Meat causes cancer? The answer is complicated and nuanced

Over the past few years, there have been numerous stories about whether meat causes cancer. Well, the answer is extremely complicated and nuanced, something doesn’t play well with clickbait headlines. People want to know The Truth™ about meat. Oh, and this article isn’t about vaccines, shocking, right?

A couple of years ago we were flooded by 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.

Then recently, we were flooded by new memes, articles, and uptight vegans when an article was published to tell us that meat was safe. Of course, that was followed up by even newer memes, articles, and uptight vegans that that new article was junk, and meat causes cancer. 

Even those of you with good scientifically skeptical minds (which includes a lot of vegans) are wondering if anyone knows anything about meat and cancer. So I sliced some excellent French sausage and ate it with my GMO crackers – then this old avian dinosaur did what he is supposed to do, he read the articles.

So, should you worry whether meat causes cancer? I agree that there are a lot of issues about people eating beef, pork, and chicken for our individual health and for the health of the planet. But that’s outside of my bailiwick, and it will garner an incredible amount of yelling and screaming.

But I’m going to be brave (or not) and just try to answer the simple question of whether meat causes cancer. And here we go.

meat causes cancer
Photo by Victoria Shes on Unsplash.

First up, cancer

One of the goals of this website is to dispel pseudoscience and misinformation about important medical issues especially with regards to vaccines and cancer – of course, there are a couple of vaccines, the HPV and hepatitis B vaccines that cross over both worlds. 

There are so many myths about cancer – I have dispelled many of them in a recent article, but here are some basics about cancer.

Simply put, cancer is a group of hundreds of unrelated diseases characterized by abnormal cell growth which can invade or metastasize to other tissues and organs. Although people use tumor and cancer interchangeably, not all tumors are cancers. There are benign tumors that do not metastasize and are not cancers.

Cancer usually requires numerous, possibly up to 10, independent genetic mutations in a population of cells before it can become a growing, metastatic cancer. Each mutation is selected, as in natural selection, because it provides some benefit to the cancer cell, such as causing blood vessels to supply the cells for nutrition and oxygen, or the ability to divide rapidly, whatever the feature is.

These mutations can have a lot of different causes. The environment (like smoking or UV radiation), viruses (hepatitis B and human papillomavirus are the most common), and genetics all can contribute to an increased risk of cancer. Furthermore, about 67% of all cancers have no cause and result from random mutations in the genome of a cell.

There are a few things you can do to prevent cancer, such as quitting smoking, staying out of the sun, getting your hepatitis B and HPV vaccinations, not drinking alcohol, keeping a low body weight, and eating a balanced diet. But even if you are a paragon of healthy living, a random mutation in some cell in your body can lead to cancer.

Four paragraphs can hardly do justice to the mountains of published scientific research data related to cancer. PubMed list nearly 4 million (yes, million) articles on cancer. I admit to not reading all 4 million, though I have read a few hundred, especially on HPV and current cancer treatments.

I am a cancer survivor. I utilized science-based medicine to treat and ultimately conquer that cancer And lucky me, there’s been no recurrence after nearly 30 years. I did not turn to fake science like vitamin C or a colon detox – I relied upon the best practices of my oncologist, surgeon, and primary care physician (along with the battalion of nurses, techs, friends, and family).

Cancer is a generic word that covers what I stated above – it’s a cell or group of cells that undergo uncontrolled cellular growth and metastasize into other cells and organs. However, cancer is not one disease.

The National Cancer Institute states that there are over 100 types of cancer.  Cancer Research UK states that there are over 200 types of cancer.  The American Cancer Society lists over 70 types of cancer (although some are more classes of cancer rather than a single type).  Wikipedia lists over 180 different cancers.

Because different cancers are not precisely defined, there is some variance over the actual number. In fact, it’s possible to argue that there are orders of magnitude more cancers because cancer in one person may have different characteristics and cellular mutations than cancer in another.

Nevertheless, each of these cancers has different etiologies (causes), pathophysiologies (development), prognosis, and treatment.

Whenever I read some internet pseudoscientists claim that there is a cure for cancer, I laugh because I know that they actually don’t understand the nature of cancer. Maybe someday well into the future, we might have some scientific “ah-ha” moment, and we find a common link between all cancers, but that seems implausible. It is not one disease.

If I see some internet quack proclaim that they have the sure thing cure for cancer, I laugh until I remember that they’re selling that junk to innocent people who think it might help.

One last point. When someone states that something causes cancer, we need to refrain from considering it an on/off switch. There are people who smoke cigarettes their whole life and never get lung cancer (see Note 1).

Meat causes cancer? WHO gives the answer

The first part of this story starts in 2015 when the World Health Organization (WHO) started the angst about meat and cancer.

WHO 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 1Carcinogenic to humans120 agents
Group 2AProbably carcinogenic to humans  82
Group 2BPossibly carcinogenic to humans311
Group 3Not classifiable as to its carcinogenicity to humans500
Group 4Probably 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 a 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.

Photo by Paul Hermann on Unsplash.

But eating meat causes cancer, right?

Again, unless you didn’t have internet in 2015, 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 led the IARC to make its classification.

The IARC published its 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 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 the 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 lifetime 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 any 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 the 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 increases in the 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 obesity 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 how they classify carcinogens. But it’s something we need in reality.

Photo by DJ Johnson on Unsplash.

Now, meat is safe?

In a new article published in the highly-ranked Annals of Internal Medicine, 14 unbiased researchers from across the world concluded that it was OK for humans to continue eating red meat and processed meat at current levels based on five new independent systematic reviews (which were also published in the Annals of Internal Medicine – here, here, here, here, and here).

The conclusions from the various systematic reviews are very similar. For example, Han et al. wrote:

The possible absolute effects of red and processed meat consumption on cancer mortality and incidence are very small, and the certainty of evidence is low to very low.

Zeraatkar et al. concluded:

The magnitude of association between red and processed meat consumption and all-cause mortality and adverse cardiometabolic outcomes is very small, and the evidence is of low certainty.

And another meta-analysis from Zeraatkar et al. stated that:

Low- to very-low-certainty evidence suggests that diets restricted in red meat may have little or no effect on major cardiometabolic outcomes and cancer mortality and incidence.

In other words, these meta-analyses, considered the pinnacle of the hierarchy of biomedical research, indicate that there appears to be either no or very weak evidence of a link between the consumption of red meat and either cardiovascular or cancer events.

Based on this research, the 14 researchers, suggesting the lack of certainty in the evidence, wrote the following statement: The panel suggests that adults continue current unprocessed red meat consumption (weak recommendation, low-certainty evidence). Similarly, the panel suggests adults continue current processed meat consumption (weak recommendation, low-certainty evidence).

Much of nutrition research has been deeply flawed. Most of it uses a case-control type of observational research. Essentially, researchers would take a group that were diagnosed with cancer or any other disease (the case) and compare it to a group that did not (the control). Then, researchers would look backward to determine what might be the differences between the two groups.

Unfortunately, the differences might be much more than meat – I could hypothesize that the control group not only did not eat meat but also exercised more, ate more grains, smoked less, or lived in an area with better water. These are confounding variables that are almost impossible to tease out of data. So if one concluded that the control group ate less meat thus we see less cancer, science would say “maybe, maybe not.”

The five systematic analyses tried to take nutrition research to a different level. First, they only included evidence that met a very strict standard for reliable evidence – they employed the GRADE system to determine which studies to include in their meta-analyses. 

The researchers generally included only high-quality clinical and cohort studies. Cohort studies are much more useful in removing confounding variables. Basically, these types of studies follow individuals to monitor their lifestyle and medical diagnoses over time. In other words, they follow these individuals after they consume meat (of course, over a long period of time).

Furthermore, since these studies are done in real-time, it’s easier to get accurate information about their food consumption instead of relying on memories that need to go back in time for decades.

Based on all of this evidence, the researchers came to this overall conclusion:

The panel suggests that adults continue current unprocessed red meat consumption (weak recommendation, low-certainty evidence). Similarly, the panel suggests adults continue current processed meat consumption (weak recommendation, low-certainty evidence).

 

Of course, some people were angry and outraged by these recommendations (and the lack of evidence supporting the dangers of meat). Julia Belluz at Vox wrote an excellent analysis of these studies and mentioned some of the dissenting voices. 

The Harvard School of Public Health — well known for trumpeting a plant-based, Mediterranean eating pattern — issued a response to the series, essentially discrediting it for discounting all the evidence showing meat’s links with poor health.

Christopher Gardner, a Stanford nutrition researcher, called the study’s GRADE approach inappropriate for nutrition. “I respect they want to have a clear-cut evidence base,” he told Vox, “but it won’t apply to lifestyle.”

I know what some of you are thinking – is science right or wrong on whether meat causes cancer? And why do they keep changing their damn minds about it? 

I’m going to repeat myself once again – science is not dogmatic, it requires evidence, and as we get more evidence, we refine our conclusions. Furthermore, press headlines do a great disservice to science – they make outlandish, unscientific clickbait statements that ignore the quality and quantity of evidence.

Moreover, much of nutritional research is just plain bad. It relies upon animal studies (which rarely ever have any clinical relevance to humans), poorly designed clinical studies (remember, cancer is not an instantaneous on/off switch, it may take decades to observe), or the aforementioned case-control studies. 

These new studies appear, after hours of reading and pouring through the data, as very powerful systematic reviews that appear to show that there is little correlation, let alone causation, between meats and various diseases, including cancer. 

Summary

So, what is the answer to the question in the headline of this article, “meat causes cancer?”

Based on the WHO recommendations, there appears to be a tiny absolute risk of colorectal cancer to eating some meats. But that data is based on the rather weak animal and case-control studies. 

But based on this panel’s extremely detailed systematic analyses (remember, five of them), there appears to be a lack of a definitive link between meat and cancer. Of course, there might be a study in the future that provides contrary conclusions – when that comes out, we need to analyze it in quality versus the research presented in this article.

When I first wrote this article, I was highly skeptical of the conclusions of the World Health Organization. It seemed very weak, although it was hard to criticize their scientific analysis. But on further review, I’m much more skeptical of it.

Human nutrition is extraordinarily complex. Whenever someone writes that they have the miracle food or diet, I know two things – the research is very weak and the conclusions are overhyped. Sometimes nutrition research reminds me of the “science” of the Loch Ness Monster – tons of testimonials, 100-year-old photos that could be anything, and not much evidence. 

Of course, all of this ignores one important issue – the contribution of our love of meat to climate change and the environment in general. That worries me a lot. Of course, I don’t eat much fresh or processed red meat, but that’s more of a personal choice rather than any moral or health issue. But that’s just me.

For now, I always try to conclude an article about food and nutrition with this piece of advice from Michael Pollan – “Eat food, not too much, mostly plants.” 

Notes

  1. Just because some people may not get lung cancer from cigarette smoking does not mean I, or anyone with more than three functioning neurons, believe that it is safe. Individuals who smoke are 25X more likely to be diagnosed eventually with lung cancer. Furthermore, smokers are much more likely to contract other deadly types of cancer, to develop long diseases, and to die of cardiovascular disease. It’s a stupid habit that will kill.
  2. Editor’s note – this article was originally published on October 2015. It has been updated to fix several formatting issues, replace broken links, and add new articles about meat and cancer.

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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!