WHO Says Meat Causes Cancer?

grilled meatsDoes 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.


“…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]


How the WHO?


diagram of rodent studies about meat and cancer

The first experimental study cited in the WHO report began with a description of 12 rodent studies showing either no association between meat and cancer or a protective effect of meat on cancer risk. None of these studies were mentioned in the WHO report. All rodents were either pre-injected with carcinogens or bred to be highly susceptible to cancer. (Illustration by Suzi Smith)





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.

infographic of 800 epidemiological studies of meat and cancer

The WHO considered over 800 epidemiological studies regarding red and processed meat and cancer. They based their findings on 56 studies relating to colorectal cancer. Of the red meat studies, more than half found no link between red meat and cancer. Of the processed meat studies, 9 were “negative” and 18 were “positive.” Epidemiological studies are not experimental and should not be viewed as conclusive evidence. The outcomes of epidemiological studies should always be tested in experimental trials to confirm that correlations are not coincidental. (Illustration by Suzi Smith)

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.


epidemiology-cause or coincidence?

Epidemiological studies implying that association is causation frequently result in misleading and conflicting headlines that leave us befuddled about what constitutes healthy eating. (Illustration by Suzi Smith)

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.]




“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.



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.

STUDY 1: Red meat in rats


The purpose of this study was to determine whether heme, the iron-containing compound responsible for the redness of red meat, might be the cancer-causing ingredient within meat. The scientists designed an experiment comparing low-heme meats like chicken to high-heme meats like blood sausage:

  • Step 1. Inject rats with azoxymethane, a powerful carcinogen. Yes, you read that correctly.
  • Step 2. Remove most of the calcium from the rats’ chow (because calcium protects cells against heme).
  • Step 3. Feed rats a 60% freeze-dried meat diet containing skinless chicken (low-heme), lean beef (moderate-heme), or low-fat pork blood sausage (high-heme) for 100 days (approximately 10 rat-years)
  • Step 4: Look for pre-cancerous changes in rats’ colons.

Results: All rats, including the chicken-fed rats, developed potentially pre-cancerous changes in the colon. The more heme the meat contained, the stronger the effect was. None of the rats actually developed cancer.

These results would seem to suggest that skinless white-meat chicken can cause potentially pre-cancerous lesions, which is not what the researchers wanted to find. So they went back and examined the chicken chow more closely and found that the chicken chow contained more arachidonic acid and toxic levels of niacin compared to the other chows, and decided to blame these differences for the unwanted results. They never went back to test these theories, so there is no way to know whether the arachidonic acid or niacin were to blame. They did not go back and subject the beef or sausage chows to additional scrutiny, presumably because the results from experiments conducted with those chows supported their desired conclusions.

TRANSLATION: If you inject yourself with a powerful carcinogen, then eat a calcium-deficient, powdered chicken, beef, or pork diet every day for 10 years, and you are a rat, your colon may start to look funny. We don’t know whether or not you would eventually develop cancer.

STUDY 2: Red meat in rats

Reference 14: Pierre FH et al 2008. Beef meat promotion of dimethylhydrazine-induced colorectal carcinogenesis biomarkers is suppressed by dietary calcium. Br J Nutr 99: 1000–06.

This experiment simply builds upon the findings of those in the above experiment by adding calcium back into the 60% meat chow and showing that it completely protects against potentially pre-cancerous lesions. The authors concluded:

“The results support the concept that toxicity associated with the excess of a useful nutrient may be prevented by another nutrient.”

TRANSLATION: Adequate dietary calcium prevents your colon from looking funny if you find yourself in the unfortunate circumstances described in Reference 13.

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?

STUDY 3: Processed meat in rats

Reference 15:  Santarelli RL et al 2010. Meat processing and colon carcinogenesis: cooked, nitrite-treated, and oxidized high-heme cured meat promotes mucin-depleted foci in rats. Cancer Prev Res 3(7):852-64.

This study attempts to understand which aspects of meat processing might be responsible for causing potentially pre-cancerous changes in rat colons. Is it the cooking temperature? The curing method? The type of packaging?

The researchers compared a variety of pork processing methods:

  • light meat vs. dark meat
  • salt curing vs. nitrite curing
  • cooking at 50 deg C (122 F) vs cooking at 70 deg C (158 F)
  • air exposure vs vacuum packing

They pre-injected rats with a carcinogen (1,2-dimethylhydrazine), added the various types of processed meat to calcium-deficient, high-sugar chow, and fed it to rats for 100 days.

The type of processed meat that caused the worst pre-cancerous changes was the dark meat that had been cured with sodium nitrite, cooked at 158 F, and left unwrapped in the refrigerator for 5 days. None of the rats developed cancer.

TRANSLATION: If you inject yourself with a powerful carcinogen, then eat a calcium-deficient, high-sugar diet containing badly-packaged cooked ham every day for 10 years, and you are a rat, your colon may start to look funny. We don’t know whether or not you would eventually develop cancer.

STUDY 4: Processed meat in rats and humans

Reference 18:  Pierre FH et al 2013. Calcium and alpha-tocopherol suppress cured-meat promotion of chemically induced colon carcinogenesis in rats and reduce associated biomarkers in human volunteers. Am J Clin Nutr 98: 1255–62.

In this study, the researchers fed the afore-mentioned badly-packaged ham not just to rats, but to a handful of humans as well. This is important, because, in the words of the researchers themselves:

“One could argue that, owing to its specific physiological diet, the rat represents a poorly fitted model to test an overloaded meat diet. Excess of meat in species such as rodents could indeed give rise to some specific toxic pathways that are possibly not reproducible in other species.”7)Pierre FH et al 2007 Carcinogenesis 28 (2):321–327.

Translation:  Rats are not humans. Rats may not be evolutionarily adapted to be able to handle a high-meat diet, the way wolves, dogs, cats, and people are. A mostly-meat diet happens to be responsible for restoring my health, but I am not a rat.

rats are not people

Rats may not be evolutionarily adapted to be able to handle a high-meat diet, and therefore, are questionable subjects in which to evaluate the implications of meat on health. People, on the other hand, have been eating meat for nearly two million years.

AnyWHO…back to the study.

Seventeen healthy men ate 6.3 oz of our friend, the badly-packaged ham, every day for four days. Their urine and stool were then tested for five different “cancer biomarkers” (substances that may indirectly be linked with cancer risk):

  • ATNC (Apparent Total N-Nitroso Compounds)
  • TBARS (ThioBarbituric Acid Reactive Substances)
  • Fecal water cytotoxicity
  • DHN-MA (DiHydroxyNonane Mercapturic Acid
  • g-H2AX (gamma-Histone 2AX)

After eating the ham, two of the biomarkers (ATNC and TBARS) had increased. [When vitamin E and calcium were added to the ham diet, both of these markers remained normal.]

Two other biomarkers (fecal water cytotoxicity and DHN-MA, considered a good marker of oxidative damage) were unaffected by the ham.

The fifth biomarker (g-H2AX, a very reliable marker of DNA damage8)Kuo LJ and Yang LX 2008 Gamma-H2AX-a novel biomarker for DNA double-strand breaks. In Vivo 22(3):305-9, may have even improved slightly after eating the ham.

But two cancer biomarkers rose—that’s scary, right? Not really.

Both of those biomarkers, ATNC and TBARS, have been called into question by scientists:

Regarding ATNC:

The carcinogenicity of ATNC formed in the gut after eating heme from red or processed meat is unknown.9)Bastide NM et al 2011. Heme Iron from Meat and Risk of Colorectal Cancer: A Meta-analysis and a Review of the Mechanisms Involved. Cancer Prev Res 4(2): 177-184

Translation: even if badly packaged ham raises levels of ATNCs in the gut, we have no idea whether ATNCs cause cancer.

Regarding TBARS:

“Although this is an easy and inexpensive method, the use of TBARS test has received wide criticism over the years. The main problem is the lack of sensitivity and specificity, since TBA reacts with a variety of compounds such as sugars, amino acids, bilirubin and albumin, producing interference in colorimetric and fluorimetric MDA measurement. Therefore, TBARS test cannot be considered representative of oxidative stress.” 10)Grotto D et al 2009 Importance of the lipid peroxidation biomarkers and methodological aspects for malondialdehyde quantification Quim Nova 32(1): 169-174

Translation: the TBARS test is useless.

Interestingly, the authors refer to a study (conducted by a different group of scientists) that found vegetarian diets resulted in higher levels of g-H2AX (a very reliable marker of DNA damage, aka mutations) than diets containing cured meat or red meat.11)Joosen AMCP et al 2009 Carcinogenesis 30 (8): 1402–1407 [This “vegetarian diets may cause cancer” study was not mentioned in the WHO report.]

TRANSLATION: If you eat 6.3 ounces of badly packaged ham for four days in a row, your urine and stool will be higher in two substances that have no proven connection to cancer.

STUDY 5: Red meat in humans

Reference 19:  Le Leu RK et al 2015. Butyrylated starch intake can prevent red meat-induced O6-methyl-2-deoxyguanosine adducts in human rectal tissue: a randomised clinical trial. Br J Nutr 114: 220–30.

The authors of this study tried to show that unprocessed red meat causes mutations in human colon cells.

Twenty-three people were asked to include 10.6 oz per day of lean red meat (cooked beef or lamb) plus two cups of orange juice or low-fat milk in their diets for four weeks. These poor volunteers then underwent rectal biopsies, which showed a 21% increase in the number of mutations of a particular type known as “O6-MeG adducts” in their colon cells. [Researchers also expected to find an increase in ATNCs 12)apparent total N-nitroso compounds but this did not occur].

As scary as this may sound, you should know that there is a special enzyme located throughout the body, including in colon cells, called MGMT, whose sole purpose in life is to go around repairing O6-MeG mutations, because these mutations occur all the time as part of daily living. It is only when these mutations go unrepaired that there may be an increase in cancer risk.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.

[In fact, in the next and last study we’ll explore, the authors chose a different kind of mutation to study because they specifically acknowledge that colon cancer risk has nothing to do with the number of O6-MeG mutations, but instead is related to low activity of the MGMT repair enzyme14)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.]

However, EVEN IF these O6-MeG mutations were to go unrepaired, and EVEN IF they were to have the potential to lead to cancer, this study wasn’t designed in a way that can tell us that red meat was to blame. The authors did not report what else the volunteers were eating over the course of those four weeks other than the red meat, milk and/or orange juice. They do tell us, though, that during the high-meat phases of the study, participants ate significantly more protein (19% more) and significantly less fiber (26% less) than during the low-meat phases of the study, therefore the study wasn’t properly controlled.

In the second phase of this study, a special fiber supplement (butyrylated high-amylose maize starch) was added to the diet, and it completely blocked the increase in O6-MeG mutations. [This fiber experiment explains why subjects were asked to add milk or orange juice to their diets—they needed something to dissolve the fiber supplement in. Milk and orange juice have very different nutritional properties (most notably sugar content). If you include beverages in your experiment, you should ask everyone in the study to drink the same beverage and ideally include a control group that doesn’t add the beverage to their diet.]

TRANSLATION: If you include 10.6 ounces of lean beef or lamb plus two cups of milk or orange juice in your usual diet every day for a month, biopsies of your rectum will show a 21% increase in the usual number of mutations in your colon cells. We do not know whether it was the red meat, orange juice, milk, higher protein intake, lower fiber intake, or some other aspect of the diet that led to the increase in mutation rate. These mutations are typically automatically repaired by the body and do not increase your risk for cancer.

STUDY 6: Red meat in humans


The purpose of this study was to try to demonstrate the effects of red meat and fiber on colon mutations in humans. Twenty-five people were confined to a metabolic ward and fed one of three diets:

  • a vegetarian diet
  • a low-fiber diet containing 14.8 oz of red meat per day
  • a high-fiber diet containing 14.8 oz of red meat per day

After 10 days, stool samples were tested for “cancer biomarker” ATNC 15)apparent total N-nitroso compounds and for a particular type of colon cell mutation called O6-CMG.

The low-fiber, red-meat diet resulted in higher numbers of O6-CMG mutations than the vegetarian diet, as well as higher ATNCs. Should we worry?

Regarding the ATNCs: we discussed earlier that their relationship to human cancer has not been established. As for the mutations, if you’ll recall, mutations happen all the time, and are not worrisome unless the body can’t repair them.

Regarding the O6-CMG mutations: this study was conducted in 2006, back when scientists thought that O6-CMG mutations couldn’t be repaired by the MGMT repair enzyme normally present in our bodies. However, in 2013, researchers discovered that MGMT is able to repair O6-CMG mutations after all, so we have since learned that an increase in the number of these mutations is not a cause for concern. 16)Senthong P et al Nucleic Acids Res. 2013 Mar; 41(5): 3047–3055.

Furthermore, there were numerous methodological problems with this study.

  • Even though the volunteers were being carefully fed in a supervised metabolic ward, the composition of their diets was not described in the paper. For example, we don’t know how much carbohydrate, protein, or fat they ate. Also, unfortunately, the low-fiber meat group was fed refined carbohydrates in place of high-fiber foods.
  • To make matters worse, if any volunteer started losing weight, he/she was fed buttered marmalade bread to restore weight. We are not told which volunteers received these additional sugary treats, nor how often.
  • There was no non-red meat animal protein control (i.e. chicken or fish). The age and health status of the volunteers were not provided.

TRANSLATION: If you eat a low-fiber, high-protein, high refined carbohydrate diet that includes 14.8 ounces of lean beef or lamb for 10 days, your colon cells will experience more mutations than usual. We have no idea what aspect of this diet is responsible for the increase in mutation rate. These mutations are typically automatically repaired by the body and should not increase your risk for cancer.

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.

Bottom Line


Human nature being what it is, believing is seeing.


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.


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?meat heart 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.

The History of All-Meat Diets

Do High-Fat Diets Cause Depression? How the inattention to laboratory animal chow ingredients render most nutrition studies conducted in rodents completely useless.

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