The media and blogosphere are abuzz with the latest report from the WHO, which classified cured and processed meats as carcinogens and put them in the same category as asbestos, alcohol, arsenic, and tobacco. But what does the research really tell us about the link between red meat and cancer?

Well, here we go again. Each year, like clockwork, the conventional medical establishment mounts an attack against red meat.

For decades, we were told not to eat it because of the cholesterol and saturated fat it contains. When that argument became less convincing, a new one was offered: we shouldn't eat red meat because it increases production of a compound called TMAO, which causes heart attacks.

Now we're being told not to eat red meat—and especially cured and processed meat—because it will give us cancer. In a recent report, the World Health Organization (WHO) ranked bacon, sausage, and other cured and processed meats as "group 1 carcinogens," which puts them in the same category as tobacco, asbestos, alcohol, and arsenic. It also placed fresh red meat in the "group 2A" category, which suggests that it is "probably carcinogenic" to humans.

Of course, this isn't a new argument; it's been around for at least 40 years. As far back as 1975, scientists speculated that the consumption of animal products was linked to cancer. (1)

How strong is the evidence linking red meat to cancer?

However, the evidence supporting this claim has never been as strong as its proponents suggest. I have critically reviewed this evidence on several occasions in the past, as have many of my colleagues. Here's a list of a few articles and podcasts I recommend reading and/or listening to if you'd like to go deep on this topic: I realize that many of you don't have the time to sift through all of that material, so I'll do my best to summarize the salient points here.
Is eating bacon the same as smoking cigarettes when it comes to cancer?
Let's start with a critical review of the evidence linking red meat to cancer that was published in one of the most prestigious scientific journals in the world (Obesity Reviews) in 2010. (2) The authors looked at 35 studies that claimed to find an association between red meat and cancer and found numerous problems. Here are some key passages from this paper, with my commentary.
Collectively, associations between red meat consumption and colorectal cancer are generally weak in magnitude, with most relative risks below 1.50 and not statistically significant, and there is a lack of a clear dose - response trend.
Translation: the association between red meat and cancer is not strong (i.e. comparing bacon to cigarettes is absurd), and in fact is often not distinguishable from chance. If red meat really did cause cancer, you'd expect to see a linear (continuous) increase in cancer rates as red meat consumption increased. But that's not what we see in many cases. In fact, in some studies you actually see a decrease in cancer rates in the people who ate the most red meat. (3)
Results are variable by anatomic tumour site (colon vs. rectum) and by gender, as the epidemiologic data are not indicative of a positive association among women while most associations are weakly elevated among men.
Translation: the studies claim that red meat causes different rates of cancer in different parts of the intestinal tract, and different rates in men and women. For example, in the study I just referenced above (#3), there was an inverse relationship between red meat intake and colon cancer (meaning people who ate more red meat had less colon cancer), but a positive relationship between red meat and rectal cancer. And in an analysis of data from the Women's Health Study, researchers found a strong (and linear) inverse relationship between red meat consumption and colon cancer. (4) Without a clear explanation of why red meat would be prevent some types of intestinal cancer while contributing to others, and have different effects in men and women, the likelihood of a causal relationship between red meat and cancer is reduced.
Colinearity between red meat intake and other dietary factors (e.g. Western lifestyle, high intake of refined sugars and alcohol, low intake of fruits, vegetables and fibre) and behavioural factors (e.g. low physical activity, high smoking prevalence, high body mass index) limit the ability to analytically isolate the independent effects of red meat consumption.
Translation: the studies linking red meat and cancer are plagued by "healthy user bias." This is a fancy way of saying that people who engage in one behavior perceived as healthy are likely to engage in other behaviors they perceive to be healthy. On the flip side, people who engage in one behavior perceived to be unhealthy are likely to engage in other behaviors perceived to be unhealthy.

In an ideal world, we would be able to conduct a randomized, controlled trial to determine whether red meat causes cancer. We'd create two groups of people that are relatively similar in age and other characteristics. Then we'd isolate them in a medical ward, strictly control their diet, exercise, and other lifestyle factors, and then feed one group more red meat and the other group less.

Unfortunately, this will never happen. Cancer can take decades to develop, so these poor souls would be living in a ward for at least 20 years. Even if we could find people to volunteer for such a study, it would be astronomically (and prohibitively) expensive.

As a result, we're left to rely on observational studies to shed light on the question of whether red meat causes cancer. The problem with this is that observational studies do not prove causality—they just demonstrate an association, or relationship, between different variables. Sometimes the association is causal, and sometimes it's not.

Let's consider red meat. Regardless of whether consuming fresh and/or processed red meat is unhealthy, it has certainly been perceived that way for the past half-century in the industrialized world. What this means is that people in observational studies that eat more red meat also have a tendency to smoke and drink more, eat fewer fresh fruits and vegetables, exercise less, and engage in other unhealthy behaviors that could influence cancer risk. This isn't just speculation; it has been shown in numerous studies. (4, 5)

For example, most Americans that eat red meat eat it with a huge bun made of white flour, with a serving or more of other refined carbohydrates (chips, fries, soda) cooked in rancid, industrially processed vegetable or seed oils. How do we know that it's the red meat—and not these other foods—that is causing the increase in cancer?

The better observational studies attempt to eliminate the influence of these other factors, but in practice that is difficult if not impossible.

You can't see what you're not looking for

What's more, there are certain factors that are likely to play a significant role in the relationship between any food that we eat and cancer, but to my knowledge, have never been adequately controlled for in any study.

One of these is the gut microbiome. Previous work has shown that the composition of the gut microbiota may directly affect the influence of dietary factors on cancer risk. (6)

For example, Streptococcus bovis, Bacteroides, Fusobacterium, Clostridia, and Helicobacter pylori have been implicated in tumor development, whereasLactobacillus acidophilus, L. plantarum, and Bifidobacterium longum have been shown to inhibit colon carcinogenesis. (7) Other studies have found that certain species of bacteria were higher in populations with high colon cancer risk, while other species were higher in populations with low colon cancer risk. (8) Finally, a recent paper compared the gut microbiota of 60 patients with colorectal cancer with that of 119 normal controls. The patients with cancer had significant elevations of Bacteroides/Prevotella (both species that are recognized as potentially harmful) when compared to the control group, and the difference was not affected by general patient characteristics (e.g., age, body mass index, family history of cancer), tumor size or location, or disease stage. (9)

We still have a lot to learn about the influence of the microbiome on health and disease, but we know enough already to conclude that it is significant. It is possible—and I would argue likely—then, that the variability we see in studies showing an association between red meat consumption and cancer may be in part due to the status of the patient's microbiome.

In other words, a patient with a dysbiotic (i.e., compromised) microbiome may be at increased risk for cancer if he or she consumes high amounts of either fresh or processed red meat. But a patient with a normal, healthy microbiome may not be.

There is, in fact, some research that hints at this possibility—though it wasn't what the study authors intended. A couple of years ago, scientists from the Cleveland Clinic published a paper linking red meat consumption with the production of a compound called TMAO, which has been associated with cardiovascular disease.

That paper was riddled with problems (which I outlined in this critique), including the most glaring one—that several foods, including seafood, increase TMAO production by a much greater margin than red meat. However, there was one section of the paper that I found to be very interesting.

It showed that omnivores who eat red meat produce TMAO, whereas vegans and vegetarians who hadn't eaten meat for at least a year do not. The researchers claimed that this means eating red meat must alter the gut flora in a way that predisposes toward TMAO production.

However, there's another explanation that I believe is much more plausible: the red meat eaters are engaging in unhealthy behaviors that lead to gut dysbiosis. This could include eating fewer fruits and vegetables and less fermentable fiber, while eating more processed and refined flour, sugar, and seed oils. All of these behaviors have been shown to be more common in the "average" red meat eater, and all of them have been associated with undesirable changes in the gut microbiota. (10, 11, 12)

Apples and oranges (or, Paleo vs. Standard American Diet)

Observational studies are useful for generating hypotheses and identifying general trends. But another limitation they suffer from, in addition to those I've described above, is that they aren't able to detect the effects of crucial differences between study participants.

Consider two different people. One follows a standard American diet, doesn't exercise much, and has a compromised gut microbiome. The other follows a Paleo-type diet, exercises regularly, and has a healthy gut microbiome. In an observational study looking at the relationship between red meat and cancer, at least 95 percent(if not more) of the red meat eaters in typical studies will fall into the former category. If the study concludes that there is a link between red meat and cancer, the 5 percent of the participants that eat a healthy diet, exercise, and have a healthy gut—and are thus highly unlikely to experience the same impact from eating red meat—will be lumped together with the other 95 percent.

Put a different way, it should be fairly obvious, given what we already know about the influence of diet, lifestyle, and the microbiome on cancer risk, that someone following a Paleo-type diet and lifestyle will not share the same cancer risk as someone following a Standard American Diet and lifestyle, even if they are eating an equivalent amount of red meat. Yet these two groups of people are always lumped together in the studies and media reports. This is a huge problem in research, and it has not been adequately addressed.

What's the bottom line?

Even if you ignore everything I've written in this article and accept the WHO report at face value, just how much would your risk of cancer increase if you eat cured and processed meats?

About three extra cases of bowel cancer per 100,000 adults. That means you have about a 1 in 33,000 chance of developing bowel cancer from eating cured and processed meats.

This is a far cry from how much smoking cigarettes, which the WHO now classifies in the same category as eating bacon and salami, increases your risk.

As Professor Ian Johnson of The Institute of Food Research said in an interview with The Guardian:
It is certainly very inappropriate to suggest that any adverse effect of bacon and sausages on the risk of bowel cancer is comparable to the dangers of tobacco smoke, which is loaded with known chemical carcinogens and increases the risk of lung cancer in cigarette smokers by around twentyfold.

Comment: It's too bad that while Chris Kresser seems to have a good bearing on the propaganda being waged against meat-eaters, he falls in line with the government propaganda on smoking. Readers might be interested in researching more on the unfounded propaganda against smokers:

What's more, the report from the WHO classified 940 other agents, along with red meat, as potential carcinogens. In the Guardian article above, Betsy Booren, the vice-president of scientific affairs for the North American Meat Institute, put it in perspective:
The IARC says you can enjoy your yoga class, but don't breathe air (class 1 carcinogen), sit near a sun-filled window (class 1), apply aloe vera (class 2B) if you get a sunburn, drink wine or coffee (class 1 and class 2B), or eat grilled food (class 2A). And if you are a hairdresser or do shift work (both class 2A), you should seek a new career.
At this point, given what the research indicates, I do not feel that modest consumption of cured or processed meat is likely to pose a significant health risk, provided you are doing other things right (i.e., nurturing your gut microbiome, eating nutrient-dense, real foods, exercising, etc.). I think there is even less evidence suggesting that we should limit consumption of fresh red meat, especially when it is cooked using gentle methods (rather than charring it) and when you eat "from nose to tail."

Okay, that's it for this year's installment of "red meat won't kill you." See you next year!