This morning, the World Health Organization revised its health guidelines around the consumption of meat. The report from the WHO’s International Agency for Research on Cancer now labels processed meats as “carcinogenic to humans,” meaning their consumption can cause cancer. The report also classifies red meat as “probably carcinogenic to humans.”
For more on what this finding means for our shopping list and diet choices, we’ve prepared a list of FAQs, as formulated by the collective minds of the PBS NewsHour.
What counts as red meat? What counts as processed meat?
Red meat is any meat that comes from a mammal. That means meat from cows (beef and veal), pigs (pork), sheep (lamb and mutton), horses, goats and bison all count as red meat.
White meats come from fish and poultry. The color difference is dictated by the amount of blood in the tissue, which plays into why red meat is more likely to cause cancer (see below).
Processed meats are any meats that aren’t fresh. People typically think of processed meat as only referring to pork and beef, but this category can also include poultry (chicken, turkey, duck) and fish. A processed meat, according to the panel, has been modified from its natural state, either “through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation.”
This includes sausages, hot dogs, corned beef, beef jerky, canned meat, meat sauces, lunch meats and bacon.
What does cancer “link” versus cancer “cause” mean?
The word “caused” sounds much more definitive than “linked,” right? Consider the news headlines surrounding this report. Here are two examples:
W.H.O. Report Links Some Cancers With Processed or Red Meat (New York Times)
“Bad Day For Bacon: Processed Red Meats Cause Cancer, WHO Says (NPR)
So which is it?
Technically, meat consumption has been linked to cancer, especially colorectal cancer, for years. It’s a correlation or “link” backed by statistical evidence. Large-scale studies from Europe, Australia, Japan and the U.S. have shown that people who consume more processed and red meat are more likely to develop cancer. Meanwhile, research in the lab has shown scientifically — in rat models and human cells (see below) — how that meat leads to the chemical shifts and genetic mutations that turn healthy cells into cancer cells.
Based on this collective evidence, the WHO panel has concluded today that processed meat can cause cancer, upgrading its threat assessment from correlative to causal.
In contrast, red meat without processing remains a probable cancer-causing agent, because there is less evidence in humans showing that it can spawn cancer.
In a separate Q&A, the WHO wrote:
In the case of red meat, the classification is based on limited evidence from epidemiological studies showing positive associations between eating red meat and developing colorectal cancer as well as strong mechanistic evidence.
Limited evidence means that a positive association has been observed between exposure to the agent and cancer but that other explanations for the observations (technically termed chance, bias, or confounding) could not be ruled out.
How does processed meat compare to smoking and other carcinogens?
The IARC deals in strength of evidence. For this agency, scientific evidence either points to a compound causing cancer in humans or it doesn’t. To classify carcinogenic status, it separates agents into five groups:
Group 1 – carcinogenic to humans
Group 2A – probably carcinogenic to humans
Group 2B – possibly carcinogenic to humans
Group 3- not classifiable
Group 4 – probably not carcinogenic
Of the millions of chemicals in the world, the WHO only only 118 agents fall into group 1. This group includes tobacco smoke, asbestos, aflatoxins (a chemical sometimes found in organic peanut butter), coal emissions from indoor stoves and as of this morning, processed meat.
However, the number of cases caused by each of these agents — or their cancer risk among the general population — varies. That’s because risk, from a public health perspective, is a statistical property. For instance, check out the Cancer Research UK’s breakdown of how smoking and meat consumption contribute to overall cancer rates in the UK:
— Cancer Research UK (@CR_UK) October 26, 2015
The risk — the statistical rate in the population — of developing cancer is greater for smoking tobacco than eating processed meat.
Ed Yong’s piece in The Atlantic does a great job of outlining why WHO’s categorical danger versus actual statistical risk can sometimes breed confusion.
My grandpa ate 7 lbs of red meat every day and lived to be 130. Why didn’t he get cancer?
It’s impossible to determine if an individual will get cancer based on a lifestyle choice, whether it’s smoking or eating processed meat. Cancer occurs when a healthy cell acquires enough mutations to start replicating uncontrollably and to spread into new organs away from its site of origin.
Those mutations vary dramatically among the types of cancer (lung, pancreatic, colorectal, etc). They can even differ within a subtype, meaning a pancreatic cancer from one person can be genetically dissimilar than a pancreatic cancer in another. Plus, a malignant tumor in a single individual is constantly evolving, and there is evidence suggesting no two cancer cells in the same tumor are the same.
A cell’s ability to acquire these mutations depends on personal genetics — whether or not you inherited a predisposition from mom and dad — and exposure to compounds that are genotoxic; that is, that can change your DNA.
The IARC is merely concluding today that processed meat can cause cancer if given the chance.
How much processed meat is safe to eat?
On an individual level, it’s hard to say. On a population level, the WHO report cites this epidemiology meta analysis, which examined colorectal cancer studies going back to 1966.
Based on that study, a person who eats 50 grams per day of processed meat has an 18 percent higher chance of developing colorectal cancer. A person who eats 100 grams has a 36 percent higher chance and so on. According to Cancer Research UK, 50 grams per day would be on par with two slices of ham. Two slices of bacon is about 75 grams.
— Cancer Research UK (@CR_UK) October 26, 2015
For red meat, cancer risk elevates by 17 percent for every 100 grams per day that is consumed.
When cancer risk levels of both processed meat and red meat were modeled together, the relationship maxed out at 140 grams per day.
Are organically raised meats as dangerous as industrial-produced?
In the context of this WHO report, yes. The danger posed by processed meat and red meat comes from chemical properties inherent to all forms of meat (see below). Organic meat labeling, at least according to the USDA, tackles issues like antibiotic use, hormone use, and access to exercise for domestic animals, which falls outside the scope of the WHO’s report.
What about white meats (chicken and fish) that are processed? Like smoked salmon? What about nitrate-free meats?
The scientific connection between smoked, salted, or cured meats comes down to how these preservation processes influence the chemistry of these foods.
Curing meats involves adding salt, sugar, nitrates like saltpeter or nitrites to preserve foods against bacteria-induced rot and maintain flavor, though the most validated suspects in the cancer drama are nitrites. Enzymes in meat convert nitrites into nitrogen oxide and nitrous acid. Both of which can chemically react with amino acids found in our proteins to form N-nitroso-compounds (NOCs), a class of carcinogenic compounds also sometimes described under the banner nitrosamines. Though saltpeter and other nitrates are a somewhat antiquated way of curing meats, these compounds can become nitrites thanks to chemical reactions mediated by our own cells, by bacteria in our guts, or by bacteria naturally found in meat. NOCs are also spawned by chemical reactions with heme, the red pigment/compound responsible for binding oxygen in blood cells.
“For the red meat, they think it is the HEME iron that is damaging,” said Alice Bender, director of Nutrition Programs at American Institute for Cancer Research.
NOCs can physically bind to genetic material — forming what are called DNA adducts — which can initiate the transformation of healthy cells into a cancer cells.
Grilling, barbecuing, frying, broiling or any form of cooking contributes to the carcinogenic potential of meat due to the accumulation of polycyclic aromatic hydrocarbons (PAHs). How this accumulation happens isn’t completely understood, but this 2005 review paper offers a general overview of the current thinking. In sum, it states that heat breaks down complex organic compounds naturally found in muscle tissue — like fats and sugars — into smaller compounds that react to form PAHs:
At high temperatures, organic compounds are easily fragmented into smaller compounds, mostly free radicals, which may then recombine to form a number of relatively stable PAHs. At temperatures below 400 °C, only small amounts of PAHs are formed. However, the amounts of PAHs increase linearly in the range 400–1000 °C.
Excessive heat can also introduce carcinogens to meat in the form of heterocyclic aromatic amines, which the WHO report abbreviates as HAAs. (The term HCA is also used and means the same thing.) Heterocyclic aromatic amines are formed when naturally-occurring compounds like creatine (found in high quantities in muscle meats), amino acids in protein and sugars become too hot. Like NOCs, heterocyclic aromatic amines are genotoxic and can yield mutations that cause cancer. Here’s what the 2005 review had to say about the formation of heterocyclic aromatic amines:
In general pan-frying and grilling produce high yield of HCAs at cooking temperatures from 200 °C and above, boiling yields little or no HCAs, and deep-fat frying, roasting, and baking procedure give variable yields. Extremely high yield of HCAs have been reported in pan residues….from frying, roasting or baking, while most commercial bouillon cubes contain modest amounts.
PAHs are also thought to be produced when fat and juices from meat hit flames over open grills. These PAHs then float up and “adhere to the surface of meat,” according to the National Cancer Institute.
Smoking introduces carcinogens to meat in the form of polycyclic aromatic hydrocarbons created in the fumes of burning wood or by heat.
These food prep processes increase the amounts of these carcinogenic chemicals in meat, regardless of whether the original animal was a mammal, a bird or a fish. However, the WHO panel primarily made their assessment based on research into processed red meats.
However, given the limited number of studies that have independently examined the influence of different preservation methods on cancer risk among the general population, the WHO can’t state yet if one is worse than another:
Different preservation methods could result in the formation of carcinogens (e.g. N-nitroso compounds), but whether and how much this contributes to the cancer risk is unknown.[Also] there were not enough data for the IARC Working Group to reach a conclusion about whether the way meat is cooked affects the risk of cancer.
Should you give up meat?
Not necessarily. Alice Bender, director of Nutrition Programs at American Institute for Cancer Research, said it’s important to remember the distinction that the WHO report makes between processed meat and red meat.
“Processed meats even in small amounts were increasing risk — a little less than 2 ounces [which is equivalent to] a hot dog or few slices of cold cuts,” Bender told the NewsHour. “For red meat, what is important to keep in mind isn’t so much that you shouldn’t eat it at all, but more the amounts. It’s large amounts that become harmful.”
Oncologist John Schoenfeld of Harvard Medical School agreed.
“There are benefits to red meat, and the findings don’t say that a balanced diet that includes red meat is bad,” Schoenfeld said. “[The report] gives pause to the part of the American diet that is high in meat consumption and red meat. There needs to be broader discussion on [red meat], weighing the benefits and the risks.”
For more of our conversation with John Schoenfeld, check out his interview with PBS NewsHour correspondent William Brangham.
If you have a question, leave it in the comments. We’ll address them ASAP.
Rhana Natour contributed to this report.
Still have questions about Monday’s news from World Health Organization? Join us at 1 p.m. EDT Wednesday for a Twitter chat. Joining us to take your questions will be Alice Bender from the American Institute for Cancer Research (@aicrtweets) and NPR Food and Culture Correspondent, Alison Aubrey (@AubreyNPRFood).
Leave your questions for these experts in the comments below, or on Twitter using the hashtag #NewshourChats.