I'm an Epidemiologist. Here's My Take on That Scary Study About Processed Meat.
Sign up for the Slatest to get the most insightful analysis, criticism, and advice out there, delivered to your inbox daily.
Hot dogs, bacon, pepperoni—processed meat, everyone seems to agree, isn't good for you. A recent study in Nature Medicine confirmed that belief and even took it one step further: Forget the occasional beef jerky; such snacks have no place at all in a healthy diet. The authors write that “even lower levels of habitual consumption of these dietary risk factors are not safe.” Given the headlines covering this study , you might think it wise to quickly cross the street if you so much as lay eyes on a piece of bacon.
I'm not convinced that processed meat is that bad. Although the evidence against it is consistent, when it comes to food research, it's still hard to draw firm conclusions about even something as well studied as processed meat.
Food is one of the most complex topics in epidemiology. There are a few reasons for this. One of the big ones is that it's very hard to measure what we eat. You have to rely mostly on study subjects reporting what they ate over the past year. People are terrible at accurately reporting their diets. The other reason is that there is no easy control group for any specific food. If you want to test whether, say, acetaminophen works, you can easily craft a placebo pill that looks exactly the same but does nothing to the body. What's the placebo control for a hamburger? You have to eat something , and that something will inevitably have some kind of impact on your health. Not to mention that your subjects will probably realize if they've been served a vegetable-based dupe of the real thing.
This new study may appear robust in a couple of ways. It's a meta-analysis, which means that the authors looked at all the studies published on a topic and combined them into one statistical model. This is, in theory, the most robust form of research, because it allows you to see the average result for a question across dozens or even hundreds of individual pieces of research.
On top of the usual meta-analysis model, the authors also performed what's called a burden-of-proof analysis. This methodology allows them to make more specific statements about the findings. In this case, they set thresholds for associations between processed meat and various diseases that went from very weak to very strong on a five-point scale.
But after aggregating the data from dozens of studies, the researchers weren't able to make a slam-dunk case against processed meat. The efforts showed that there was a weak (2 out of 5 on their scale) relationship between processed meat and diabetes. The association between processed meat and colorectal cancer just scraped 2 out of 5 on the scale as well, while heart disease was only very weakly (1 out of 5) associated with processed meat consumption.
The meta-analysis also looked at sugar-sweetened beverage consumption: soda, pop, soft drinks, etc. This was also weakly associated with increased risks of diabetes, cancer, and heart disease.
Why are all these associations so weak? The main reason is that the evidence was inconsistent. Some studies show an increased risk of even small amounts of processed meat; some don't. Some data indicates that processed meat gives you diabetes; other data show that it's fine. The research is scattershot simply because food studies are hard to do in an airtight way. Such studies rarely give us enough information to confidently tie together cause and effect; aggregating a bunch of flawed studies together doesn't necessarily help all that much.
In addition, there is this question: What even is “processed meat”? It's a category so broad that it's impossible to capture in a single paragraph, let alone a study in which the goal is to minimize as many variables as possible. A cheap hot dog at a baseball game and $5,000 Serrano ham are both processed meat, but they are drastically different products in terms of how they are made and who buys them to eat. The only thing the fancy smoked sausage from the farmers market and the chicken nuggets in your freezer have in common is that they are “processed,” but everything else—including the method of processing—is totally different. So when the take-home message is that processed meat is not safe in any amount, it's worth asking: Well, which processed meats are we talking about? The research doesn't get into that.
We have also never really identified what it is about processed meat that makes it so bad for our health. Depending on whom you ask, it is either the nitrite preservatives used in these products or the high levels of fat and salt. But there are plenty of processed meats that don't have these things added to them, and the research showing higher associated risks doesn't differentiate, so it's hard to know, from a theoretical perspective, whether processed meat could even give people cancer.
What we can say from this recent study is that the overall evidence that we've got across dozens of pieces of research is fairly consistent . People who say they eat more processed meat are consistently less healthy in a variety of ways than people who say they eat less of the stuff. The extent of the negative effects, and the risk for various diseases, varies from study to study, but pull back and the evidence shows a clear trend: If you report eating a lot of processed meat, you are more likely to get diabetes, heart disease, cancer, and a whole range of other problems.
At the end of the day, it's still difficult to know whether it's processed meat itself that is causing these issues or if there is something about the group of people who tend to buy more processed meat in high-income countries that is simply very different from everyone else. We have not properly eliminated the possibility, for example, that those who earn more money buy less processed meat and are less likely to get cancer for reasons unrelated to processed meat consumption.
It's probably never going to be a good idea to gorge on bacon. If nothing else, processed meat tends to be very high in calories, due mostly to containing a lot of fat, and we know that eating too much of that is bad for you in many ways. But the idea that any hot dog consumption is inadvisable isn't supported by strong evidence. I, for one, will keep having the occasional slice of salami.
Sign up for Slate's evening newsletter.
