Eating meat causes cancer – let’s get this right

Oh here we go again. Everything causes cancer. Time to move into a a giant bubble and eat nothing but GMO, 100% gluten wheat bread. Now, eating meat causes cancer.

Unless you don’t have a Facebook and Twitter account (and who doesn’t these days), your timelines were flooded, literally, with memes, articles, and uptight vegans laughing at everyone because the World Health Organization stated that eating meat causes cancer – that’s either processed meat, like a good French sausage, or unprocessed red meat.

Back to my giant bubble.

For all of you with good scientifically skeptical minds, this story has already been parsed and dissected, and you’ve moved on. That’s what I did, as I sliced some excellent French sausage and ate it with my GMO crackers.

But then I thought, well if this is a thing, should I be worried? Should my non-vegan carnivore readers be worried?

Let’s look at this every way I can. Maybe there’s enough evidence to convince some or all of us to modify our behavior. Or not.

I’m charging ahead, brave readers – read on.

The WHO and cancer research

Let’s look at what happened and what was said. The World Health Organization has a research group, called the International Agency for Research on Cancer, or IARC for short. The role of IARC is an important one for cancer research.

As they say in their mission statement,

The objective of the IARC is to promote international collaboration in cancer research. The Agency is inter-disciplinary, bringing together skills in epidemiology, laboratory sciences and biostatistics to identify the causes of cancer so that preventive measures may be adopted and the burden of disease and associated suffering reduced.

This is an important function. The IARC brings together data gathered everywhere in the world, and uses the evidence to make recommendations on what is known and not known about cancer. This is good information.

But one of their most important roles is to look at what may cause cancer and what may prevent it.

Causes and prevention of cancer Emphasis is placed on elucidating the role of environmental and lifestyle risk factors and studying their interplay with genetic background in population-based studies and appropriate experimental models.

This emphasis reflects the understanding that most cancers are, directly or indirectly, linked to environmental factors and thus are preventable.

One of the more important responsibilities of the IARC is to classify cancer risks (anything that touches the body from airborne chemicals to foods to light to anything). IARC places these compounds or agents into 5 groups:

Group 1 Carcinogenic to humans 120 agents
Group 2A Probably carcinogenic to humans   81
Group 2B Possibly carcinogenic to humans 299
Group 3 Not classifiable as to its carcinogenicity to humans 502
Group 4 Probably not carcinogenic to humans     1

A side note – you all are probably wondering, like me, what’s the only compound in Group 4, “probably not carcinogenic to humans.” Well, it’s caprolactam, a component of nylon. So there you go, wear and use nylon whenever you want.

It’s important to note one major flaw to this classification system – it doesn’t address risk. It’s almost a binomial system, that is there’s evidence that something causes cancer or it doesn’t. Groups 2A, 2B, and 3 exist because data isn’t sufficient to move it to group 1 or group 4.

Moreover, the grouping doesn’t give you any information about relative or absolute risk. The Group 1 classification, “carcinogenic to humans,” is only partially useful to a person. Those 120 agents in Group 1 have widely varied risk profiles. Cancer really isn’t a binomial disease – risk for each different agent increases over time and hazard amount. And sometimes, certain cancers result from random mutations, genetic abnormalities or infectious diseases.

As Cancer Research UK explains,

As Professor Phillips explains, “IARC does ‘hazard identification’, not ‘risk assessment’. “

That sounds quite technical, but what it means is that IARC isn’t in the business of telling us how potent something is in causing cancer – only whether it does so or not”, he says.

To take an analogy, think of banana skins. They definitely can cause accidents, explains Phillips, but in practice this doesn’t happen very often (unless you work in a banana factory). And the sort of harm you can come to from slipping on a banana skin isn’t generally as severe as, say, being in a car accident.

But under a hazard identification system like IARC’s, ‘banana skins’ and ‘cars’ would come under the same category – they both definitely do cause accidents.

Remember, all the IARC has stated is that meat is a carcinogen in a broad group – that classification doesn’t tell us whether 1 steak per year will give us cancer or 1 steak every day.

Given all of this information or lack thereof, let’s look at IARC’s research and conclusions about red meat.

Eating meat causes cancer?

Again, unless you didn’t have internet these past few days, headlines have been blasting everywhere about red meat, processed meat and cancer. But I’m a good scientific skeptic, so headlines and popular press barely mean anything to me scientifically. I want good published evidence.

Let’s look at what the IARC said, and the underlying evidence that lead the IARC to make its classification.

The IARC published it’s recommendations (pdf, registration required) in Lancet Oncology, a respected peer-reviewed journal dealing with cancer biomedical science. A “Working Group” of 22 cancer scientists from across the world. They reviewed from over 800 published epidemiological studies that analyzed the relationship between the consumption of red meat (or processed meat) and cancer.

Their conclusions were:

Overall, the Working Group classified consumption of processed meat as “carcinogenic to humans” (Group 1) on the basis of sufficient evidence for colorectal cancer. Additionally, a positive association with the consumption of processed meat was found for stomach cancer.

The Working Group classified consumption of red meat as “probably carcinogenic to humans” (Group 2A). In making this evaluation, the Working Group took into consideration all the relevant data, including the substantial epidemiological data showing a positive association between consumption of red meat and colorectal cancer and the strong mechanistic evidence. Consumption of red meat was also positively associated with pancreatic and with prostate cancer.

The Working Group classified processed meat in Group 1, “carcinogenic to humans”, and red meat in Group 2A, “probably carcinogenic to humans.”

Just to clarify, “red meat” is any meat that’s a dark red color before it’s cooked, specifically any mammalian meat from cattle, sheep, goats, pigs, and a few others.

“Processed meat” is any mammalian meat that’s not sold fresh, but instead has been cured, salted, smoked, or otherwise preserved in some way (so things like bacon, sausages, hot dogs, ham, salami, and pepperoni).

Since someone asked the question, “burger” meat is not processed (grinding doesn’t count), and it is considered a red meat.

I found that the breadth and depth of the IARC’s research, their own meta analyses, and the use of other meta analyses to be beyond reproach. I found no evidence, unless we choose to cherry pick a study here and there, that their conclusions were wrong.

Cancer Research UK, a fairly important organization in cancer research, concurs with the IARC conclusions, but only to a point. They state that there’s “now a large body of evidence that bowel cancer is more common among people who eat the most red and processed meat.” (Bowel cancer is also known as colorectal cancer in most areas of the world.)

There is a lot of convincing evidence that links consumption of meat to colorectal cancer. A recent powerful meta analysis is one of the more convincing ones.

I need to be consistent. Even though I personally love bacon and a good grilled steak, I’m not going to kid myself about the cancer link. The evidence strongly supports the cancer link.

But…because there is always a but.

How do we interpret this IARC decision?

First of all, it’s important to note that the IARC determined that red meat and processed meat are not equivalent with respect to cancer. Processed meat was worse.

Secondly, we need to look at the actual risk. The findings of the IARC showed that those who ate the most processed meat had an approximately 18% higher risk of developing colorectal cancer, compared to those who ate the least.

But what does that 18% actually mean? It is a relative risk, which seems to be really high. If I could reduce my risk of a particular cancer by 18%, I’d be on board in a second.

Except when you convert that number to an absolute number, things look very different. According to the National Cancer Institute, the lifetime risk of colorectal cancer in the USA is about 45 per 1,000 men and women.

If you reduced your consumption of processed meat to the lowest level, your lifetime risk of colorectal cancer would be about 36.9 per 1,000 people. In other words, your lifetime risk might be reduced by reducing consumption of processed meat, nevertheless, you still have a significant risk of colorectal cancer.

An article in the Atlantic, which also makes this point about risk, reminds us that:

Two risk factors could be slotted in the same category if one tripled the risk of cancer and the other increased it by a small fraction. They could also be classified similarly even if one causes many more types of cancers than the other, if it affects a greater swath of the population, and if it actually causes more cancers.

So these classifications are not meant to convey how dangerous something is, just how certain we are that something is dangerous. But they’re presented with language that completely obfuscates that distinction.

This is what you have to understand. The IARC dumps anything that has been shown to cause cancer into Group 1. But it doesn’t distinguish between the different compounds in Group 1.

For example, is smoking worse than eating meat? By far. Smokers don’t just have a few percent increase in risk of cancer compared to non-smokers, they are 1500-3000% more likely to contract cancer over those who never smoked.

For those of you who want to create some nonsensical strawman argument that cigarettes and red meat are equivalent on the scale of cancer risk factors. Don’t. They aren’t.

Moreover, meat generally causes just one type of cancer – colorectal. Cigarette smoking can cause cancer almost anywhere in the body. Cigarette smoking causes cancers of the lung, mouth, nose, throat, voicebox (larynx), esophagus, liver, bladder, kidney, pancreas, colon, rectum, cervix, stomach, blood, and bone marrow (acute myeloid leukemia).

But in the world of IARC classifications, they look the same. But it’s the risk that matters, and IARC doesn’t get into that (which is problematic if they really want to drive the conversation about cancer preventions).

But I have a slightly more troubling concern with this particular IARC statement. Consumption of foods is a lot more complicated than just eating or not eating meat.

For example, even though there is no real evidence that eating a high fiber diet may decrease colorectal cancer, there may be some protective effect on those who eat high red and processed meat diets.

Furthermore, the link between morbidity and cancer has been established, so maybe those who eat higher amounts of meat may be more obese, contributing to the cancer risk.

Though confounding factors probably were considered by the IARC Working Group, they aren’t really an important factor in the how they classify carcinogens. But it’s something we need in reality.

Summary – eating meat causes cancer?

I’m not sure what I think of the IARC pronouncement. I generally have low risks for cancer, as I don’t smoke, I stay out of the sun, and I don’t drink, some of the higher risk factors for cancer. My consumption of red meat is already way below the amount considered to be risky, so I’m not going to worry too much.

Moreover, a slightly increased risk of one (out of 250 different cancers) probably won’t worry me.

In reality, we need to carefully review what increases risk of cancer. Smoking, inhaling asbestos, sitting in a tanning booth for hours – those risk factors contribute mightily to increased risks of certain types of cancer. I do everything I can to avoid those risks, including snarling at smokers who think it’s alright for them to smoke around me.

Everything we eat causes cancer. Sort of. It’s important to understand absolute risks versus relative risks. And how the relative and absolute risk from eating red meat compares to significantly more scary environmental issues. Like smoking.

And that old adage about “everything in moderation” – it’s applicable to eating meat and cancer. A little every once in a while probably has a negligible effect on your risk. Eat a lot every day, and it has an effect, but on the scale of cancer risks, it’s not that high.

And here’s one more thing. Big Colonoscopy has done a good job in getting us (at least in the USA) to check your plumbing for cancer. We do a great job in early diagnosis which increases the chances of preventing or surviving colorectal cancer.

So, I’m not going to lose much sleep over eating red meat, processed meat, or raw meat (OK, not so often). I don’t think you should either. Remember, a meme on Facebook has all of the scientific relevance of pointing at a 100 year old photo of the Loch Ness Monster and saying “yup, that’s evidence.”

Editor’s note – this article was originally published in October 2015. It has been updated to fix several formatting issues, replace broken links, update information, and clean up some verbiage.

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The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!
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  • I think you missed a few things here, in the bigger picture:

    1 – Colorectal cancer is the 2nd or 3rd (?) leading cause of cancer death, not something to take too lightly.

    2 – The difference between processed red meat and non-processed red meat isn’t large, this is true. But the big picture is that a pescovegetarian diet is actually correlated with a *significant* drop in bowel cancer rates.

    It’s enough difference to cause me to shift my diet. Unlike many cancer risks, this is one I can do something about…

  • medicalpeereview

    meat isn’t the problem, it is the sodding food scientists and their preservatives that make it poisonous. They gave us BSE by suggesting it was ok to feed dead cows to cows.

  • zapp7

    Hypothetically, what would the risk of cancer from processed meats have to be for you to decide to stop consuming it?

    • Chris Preston

      The trouble with answering a question like that is that it is all a sliding scale. The number of deaths due to colon cancer each year are less than deaths due to lung, liver, and stomach cancers. Processed meat only contributes a small amount to colon cancer incidence.

      When you look at causes of cancer deaths, the big factors are tobacco, obesity, HPV and HBV infections, low fruit and vegetable intake, alcohol use, air pollution and radiation exposure (including the sun). Processed meat ranks a long way down in the scale.

      The most appropriate approach to announcements like this is to protect yourself from exposure to the high level risks: tobacco, viral infections, alcohol, obesity, poor diet, sun exposure, etc. and cut down processed meat consumption to a reasonable amount. If 50 g per day of processed meat double the risk of colon cancer, I might contemplate whether I reduced my consumption below that level. To put this in context, smoking increase the risk of lung cancer about 25-fold.

      • zapp7

        Thanks for the reply. I’m always interested in the question as different people have different answers. Some people will dismiss any and all evidence to avoid reconsidering their habits.

        I don’t think that the more dangerous cancer promoters have much to do with addressing the question of what threshold of processed meat cancer risk should alter one’s habits. For example, if there were ten things that were linked to lung cancer more strongly than cigarettes, I wouldn’t be any more inclined to smoke. The dangers of cigarettes are what they are, regardless of other factors.

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