If you paid attention at all to the news yesterday, you surely saw the headlines proclaiming that bacon causes cancer. The news came out of the ruling of a committee (the International Agency for Research on Cancer, IARC) for the World Health Organization. (By the way, this is the same committee a couple months ago that made news when they announced glyphosate - aka Roundup - was carcinogenic).
What follows is a pointer to two of best analyses of the announcement I saw followed by my own thoughts.
Here's Ed Yong at the Atlantic.
“Here’s the thing: These classifications are based on strength of evidence not degree of risk.
Two risk factors could be slotted in the same category if one tripled the risk of cancer and the other increased it by a small fraction. They could also be classified similarly even if one causes many more types of cancers than the other, if it affects a greater swath of the population, and if it actually causes more cancers.
So these classifications are not meant to convey how dangerous something is, just how certain we are that something is dangerous.
But they’re presented with language that completely obfuscates that distinction.”
Then, in what is sure to become a classic line, Yong wrote:
“Perhaps we need a separate classification scheme for scientific organizations that are “confusogenic to humans.””
Over at Grist, Nathanael Johnson wrote the following
“What if you just want a sausage once every other week or so? The thing to keep in mind here is that IARC’s job is to figure out if substances can cause cancer, not if they’re likely to. It’s findings aren’t that useful to normal people looking for advice on how to live their lives.”
So, we need to keep in mind that the old adage that the dose makes the poison. And, we need to look at relative risks. Such as this one offered by Johnson:
“If today you start eating 50 grams a day (about three strips of bacon) more processed meat than usual, your risk of cancer increases 18 percent. For comparison, if you are a nonsmoker who starts smoking three cigarettes a day, your risk of lung cancer increases 600 percent.”
One thing rarely communicated in these sorts of reports is the baseline level of risk. Let's use Johnson's example and suppose that eating three pieces of bacon everyday causes cancer risk to increases 18%. From what baseline? To illustrate, let's say the baseline risk of dying from colon cancer (which processed meat is supposed to cause) is 2% so that 2 out of every 100 die from colon cancer over their lifetime (this reference suggests that's roughly the baseline lifetime risk for everyone including those who eat bacon). An 18% increase means your risk is now 2.36% for a 0.36 percentage point increase in risk. I suspect a lot of people that would accept a less-than-half-a-percentage point increase in risk for the pleasure of eating bacon.
Now, let's suppose instead that the baseline risk was 10% (10 out of 100 die from cancer). In this case, an 18% increase means your risk of cancer is now 11.8% for a 1.8 percentage point increase in chance of dying of colon cancer. Thus, the same percentage increase in risk (18%) results in very different changes in absolute likelihoods of dying (an increase of either 0.36 percentage points or 1.18 percentage points) depending on the baseline starting point. In a population of 1000 people who started eating 3 pieces of bacon, in one case we'd have about 3.6 extra people die and 11.8 extra people die in the other. That's more than three times as many people dying in the high-baseline risk case than in the low-baseline risk case. In short, studies that say that eating X causes a Y% increase in cancer are unhelpful unless I know something about my underlying, baseline probably of cancer is without eating X.