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In the fall of 2015, the World Health Organization's International Agency for Research on Cancer (IARC) caused a stir by declaring that there was enough evidence to say that processed meats - bacon, sausage, and other cured delicacies - cause cancer.
In the study that included these results, the agency also said that red meat was probably carcinogenic.
Now, a Reuters investigation into how exactly IARC decides something causes cancer adds some new wrinkles to that story.
Notably, an anonymous source quoted by Reuters, who observed the closed proceedings, alleged that "the expert panel reviewing the scientific evidence appeared to aim for a specific result" and that the data on just how much meat consumption raised cancer risk seemed "to come from nowhere, overnight."
We should note that even if the source is right that the panel "appeared to aim for a specific result," the idea that processed meat can eventually cause cancer is not a controversial one - the controversy comes more from the question of just how risky that meat is and how that information was communicated.
But the Reuters investigation does reveal some interesting things about IARC's process of communicating risk.
First of all, they're reluctant to say anything is definitely safe. Of the 989 things they've investigated over the past 40 years, only one has been found to "probably not" cause cancer.
Second, there are critics who dispute part of IARC's review process, especially because many of the people who make the final decision may have been involved in the very research they are citing to support it.
And finally, it's important to note that when IARC declares something a "known carcinogen," they are not making a statement about how risky it is. Something is considered a carcinogen even if it increases your risk of cancer just a little bit.
Bacon, sunbathing, alcohol, and plutonium are all "known carcinogens," but one of those is going to give you cancer much more quickly than the others.
So is bacon as dangerous as smoking or other known carcinogens?
Our first question to researchers when we heard about the processed meat decision was to ask if that declaration was a surprise.
"No, it's not," said Dr. Jiyoung Ahn, an associate professor of population health and environmental medicine at NYU's Perlmutter Cancer Center.
For a "long time," she said, "multiple studies have reported the relationship between meat intake and bowel cancer."
And it's true. Researchers have repeatedly reported a connection between processed meat and colorectal cancer (also called bowel cancer) and red meat and the same cancers. A connection to prostate cancer has long been suspected, too.
But a common reaction to the IARC decision was to call bacon "the new smoking:"
Eating bacon is now as bad as smoking! Jesus!!!
- Stephen Mulhern (@StephenMulhern) October 26, 2015
This is not the case.
It's true that the WHO's IARC put bacon onto the list of known human carcinogens, along with tobacco, asbestos, and other scary substances. Once there is enough evidence that researchers can say, yes, a substance causes cancer, it would be wrong for them not to put it on that list. Alcoholic beverages have been listed as a carcinogen since 2012.
But that doesn't mean that everything on that list is equally dangerous and equally likely to cause cancer. Things are included if we know they can cause cancer, but that doesn't mean they all automatically do cause cancer. Exposure to the sun's ultraviolet rays can cause cancer, for example, but doesn't always do so - it depends on dose and other factors.
Eating an additional 50 grams of processed meat a day increases your risk of colorectal cancer by 18%, according to the IARC's report. An increased risk of almost 20% sounds like a lot - and it's definitely something worth paying attention to. But your overall risk of getting these cancers in the first place is still low, so increasing a low number by 20% is less scary.
As Dr. Kurt Straif, Head of the IARC Monographs Programme, put it in a statement: "For an individual, the risk of developing colorectal cancer because of their consumption of processed meat remains small, but this risk increases with the amount of meat consumed."
Cancer Research UK compares what we know about the effects of eating meat to the effects of smoking in this helpful graphic:
Both smoking and eating red or processed meat can cause cancer (or can probably cause cancer, in the case of red meat), but smoking is responsible for a greater percentage of lung cancers than meat is responsible for colorectal cancers. High meat consumption is just one of many risk factors for colorectal cancer; others include having Crohn's disease, having a family history of cancer, and - surprise - smoking.
To look at the statistics another way, the researchers say that eating processed meats probably causes 34,000 people to die of cancer around the world every year. If red meat causes cancer (and isn't just associated with cancer deaths coincidentally), then it would likely be the cause of an additional 50,000 global cancer deaths every year. Those numbers are significant and they should give pause to anyone who consumes a lot of processed or red meat.
But those numbers aren't as bad as others we could look at. Alcohol is responsible for 600,000 cancer deaths every year (including a number of colorectal cancers), and smoking kills one million people each year. Air pollution causes another 200,000 deaths.
So yes, bacon causes cancer. But no, it's not the same as smoking.
As Tom Chivers of BuzzFeed News UK puts it, over your lifetime, the UK's National Health Service calculates your risk of colorectal cancers at 5%. An additional 50g of processed meat a day raises that to 6%.
So can I eat some bacon now?
This isn't meant to absolve or not absolve the consumption of these meats for health reasons (there are other moral considerations for eating industrially produced meat too).
Many interpret these findings to be further confirmation of already-existing health advice: Don't eat too much processed or red meat. Eating these meats every day can quickly put anyone over the amount that's known to significantly increase cancer risk. And that's before considering the other potential health risks of eating large amounts of processed meat, which has also been associated with an increased risk for heart failure in men and women and a higher rate of earlier death from all causes.
Some, like NYU's Ahn, who is also a registered dietitian, would say to avoid these meats altogether. "I wouldn't advise eating processed meat," she said. "We have other excellent sources of protein," like chicken, fish, or tofu.
But not everyone goes so far.
"These findings further support current public health recommendations to limit intake of meat," Dr Christopher Wild, Director of IARC, said in a statement. But he refrained from saying that meat should be avoided altogether, and noted that there are healthy compounds in meat too.
We don't know exactly what causes the increased cancer risk, though the researchers noted that the creation of processed meats often introduces chemical compounds that are carcinogenic. Cooking meat to a charred point also creates carcinogens, according to the report. Ahn is investigating whether gut bacteria make certain people more vulnerable to cancer if they consume red or processed meat.
For now, as always, Michael Pollan's seven-word piece of advice for healthy eating remains relevant: "Eat food. Not too much. Mostly plants."
So go ahead and eat some bacon. Maybe just don't eat all of the bacon.