also announced that eating unprocessed red meat, such as beef or lamb, can "probably" cause cancer, but the evidence here is less convincing.
  3. This is the direst warning on meat yet, although other major health organizations, including the scientific advisory committee for the US Dietary Guidelines, had previously recommended that Americans cut down on their meat consumption.
  4. The evidence of harm mostly relates to colorectal (colon and rectal) cancers in people who regularly eat meat. While the overall cancer risk is small, it increases depending on the amount of meat a person eats.

Why the WHO concluded that meat can cause cancer

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 research evidence, the agency then classifies these items and behaviors as either definitely, probably, or possibly cancer-causing in humans.

On Monday, the IARC announced their findings on meat and cancer in the journal Lancet Oncology. They focused mainly on two types of meat: 1) Processed meats, which are transformed by salting, curing, or fermentation. This includes everything from hotdogs and bacon, to lunch meats like salami and prosciutto; 2) Unprocessed red meats, such as beef, veal, pork, lamb, and goat.

They determined that processed meats fall into the same cancer-risk 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. (As Vox's Brad Plumer explains, that's not the case at all.) 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 relative risk of colorectal cancer by 18 percent. Since a person's lifetime risk of colorectal cancer is about five percent,  daily meat consumption seems to boost that absolute risk by one percentage point to 6 percent (or 18 percent of the 5 percent lifetime risk).

Meanwhile, the IARC concluded that unprocessed red meat 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 the same small amount.

Why you can still eat meat if you want

These cancer risk increases are indeed small but they're not nothing, and a body of evidence has accumulated linking the heaviest meat eaters to bowel cancers — hence the IARC's warning today.

But 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.

This IARC doesn't imply that the occasional burger or pork chop will definitely increase your cancer risk. Instead, the researchers are warning that lots of meat can have potential downsides when it comes to cancer. So if you're eating a hot dog a day, there's more to worry about than if you have one per month. As the panel notes, a person's cancer risk "increases with the amount of meat consumed." And again, the absolute risk is small even among the heavier meat consumers.

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.

How researchers think processed meat leads to cancer

bacon Sahua d/Shutterstock

Don't despair — you can still eat the occasional bacon strip. (Sahua D/Shutterstock)

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 and broken down in the gut can be problematic.

Processed meats contain chemicals that can convert in the gut into  carcinogenic chemicals, specifically N-nitroso compounds and polycyclic aromatic hydrocarbons. According to UK Cancer Research, the N-nitroso compounds damage the cells that line the bowel, requiring them to replicate in order to heal. "And it's this ‘extra' replication," they write, "that can increase the chance of errors developing in the cells' DNA — the first step on the road to cancer."

Meanwhile, simply cooking red meat can also introduce these carcinogens. "Cooking improves the digestibility and palatability of meat," the IARC 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."

The evidence on red meat will no doubt be controversial

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 meat avoiders, 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. (For more on the strengths and limitations of different types of evidence, see here.)

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. "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.

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