This week’s news that processed meats are carcinogenic needs a little closer look. The TL;DR is pretty much as you’d expect.
Well, the newspapers are shouting it from the mountaintops: “Processed Meats Rank Alongside Smoking As Cancer Causes – WHO,” “Hot dogs, bacon and other processed meats cause cancer, World Health Organization declares,” “Processed Meats Are ‘Carcinogenic to Humans’ Says WHO.”
But, as always, I’m skeptical.
So what’s the story? Well, the World Health Organization put together a panel of experts and reviewed over 800 studies, some good, some very likely not so good, that looked at whether eating “red meat” and “processed meat” cause cancer. They have concluded that, despite the paucity of direct evidence, “red meat” should be classified as a type 2A carcinogen, and “processed meat” a type 1.
What does that mean?
First, let’s take a look at the study — well, I would if I could. As far as I can see it hasn’t been published yet, and it won’t be for a little while. When it is, you’ll be looking for a publication labeled this: “International Agency for Research on Cancer. Volume 114: Consumption of red meat and processed meat. IARC Working Group. Lyon; 6–13 September, 2015.” It’ll be a full workup of the meta-analysis, which they performed over the course of their week-long conference. For now, at least as far as I can tell from the use of the future tense in the words “will be published,” all we have is a two-page write-up in The Lancet Oncology, which you can find here.
Okay, so what we have is a bare-bones explanation of what they did, the basic logic they used, and the conclusions they drew. Let’s walk through it.
The first question is what, for their purposes, are “red meat” and “processed meat?”
“Red meat refers to unprocessed mammalian muscle meat—for example, beef, veal, pork, lamb, mutton, horse, or goat meat—including minced or frozen meat; it is usually consumed cooked. Processed meat refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation. Most processed meats contain pork or beef, but might also contain other red meats, poultry, offal (e.g. liver), or meat byproducts such as blood.”
And so here’s the first sticking point. Beef and pork and lamb are, as you might guess, a little different from one another. So are salting, curing, fermenting, smoking (!!) and “other processes to enhance flavour or improve preservation.” When the things you’re identifying as “carcinogenic” are this broad, it’s very difficult to know what part of them actually causes cancer. So: do they get more in-depth? Yes, a little.
So what, you may next ask, are the proposed ingredients/compounds and mechanisms of action?
“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 panfrying, grilling, or barbecuing generally produces the highest amounts of these chemicals.”
Again, here’s something that makes me stop and question what’s going on. NOCs, PAHs, and HAAs are known to be carcinogenic to humans in high enough doses. NOCs are, by and large, the product of curing meats with sodium nitrite or in the presence of nitrite. So that’s things like spam, bacon, and smoked meats. PAHs and HAAs are the product of, basically, burning your meat. That “char” on well-done or barbecued meats (including chicken, by the way, which wasn’t included even though pork was), the nice “sear” on the outside of a steak — those parts contain PAHs and HAAs. PAHs and HAAs are known to cause cancer in animals at very high doses (but, again, so are a whole lot of things that are safe). The other compound they looked at for which they say there was evidence of a mechanism was “haem iron,” (pronounced “HEEM” and also spelled “heme”) which is the more easily-absorbed form of iron we get from meat consumption (non-haem iron is much less bioavailable, which is why iron intake guidelines for vegetarians are 1.8x higher). Haem iron consumption is, again at certain levels, associated with a colorectal cancer risk, possibly because it causes formation of NOCs and certain aldehydes.
If we knew all this, what did the panel actually do?
Mostly, they looked at 800 studies and looked for correlations. Chief among these (those with the most concrete associations with some form of cancer and what led them to their classifications) were 14 cohort studies and 15 case-control studies that looked at red meat consumption and colorectal cancer risk; plus 18 cohort studies and 9 case-control studies that did the same for processed meat. For the red meat data, 7/14 cohort studies showed “positive associations […] with high versus low consumption,” and 7/15 case-control studies “reported positive associations of colorectal cancer with high versus low consumption.” For the processed meat data the numbers were higher: 12/18 and 6/9. They also pointed to a meta-analysis done in 2011 (Chan DS, Lau R, Aune D, et al. “Red and processed meat and colorectal cancer incidence: meta-analysis of prospective studies.” PLoS One 2011; 6: e20456) which found a dose-dependent risk for consumption of both (and which is where the media are getting their “50g=18%+ risk” stat.)
Here, again, is another issue I’m having (and which, I’m sure, the US meat industry will have a field day with). Because we don’t have the full data, we don’t know if the half of the red meat studies (or the third of the processed meat ones) which didn’t show any “positive associations” were particularly well done or not (pun fully intended).
Based on these data — the apparent correlations with colorectal cancer incidence and the presence of known potential carcinogens (depending on dose) — the panel labeled “processed meat” as a type 1 carcinogen and “red meat” as a type 2A.
And here’s where I’m left scratching my head, and where other health organizations will likely disagree with the WHO.
The IARC (the folks responsible for this study as well as the classification system) describes a type 1 carcinogen as follows: “the agent (mixture) is carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.” That is to say that they’re saying (a) “red meat” is carcinogenic to humans and (b) that when we are exposed to it, we are exposed to carcinogenic levels of it. The first half I’ll maybe buy, the second I just don’t think they’ve proven yet.
Why not? Because right now all we know is that their strongest case is for processed meats and colorectal cancer, and that the only study that looked at any kind of dose-dependence (at least as far as I can tell from their short paper) showed a a 17% increased risk (95% CI 1·05–1·31) per 100g per day of red meat and an 18% increase (95% CI 1·10–1·28) per 50g per day of processed meat. That’s one study, and even if we take its results as gospel (something I’m loath to do), that means you’re 18% more likely to get colorectal cancer for every 2.5 pieces of bacon you eat per day.
If you eat around 18 pieces of streaky bacon a week, day in, day out, then you’re meeting the requirements for a possible 1/5 increase in your risk of one type of cancer.
I like bacon and all, but that seems like a lot of bacon. Just to be clear, to see the dose response of that one study, you’d need to be eating 18.25kg (over 40 lbs) of bacon a year to raise your lifetime risk of colorectal cancer from the “normal” 5% to 6% (well, it might be lower in the absence of processed meats — America does consume a lot of bacon, I suppose…).
And that’s where the buck really stops for me.
Because, regardless of the “same category as smoking” claim, this finding is of very limited usefulness. A LOT of things cause cancer. “Solar radiation,” “outdoor air pollution,” and “oral and hormonal contraceptives” are also on the IARC list of type 1 carcinogens. And they do cause cancer. So we limit our exposure in some cases. But they also have good things about them, like raising your vitamin D levels which might be good for your immune system, or preventing pregnancy (which is far more life-threatening than the pill) … okay, air pollution not so much, but still.
Plus, we already know you shouldn’t be eating two or three strips of fried bacon every day, in the same way that we know you shouldn’t be going to tanning salons or breathing outdoors in Beijing. If you’re eating fried, braised, or otherwise charred red meat or processed meat every day, you probably have other, more pressing concerns — like obesity, heart disease, high cholesterol levels, hypertension, and so on. And red meat at least actually has things in it that are actively good for you (like haem iron, which is also bad for you, and protein, B vitamins, etc. which are great unless you have gout and then maybe ease off a little on the protein shakes, yeah?)
So this is what I’m going to draw from this study, at least in its present form. If you scrolled down for the TL;DR, for follow-at-your-own-risk health advice from an armchair skeptic and non-medical professional who just reads a lot of studies, this is it, just for you. Are you ready? Here goes:
Eat plenty of fruits and vegetables. Get some grains in your diet. Exercise a little each day. Get a good night’s sleep. Eat less sugar. Go see a doctor when you’re sick. And yeah, you’re probably okay eating “red” meat and processed meat in moderation. Just try not to burn it too much, or too often.
Oh, and when the actual study comes out, I’ll be reading it.
Edit: I’ve received some flack from folks at /r/skeptic who point out that I’m being dismissive of the work done by professionals. I’d like to take a moment to respond here with a mea culpa. I am an amateur. A layman. I am not a scientist. I am not an expert. But part of the project I’m engaging in, part of the experiment in writing this blog, is to practice applying reasoning to even things I don’t fully understand, in an attempt to understand them better. I do it in public because I want to encourage others to do so as well. Sometimes it goes better than others. This post was intended as a counterpoint to the flailing about on the internet that took place, as people compared eating bacon to smoking or called for labeling of meat as carcinogenic on packaging. I might have gone too far in the other direction; it’s not a perfect post. Self-reported surveys and animal models are probably the best we’re going to get when it comes to some of the carcinogens in meat, because we can’t just give people things to see if they get cancer. And there are indeed studies that show a strong correlation between cooked meats and certain cancers.
Maybe soon we’ll be able to grow human colons like we’re starting to grow human kidneys for testing compounds on to get even more specific evidence than animal models. In the meantime: don’t panic. And maybe eat a little less bacon.
Richard Ford Burley is a writer and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and techno-futurism here at This Week In Tomorrow.
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