Monday, October 26, 2015

Do Processed and Red Meat Cause Cancer?

Today, the World Health Organization's International Agency for Research on Cancer published a statement in The Lancet detailing its position on the carcinogenicity of processed and red meat (1).  The statement, resulting from a meeting of 22 scientists from 10 countries, concluded that processed meat is a group 1 carcinogen, meaning that it is "definitely carcinogenic to humans".  They also judged that red meat is a group 2A carcinogen, meaning that it probably causes cancer but the evidence isn't as strong.  They're mostly referring to the links between processed and red meat and digestive tract cancer, particularly cancers of the colon and rectum.

These statements were met with a media frenzy, and the expected furor from the meat industry.  The most surprising thing, for me, is that anyone would be surprised by the IARC's statement.

Even the 2010 USDA Dietary Guidelines, as influenced by industry lobbying as they are, state (2):
...moderate evidence suggests an association between the increased intake of processed meats (e.g., franks, sausage, and bacon) and increased risk of colorectal cancer and cardiovascular disease.
Although to be fair, the Guidelines do beat around the bush quite a bit, never directly stating that we should avoid processed meat.  Instead, they recommend avoiding "solid fats", then list processed and red meats as possible sources of such fats.

As I detailed in my series "Is Meat Unhealthy?", there is abundant evidence that processed meat increases the risk of digestive tract cancers.  We have consistent evidence from observational studies, human biomarker trials, and animal studies.  And we have a clear mechanism.  Science is never 100 percent conclusive, but this is about as close as it gets.

As far as fresh red meat is concerned, the risk is less clear, but I think the IARC's conclusion that it probably contributes to colorectal cancer risk is reasonable based on the evidence.

Some people are comparing the risk of eating processed and red meat to the risk of smoking cigarettes.  This is utter nonsense.  Smoking cigarettes is far more harmful to health than eating processed meat, and infinitely more harmful than eating fresh red meat.  On average, cigarette smokers have three times higher mortality rates than non-smokers, and they die an average of 10 years earlier than non-smokers (3)!

In contrast, if you squint at the data hard enough, people who eat a lot of processed meat have a slightly higher mortality rate than people who don't.  And people who eat a lot of fresh red meat have about the same mortality rate as people who avoid it.  Also, fresh red meat is a highly nutritious food, whereas cigarettes don't supply any essential nutrients as far as I know...  So although these foods probably do carry some risk, let's keep it in perspective and not get carried away.

My conclusion: eat bacon if you want to-- but don't think you're doing your health any favors.


Brandon Berg said...

"On average, cigarette smokers have three times higher mortality rates than non-smokers"

I'm not a big fan of this kind of phrasing. In the long run, of course, everyone has the same mortality rate. I know that you mean three times the morality rate over a specific age interval, but without reading the footnoted study, I don't know what that interval is. Simply saying "three times the mortality rate" doesn't really tell me much.

dianaedd said...

This comparing it to cigarettes really annoyed me. Cigarettes are addictive and the poisons enter the bloodstream directly through the lungs. People smoke a lot more cigarettes than probably eating a similar amount of processed meat. Not that I believe processed meat is healthy, because it likely isn't. I guess the idea is just to scare people more in the hope they change some habits? However, one needs to avoid extreme statement, IMO.

jespersis said...

And then there's Susannah Mushatt Jones!

Puddleg said...

"We have consistent evidence from observational studies, human biomarker trials, and animal studies."

Is the animal study evidence consistent, and does it speak to meat per se being a stand-alone carcinogen? Every animal study I've seen involved feeding meat alongside a carcinogen, with added heme, a vitamin or mineral deficient diet, in some combination. When bacon reduced the rate of cancer in rats fed carcinogenic chemicals and beef had no effect this obviously wasn't the result expected so the diet had to be tweaked till the desired result was achieved.
The problem is that the people who rely most on processed and red meat in our societies are the people who are exposed to carcinogens and whose diets are most nutritionally inadequate (and would be even worse without meat). I've never seen a prospective epidemiological study that controlled for environmental and workplace carcinogen exposures or did much to account for nutritional deficiencies.
Further, bacon - a cured sliced meat - is hardly comparable to products made from finely ground meats with grain product fillers, Fe dyes, and MSG flavourings. A meat sausage which is just mince in a skin shouldn't be different from the same mince on the plate.
A quick search found many things which are associated with colon cancer more than red meat or processed meat, including.
- High fasting insulin
- low HDL
- low serum PLP and low pyridoxine intake (note that processed meats tend to be very poor sources of this vitamin, unprocessed red meat is a good source).
Korea has the world's highest incidence of colon cancer and Japan is in the top 20, whereas Germany, a nation of sausage eaters, is not. Yet red meat is not associated with cancer in Asian epidemiology. Go figure. It might have something to do with beef being considered a healthy food in those countries and thus eaten more evenly across social strata and not as a substitute for a nutritionally adequate diet.

Tina said...

In my opinion, the increase from 56 to 66 cases in 1000 (10/1000 = 1% increase) is insignificant considering the scope and uncertainties prevalent in the "evidence".

And, I'm with Brandon (above) - any article that tells me I have a higher risk of death by doing X... loses me. We all have a 100% risk of death, that's just the risk we all take by being alive.

tomR said...

"processed meat is a group 1 carcinogen" - to debunk this one just needs one type of processed meat that is not carcinogenic. Including the way of processing to be invented in the future... False grouping too much things into one word perhaps? Some recipies that torture meat look carcinogenic, eg. "burnt ends":

On the other hand - the longest living eats eggs and bacon:

Unknown said...

Well, as a diabetic...meats are something I can eat that won't spike my bloodsugar like horrible carbohydrates do.

I'll take my chances with the meat, thanks.

Kathleen said...

With so many variables at stake how can anyone be certain that any individual contracted bowel cancer because they ate either processed meats or red meat? Could it be because they had a genetic predisposition to bowel cancer? Because they ate other junk food alongside processed meat and red meat? Because they failed to eat food brimming with naturally present beneficial microorganisms? Another look at the diets of those involved in the studies may highlight that they probably relied too heavily on convenience foods to form a substantial part of their diet, of which cheap red meat and cheap processed meats were just another component. To list processed meat as carcinogenic and dangerous is pure scare-mongering. Earlier generations ate cured meats and sausages regularly and the incidence of bowel cancer was significantly lower than it is today. Indeed, in the absence of refrigeration cured meats (including bacon) was often the only way to preserve meat so it would have formed a substantial part of traditional diets. Ditch fake, novel ingredients and convenience foods. Find a good traditional butcher and enjoy your traditional meats!

Walkitout said...

Well the processed meats and red meat are not exactly exactly different. The main promoters of red meat paleo promoters are pretty happy by still promoting red meat and bacon.

Unknown said...

Once again, thank you for putting a sane voice on this. While I am perfectly capable of doing the work, I just don't have time right now! Of course I will read more but for the moment those grass-fed steaks in my refrigerator will be my delicious dinner.

Nigel Kinbrum said...

There's haem iron & Neu5Gc in red meat. Both may cause cancer.

The iron can be neutralised by chlorophyll, as per

The Neu5Gc? Dunno.

Stephan Guyenet said...

Hi Tina,

Yes, we will all die eventually, but all else being equal, I'd rather not get colon cancer.

Hi Kathleen,

This is why researchers control their variables. The argument you're making could be applied to any food, in which case it would be impossible to know if anything were healthy or unhealthy.

Earlier generations (at least since the 1940s) had higher age-specific colon cancer rates than we do today. Since the 1970s, colon cancer incidence and mortality have been declining in the US. I haven't been able to find trends for processed meat intake, but this decline in colon cancer incidence corresponds with a sharp decline in beef intake. So I don't buy the argument that we didn't see this effect in our grandparents. They also had much higher rates of stomach cancer than we do today.

T33CH said...

Can't part of the decline of stomach and colon cancer be due to the decline of cigarette smoking?

Cigarette Smoking and Colorectal Cancer Mortality in the Cancer Prevention Study II
Ann Chao, Michael J. Thun, Eric J. Jacobs, S. Jane Henley, Carmen Rodriguez and Eugenia E. Calle

"Our main finding is that men and women who smoked cigarettes for 20 or more years at study enrollment experienced higher colorectal cancer death rates, even when we adjusted for multiple potential confounders. "

Bottom line: Don't smoke and avoid processed foods made from plant or animal products.

Unknown said...

People will walk away with the false belief that red meat is the sole cause of digestive cancer - I know people who have become vegan/plant based and fearful of meat because of this fear mongering and have still ended up getting colon cancer.

Colon cancer is highly associated with vitamin D deficiency, diabetes, obesity, iron overload etc. - all these issues are possible on a diet high in plants and devoid in meat. If flour is refined and fortified, and a staple in your diet you can easily get iron overload. There's also a genetic factor to it.

As for colon cancer rates going down in the US that data is also confounded because people are smoking less. Could also be because of colonoscopies - colon cancer doesn't develop overnight.

It seems like you have the position that those critical of this are just crazed people who love bacon - but I feel recommendations that come out like this without sufficient context just undermine the public's trust in science.

Stephan Guyenet said...

Hi T33CH,

Yes, I bet that's part of the explanation. It's probably a complex stew of factors. My only point is to say that we can't make the argument that previous generations ate processed/red meat and didn't get colon cancer.

Hi Vikram,

I think most people are able to understand that "increases risk" doesn't mean the same thing as "is the sole cause of risk".

You said "It seems like you have the position that those critical of this are just crazed people who love bacon". Not everyone is like that, but to be frank, yes there are people who are irrationally attached to bacon and will flail around at anything that's critical of it. I don't understand that perspective but it seems pretty common in the LC/Paleo community.

glib said...

Black pepper is certainly the most inflammatory substance I have tried, for my colon. It is present in many processed meats, such as bacon and salami. I would love to see a study done with prosciutto only. In point of fact, why even look? I have a small experiment. Most members of my original family (about 50 people) eat prosciutto every day, and no one has had any problems.

Unknown said...

As I understand from the blog post at the link, the likely mechanism by which processed meat is dangerous, is the cooking of meat with nitrites in it. Should I therefore feel safe by eating uncooked raw ham? (I usually eat only about 35g/day of it, anyway.)
OK, I also have eggs+bacon about twice a week. Maybe I'll cook the eggs without the bacon from now on, and add some raw ham on top afterwards.

Peter said...

I looked up,colon cancer in India, thinking that since they have a religious prescription against eating beef. They had the lowest colon cancer incidence of any country on the chart. Proves nothing, but makes you wonder.

tomR said...

@Snorri Godhi - when it comes to "processed meat" and cancer it's not just nitrites, there are multiple dangers involved. Including processes like smoking, or preservation via salting. That's for traditional items.

Industrial style processing itself involves putting large amount of input products into very large bowls. Some tiny part of inputs are going to be contaminated with stuff. Basically mixing lots of input material together means there's going to be a low-level contamination of the output - may be causing something like low-level inflammation that cancers like. If there's no mixing you can just throw out bad food item, in industrial processing it ends up being a tiny percent of the mix...

In case of some hot-dogs or sausages it's also the quality of input material that goes in them that is questionable - such things just wouldn't sell as fresh meat, while processing allows them to be sold with the help of artificial flavors or colorings.

There's also the topic of of too rapid ingestion of amino acids, spiking their levels in the blood to the levels higher than for more slowly digested normal meat (similarly to processed carbs spiking glucose); and amino acids help cancer - methionine is a growth factor, glutamate helps metastasing, and some cancers are dependant on glutamine ("glutamine addiction"). An impulse for existing cancer to grow.

It's also not like multiple chemicals put in "processed meat" are helping...

I've purposedly put "processed meat" in hyphens, as the term is quite misleading, a false grouping perhaps, - too wide and divergent group, with many processing methods not having anything in common with each other. Such grouping of products unfairly punishes those who processs meat, but with some safe/safer methods, than the majority thad does harmful products. It's also not persistently defined, with new processing methods being invented frequently, some old ones abandoned, changed, or even banned.

tomR said...

@George Henderson - within Korea meat and salting it (or salting something else) is a risk factor for cancer

"An increased risk of stomach cancer was noted among people who frequently consume broiled meats and fishes, salted side dishes (salted/fermented fish products) and salty stewed foods, such as soybean paste thick stew. Frequent consumption of mung bean pancake, tofu, cabbage, spinach and sesame oil decreased the risk. [...] For meat and fish, pan frying was associated with decreased risk, whereas stewing or broiling was associated with increased risk. Pickled vegetables increased the risk, whereas fresh vegetables did not."

"In addition, frequent meat intake was associated with proximal colon cancer risk among men and risk of proximal colon and rectal cancer among women [21]."

Walkitout said...

". Not everyone is like that, but to be frank, yes there are people who are irrationally attached to bacon and will flail around at anything that's critical of it. I don't understand that perspective but it seems pretty common in the LC/Paleo community."

Yes, It is isn't it Stephen?

Yet you have promoted and still do promote on your recommended reading blog. Possibly one of the biggest "paleo" influences and online defenders of red meat.

His blog post on the insulin index is pretty comical reading if youget around to re-reading it. He also ends the blog post with

"Now, pass the bacon".

Unknown said...

Tom: thanks for your feedback. That was exactly my concern, that "processed meat" is too heterogeneous a group for statistics of increased risk to mean much. As i said, i favor raw ham, so of the risk factors you list, i should be concerned only about the salt and maybe the smoking. Since i take a sauna at least twice a week, i probably ingest too little salt anyway.

Pando_DePuntaAPunta said...

I do agree that processed meat could cause cancer and other diseases such as CVD.
What I do not see is the evidence for fresh red meat. All the possible proposed mechanisms failed to explain the differences between the outcomes when comparing red meat and processed meat.
- Heme iron content is very similar between processed and unprocessed meat, yet the evidence of higher risk is stronger for processed meat.
- HCA's are formed at high cooking temperatures, both types of meat can be cooked at high temperatures but in the group of unprocessed meat we have cold cuts not subjected to this type of cooking. If this were the reason wouldn't it be the evidence higher for raw meat.

And we can say the same about other suspicious compounds such as Neu5Gc, IGF-1 or L-carnitine (in the case of CVD risk).
The only difference in composition and/or treatment is the use of additives.

I really think the evidence for red meat alone as a risk factor for cancer and/or other diseases is limited and new findings suggest so (like the strong EPIC Cohort).

tomR said...

One of the many reasons for more cancer risk after typical processing of raw meat than before may be addition of glutamate - a popular additive in processed industrial food products. Added glutamate comes mostly from non-meat sources (yeast extract, soy protein), so you end up with more that was present in the raw meat alone. Logically meat + glutamate from yeast/plants/bacteria should be more of a risk for cancer that meat itself.

"Inhibition of extracellular glutamate release or glutamate receptor activation via competitive or non-competitive antagonists decreases growth, migration and invasion and induces apoptosis in breast cancer, melanoma, glioma and prostate cancer cells.
In addition to the growing number of studies reporting GluR expression in malignant disease, Glu antagonists have been shown to have anti-proliferative and anti-migratory effects both in vitro and in vivo.
However, these data are important in that they establish what is emerging as a general phenomenon: Glu antagonists and blockade of Glu release from cells has anti-tumor effects.
The role of glutamatergic signaling in the progression of glioma is well established
We recently reported that serum Glu levels correlate with primary PCa aggressiveness and that Glu antagonists decrease PCa cell proliferation"

tomR said...

Some of the steaks one gets at the reastaurant might actualy be processed meat by WHO definition "Processed meat refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation.(cut here)"

Aging is a process, it transforms meat, and it's role is to improve flavor. It even has some suspicious stuff - like fungi - included.

"The process changes beef by two means. Firstly, moisture is evaporated from the muscle. The resulting process of desiccation creates a greater concentration of beef flavour and taste. Secondly, the beef’s natural enzymes break down the connective tissue in the muscle, which leads to more tender beef.

The process of dry-aging usually also promotes growth of certain fungal (mold) species on the external surface of the meat. This does not cause spoilage, but actually forms an external "crust" on the meat's surface, which is trimmed off when the meat is prepared for cooking. These fungal species complement the natural enzymes in the beef by helping to tenderize and increase the flavor of the meat. The genus Thamnidium, in particular, is known to produce collagenolytic enzymes which greatly contribute to the tenderness and flavor of dry-aged meat."

Unknown said...

Here's a good post on this topic from across the pond by another obesity researcher/food blogger: