Does Meat Cause Cancer?
Last October, the World Health Organization (WHO) released a two-page report entitled Carcinogenicity of Consumption of Red and Processed Meat, warning the planet that processed meat definitely causes colorectal cancer in humans, and that red meat “probably” causes colorectal cancer in humans. The report listed a total of 20 scientific references. WHO’s frightening anti-meat proclamation made headlines worldwide and had a major impact on how people think about meat and health. While plenty of pro-meat critiques of the WHO report have been published, the majority of those I read took the WHO’s findings at face value and emphasized that the statistical risk associated with eating processed and red meat is very small.
I strongly disagree. I read the report and all of the experimental studies cited in the report. I found no scientific evidence to support the WHO’s anti-meat cries, and I think it is important to set the record straight.
Let me disclose my biases from the start (something WHO committee members should also be required to do). Eight years ago I changed from a low-fat, low-cholesterol, low-carbohydrate, high-fiber/high-plant diet to a mostly-meat diet, loaded with fat and cholesterol and quite low in fiber, and it reversed every health problem I ever had (read my story on the About page). Naturally I was worried that my new meaty menu was going to kill me, so I began digging into the science for myself and lo and behold: there was no evidence that meat increases risk for heart disease, obesity, or diabetes after all. I came to believe, based on my powerful personal experience and my reading of the research, that animal foods (meat, poultry, and seafood), complete with their natural cholesterol and fats, are good for people.
But what about cancer? Is my meat-based diet, which is working so well for me right now, putting me at risk for cancer down the road?
I am a scientifically curious psychiatrist. I love getting to the bottom of things, and my life’s work centers around helping people confront reality, no matter how complicated or unwelcome it can sometimes be. If the truth is that meat makes me feel great now but is ultimately going to do me in, I want and need to know that. Whether you eat meat only occasionally, every day, or are an all-meat zero-carber, you need to know it too, so I dove into the WHO report to see what’s what.
What I discovered was that THE WHO REPORT IS NOT A SCIENTIFIC DOCUMENT. IT IS A POLITICAL DOCUMENT. Politicians can get away with making sweeping statements to the general public that stand on shaky ground. Scientists are held to a higher standard. They are supposed to show their work, and defend their positions as objectively and honestly as humanly possible. After reading the studies upon which the WHO’s anti-meat proclamations are made, I concluded that there simply is no scientific evidence that meat causes cancer in humans.
And I am not alone.
In November 2013, 23 cancer experts from eight countries gathered in Norway to examine the science related to colon cancer and red/processed meat. They concluded:
“…the interactions between meat, gut and health outcomes such as CRC [colorectal cancer] are very complex and are not clearly pointing in one direction….Epidemiological and mechanistic data on associations between red and processed meat intake and CRC are inconsistent and underlying mechanisms are unclear…Better biomarkers of meat intake and of cancer occurrence and updated food composition databases are required for future studies.” 1)Oostindjer M et al 2014. The role of red and processed meat in colorectal cancer development: a perspective. Meat Science 97: 583–596. To read the full report: http://www.ncbi.nlm.nih.gov/pubmed/24769880 [open access]
Translation: we don’t know if meat causes colorectal cancer. Now THAT is a responsible, honest, scientific conclusion.
How the WHO?
How could the WHO have come to such a different conclusion than this recent international gathering of cancer scientists? As you will see for yourself in my analysis below, the WHO made the following irresponsible decisions:
- The WHO cherry-picked studies that supported its anti-meat conclusions, ignoring those that showed either no connection between meat and cancer or even a protective effect of meat on colon cancer risk. These neutral and protective studies were specifically mentioned within the studies cited by the WHO (which makes one wonder whether the WHO committee members actually read the studies referenced in its own report).
- The WHO relied heavily on dozens of “epidemiological” studies (which by their very nature are incapable of demonstrating a cause and effect relationship between meat and cancer) to support its claim that meat causes cancer.
- The WHO cited a mere SIX experimental studies suggesting a possible link between meat and colorectal cancer, four of which were conducted by the same research group.
- THREE of the six experimental studies were conducted solely on RATS. Rats are not humans and may not be physiologically adapted to high-meat diets. All rats were injected with powerful carcinogenic chemicals prior to being fed meat. Yes, you read that correctly.
- Only THREE of the six experimental studies were human studies. All were conducted with a very small number of subjects and were seriously flawed in more than one important way. Examples of flaws include using unreliable or outdated biomarkers and/or failing to include proper controls.
- Some of the theories put forth by the WHO about how red/processed meat might cause cancer are controversial or have already been disproved. These theories were discredited within the texts of the very same studies cited to support the WHO’s anti-meat conclusions, again suggesting that the WHO committee members either didn’t read these studies or deliberately omitted information that didn’t support the WHO’s anti-meat position.
Does it matter whether the WHO gets it right or wrong about meat and cancer? YES.
“Strong media coverage and ambiguous research results could stimulate consumers to adapt a ‘safety first’ strategy that could result in abolishment of red meat from the diet completely. However, there are reasons to keep red meat in the diet. Red meat (beef in particular) is a nutrient dense food and typically has a better ratio of N6:N3-polyunsaturated fatty acids and significantly more vitamin A, B6 and B12, zinc and iron than white meat (compared values from the Dutch Food Composition Database 2013, raw meat). Iron deficiencies are still common in parts of the populations in both developing and industrialized countries, particularly pre-school children and women of childbearing age (WHO)… Red meat also contains high levels of carnitine, coenzyme Q10, and creatine, which are bioactive compounds that may have positive effects on health.” 2)Oostindjer M et al 2014. The role of red and processed meat in colorectal cancer development: a perspective. Meat Science 97: 583–596.
The bottom line is that there is no good evidence that unprocessed red meat increases our risk for cancer. Fresh red meat is a highly nutritious food which has formed the foundation of human diets for nearly two million years. Red meat is a concentrated source of easily digestible, highly bioavailable protein, essential vitamins and minerals. These nutrients are more difficult to obtain from plant sources.
It makes no sense to blame an ancient, natural, whole food for the skyrocketing rates of cancer in modern times. I’m not interested in defending the reputation of processed meat (or processed foods of any kind, for that matter), but even the science behind processed meat and cancer is unconvincing, as I think you’ll agree.
Ready? Hold your nose, we’re going in.
The Epidemiological “Evidence” Against Meat
The WHO looked at more than 800 “epidemiological” (more about that word in a moment) human studies of red/processed meat and cancers of all kinds. Of the 16 types of cancer explored, the WHO chose to base its doomsday decree on studies of colorectal cancer only (presumably because the evidence related to other kinds of cancer was lacking).
The Epidemiology of Red Meat and Cancer
Of those 800+ epidemiological studies, a mere 29 were put forth by the WHO as “informative” about the connection between unprocessed red meat and colorectal cancer.
Of those 29 studies, 14 suggested that red meat was associated with a higher risk for colorectal cancer in humans; 15 of them did not.
The Epidemiology of Processed Meat and Cancer
As for processed meat, the WHO chose 27 of the 800+ studies to make its case for the cancer connection.
Of those 27 studies, 18 suggested that processed meat was associated with a higher risk for colorectal cancer in humans; 9 did not.
The Problem with Epidemiological Studies
Epidemiological studies are not experiments; they are untested hypotheses (guesses), and are therefore completely impotent when it comes to the ability to show cause-and-effect relationships between any two things, including things like meat and cancer. The scientific method demands that these guesses then be tested in clinical studies to see whether or not they are accurate.
Here’s an example: let’s say you are interested in understanding what causes alcoholism. You interview 10,000 alcoholics and 10,000 non-alcoholics by giving them questionnaires about their daily habits. You wonder if pretzels have something to do with drinking because your alcoholic grandfather often stumbles in late at night with pretzel crumbs on his shirt. So in your study you include the following question: “How often have you eaten pretzels in the past two years?” If you find that alcoholics reported eating significantly more pretzels than the teetotallers, the next day the following headline might appear in the Huffington Post: “Eating pretzels increases risk of alcoholism.” The story that follows the headline might advise people to eat fewer pretzels to reduce their risk of alcoholism.
Association is not causation. It could be that pretzels cause alcoholism, but it could also be that alcoholics spend more time in bars where there are lots of free pretzels. The only way to know for sure what’s going on is to do an experiment. Feed some non-alcoholics pretzels every day and watch what happens to them compared to a similar group who is banned from eating pretzels. [For an excellent review explaining the limitations of epidemiological studies of meat and human health, please see this article authored by the USDA’s National Program Leader for Human Nutrition, David Klurfeld PhD: Klurfeld DM 2015 Research gaps in evaluating the relationship of meat and health. Meat Science 109: 86–95.]
Regardless, even if you believe in the (non-existent) power of epidemiological studies to provide meaningful information about nutrition, more than half of the 29 epidemiological studies did NOT support the WHO’s stance on unprocessed red meat and colorectal cancer.
It is irresponsible and misleading to include this random collection of positive and negative epidemiological studies as evidence against meat.
The following quote is taken from one of the experimental studies cited by the WHO. The authors of the study begin their paper with this striking statement:
“In puzzling contrast with epidemiological studies, experimental studies do not support the hypothesis that red meat increases colorectal cancer risk. Among the 12 rodent studies reported in the literature, none demonstrated a specific promotional effect of red meat.” 3)Pierre FH et al 2004. Beef meat and blood sausage promote the formation of azoxymethane-induced mucin-depleted foci and aberrant crypt foci in rat colons. J Nutr 134: 2711–16.
[Oddly enough, none of these twelve “red meat is fine” studies, which the authors went on to list and describe within the text of the introduction to this article, were included in the WHO report].
I cannot emphasize enough how common it is to see statements like this in scientific papers about red meat. Over and over again, researchers see that epidemiology suggests a theoretical connection between some food and some health problem, so they conduct experiments to test the theory and find no connection. This is why our nutrition headlines are constantly changing. One day eggs are bad for you, the next day they’re fine. Epidemiologists are forever sending well-intentioned scientists on time-consuming, expensive wild goose chases, trying to prove that meat is dangerous, when all other sources–from anthropology to physiology to biochemistry to common sense—tell us that meat is nutritious and safe.
The Experimental Evidence against Red Meat
A grand total of SIX experimental studies were cited in the WHO report (references 13-15, and 18-20) as evidence that meat causes cancer, four of which were conducted by a single research group (Pierre FH and/or Santarelli RL). Three are rat studies, two are human studies, and one is a rat/human study [a study of rats and humans, not of hybrid rat-human creatures].
Let’s look at each of these carefully to see if there is cause for alarm. If you’d rather cut to the chase, click here to go directly to the summary.
The WHO offers three possible theories about how processed meat could cause cancer:
“Meat processing, such as curing and smoking, can result in formation of carcinogenic chemicals, including N-nitroso-compounds (NOC) and polycyclic aromatic hydrocarbons (PAH). Cooking improves the digestibility and palatability of meat, but can also produce known or suspected carcinogens, including heterocyclic aromatic amines (HAA) and PAH. High-temperature cooking by pan-frying, grilling, or barbecuing generally produces the highest amounts of these chemicals.” 4)http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045%2815%2900444-1/fulltext
The authors of the next study begin by shooting holes in two of these theories:
Theory 1: Processing meat leads to the formation of “N-nitroso compounds”, which may cause cancer.
Problem with theory 1: This study’s authors point out that when rats are fed a bacon-based diet, which is high in N-nitroso compounds, they DO NOT develop signs of cancer.5)Parnaud G et al 2000. Endogenous N-nitroso compounds, and their precursors, present in bacon, do not initiate or promote aberrant crypt foci in the colon of rats. Nutrition and Cancer 38(1):74–80. [This “bacon doesn’t cause cancer in rats” study was not included in the WHO report].
Theory 2: When meat is cooked at high temperatures, compounds called “heterocyclic amines” (HAAs) can form, and these may promote cancer in rodents and monkeys.
Problems with theory 2: This study’s authors explain that chicken cooked at high heat also contains heterocyclic amines, but chicken is not associated with cancer risk (and the same is true for fish). Also, the doses of these compounds that cause cancer in animals are 1,000 to 100,000 times higher than doses found in human food.
Both of these highly questionable theories are nevertheless cited in the WHO report as evidence against processed meat.
As for the PAH theory, in case you were wondering, ALL charred, smoked, baked and toasted foods contain PAHs, including grilled vegetables, breads and cereals. In fact, breads and cereals contribute the highest amounts of PAHs to the average person’s diet,6)Bansal V and Kim K 2015 Environment International 84:26–38 but nobody studies the potentially cancerous effects of those, do they?
The Experimental Evidence: Should you Worry?
I personally think the first three studies of carcinogen-injected rats are not only ludicrous, but irrelevant to human health, and can be dismissed.
I also find the rat-human study of ham unconvincing; the biomarker results were all over the place, and I don’t care about the health effects of dried-up ham.
In my opinion, only the final two studies cited are worth considering, because a) they are human studies and b) they use unprocessed red meat. They both propose that heme, which is what makes red meat red, can increase mutations in colon cells.
Unfortunately, these two studies were not designed in a way that could prove that it was red meat that caused mutation rates to increase, but let’s just say for the sake of argument that red meat increases mutations. There still may not be any cause for alarm, because:
- Mutations occur constantly as a normal part of everyday life.
- Mutations are caused by a wide variety of natural stimuli– from within our bodies, from the foods we eat, and from the environment.
- Our bodies have evolved numerous, sophisticated mechanisms to neutralize mutations.
- Mutation rates may have nothing to do with cancer risk. [Please see my series What Causes Cancer?].
However, just because these two studies were poorly designed doesn’t mean we should discard the theory that heme might pose health risks to humans. Could heme be the cancer-causing culprit lurking within red meat?
Not according to this 2015 statement written by David Klurfeld, PhD, the USDA’s National Program Leader for Human Nutrition:
“While heme iron can increase cell proliferation in the colonic mucosa of mice and catalyze the formation of N-nitroso compounds in rats, there is no data that normal levels of heme in human intestine contributes to any harm.”17)Klurfeld DM 2015 Research gaps in evaluating the relationship of meat and health. Meat Science 109: 86–95
Interestingly, Dr. Klurfeld was one of the authors of the WHO report.
When you get right down to it, the only plausible evidence to suggest that red meat might be risky to human colon health is contained in two, that’s TWO, human studies, both of which were very small and poorly designed, and therefore unable to give us useful information about the effects of red meat on cancer risk. These studies are inconclusive at best, and worthless at worst.
Human nature being what it is, believing is seeing.
People looking for reasons to avoid red meat may view these two studies as concerning.
People looking for reasons to eat red meat may view these two studies as reassuring.
It is your choice, of course! I just want you to have the facts so you can make an informed decision.
Trumpeting to the world that meat causes cancer on the basis of these two studies is ridiculously irresponsible and makes a mockery of the WHO. There is ample information to suggest that the WHO’s report is biased, incomplete, and scientifically dishonest.
To reward yourself for making it to the end of this convoluted scientific journey, I invite you to enjoy a more light-hearted take on the WHO report—a poem written in the style of the beloved Dr. Seuss.
If found this article interesting, you might also enjoy the following posts:
Does Carnitine from Red Meat Cause Heart Disease? An example of how experimental scientists twist themselves into pretzels trying to connect meat with human health problems—and fail miserably.
New Dietary Guidelines Hazardous to your Health? A critique of the USDA dietary guidelines generation process, including clear evidence of anti-meat bias.
Do High-Fat Diets Cause Depression? How the inattention to laboratory animal chow ingredients render most nutrition studies conducted in rodents completely useless.
References [ + ]
|1, 2.||↑||Oostindjer M et al 2014. The role of red and processed meat in colorectal cancer development: a perspective. Meat Science 97: 583–596.|
|3.||↑||Pierre FH et al 2004. Beef meat and blood sausage promote the formation of azoxymethane-induced mucin-depleted foci and aberrant crypt foci in rat colons. J Nutr 134: 2711–16.|
|5.||↑||Parnaud G et al 2000. Endogenous N-nitroso compounds, and their precursors, present in bacon, do not initiate or promote aberrant crypt foci in the colon of rats. Nutrition and Cancer 38(1):74–80.|
|6.||↑||Bansal V and Kim K 2015 Environment International 84:26–38|
|7.||↑||Pierre FH et al 2007 Carcinogenesis 28 (2):321–327.|
|8.||↑||Kuo LJ and Yang LX 2008 Gamma-H2AX-a novel biomarker for DNA double-strand breaks. In Vivo 22(3):305-9|
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|10.||↑||Grotto D et al 2009 Importance of the lipid peroxidation biomarkers and methodological aspects for malondialdehyde quantification Quim Nova 32(1): 169-174|
|11.||↑||Joosen AMCP et al 2009 Carcinogenesis 30 (8): 1402–1407|
|12, 15.||↑||apparent total N-nitroso compounds|
|13.||↑||Christmann M et al 2011 O6 -Methylguanine-DNA methyltransferase (MGMT) in normal tissues and tumors: Enzyme activity, promoter methylation and immunohistochemistry. Biochimica et Biophysica Acta 1816: 179–190.|
|14.||↑||Lewin MH et al 2006. Red meat enhances the colonic formation of the DNA adduct O6-carboxymethyl guanine: implications for colorectal cancer risk. Cancer Res 66: 1859–65.|
|16.||↑||Senthong P et al Nucleic Acids Res. 2013 Mar; 41(5): 3047–3055.|
|17.||↑||Klurfeld DM 2015 Research gaps in evaluating the relationship of meat and health. Meat Science 109: 86–95|