Many media reports are reporting it incorrectly or confusingly.
For example, in New Zealand’s TV3 News starts out1
World Health Organisation (WHO) experts have put processed meat in the same cancer-causing category as cigarettes …
… it has ranked processed meat in the highest cancer risk category, alongside asbestos and even smoking.
Sorry, but they didn’t.
The results are presented by IARC, the International Agency for Research on Cancer. They classify their results in five levels.
These levels do not say how much something increases your chance of getting cancer.
The levels reflect how confident they are in saying it has some level of increasing the chance of getting cancer, whether that increased chance of getting cancer is small or large.
The categories give how strong the evidence is: definitely, probably, possibly, can’t determine, probably not.
Imagine you have studies suggesting there is a tiny increase in cancer from some activity, but the evidence for that small increase in cancer rates is very strong. You’d put that in their highest category because you are very confident in the small increase in cancer risk.
Or, as IARC put it in their Q&A (PDF file):
Q. Processed meat was classified as carcinogenic to humans (Group 1). Tobacco smoking and asbestos are also both classified as carcinogenic to humans (Group 1). Does it mean that consumption of processed meat is as carcinogenic as tobacco smoking and asbestos?
A. No, processed meat has been classified in the same category as causes of cancer such as tobacco smoking and asbestos (IARC Group 1, carcinogenic to humans), but this does NOT mean that they are all equally dangerous. The IARC classifications describe the strength of the scientific evidence about an agent being a cause of cancer, rather than assessing the level of risk.
IARC are saying there is strong evidence that processed meat is associated with some level of increase in cancer and that there is moderate evidence that red meat is associated with some level of increase in cancer.
What are the levels of risk, then?
The level of risk depends on how much processed meat you eat: the more you eat, the higher the risk.
Starting with a modest level of cancer risk, each 50 grams more of processed meat each day raised the risk of colorectal cancer2 by 18%. The risk of cancer to a person who eats more is still modest because it’s an increase in a small number.
Similarly, if the link between red meat and cancer were confirmed, starting with a modest level of cancer risk, each 100 grams per day of red meat would raise the risk of colorectal cancer2 by 17%.
(Processed meat is that has been cured or treated in some way.)
By contrast, the risk of lung cancer from smoking is high.
Professor Kevin McConway (Applied Statistics, The Open University) compared the increase in cancer risk for smoking and eating processed meat –
Of 100 lifetime non-smokers in the UK, around 1 will get lung cancer. For 100 smokers of a pack a day, more than 20 will get lung cancer. That’s a huge increase in risk. Again in the UK, about 6 people in every 100 will get bowel cancer in their lives. According to the IARC data, if these 100 people in the UK start eating an extra 50g of processed meat a day, then 7 of them will get bowel cancer.
A diet with very large amounts of red or processed meat isn’t the brightest of ideas, but it’s not the same risk as for, say, smoking cigarettes (which is best described as a plainly dumb idea).
Those who are interested in this should try read the coverage at Cancer Research UK – it’s longer, but covers most of the aspects.
It’s worth remembering that this new report mostly reaffirms what is already known, rather than presents something new: their report gathers previous research and summarises what had be drawn from them. That red meat or processed meat has been linked to an increased incidence in cancer at some level has been known for a while. Furthermore, they note that many other factors might play a role in the risk level – for example to how and to what extent the meat is cooked.
A more detailed account is presented in the medical journal, Lancet. (If you don’t have access, you can register to get a copy of the article.)
Many media, (including TV3 News) have reported it incorrectly, or at least in a confusing way, and those with better science communication skills are left trying to put right the resulting confusion. Unfortunately that story not a new story.
All of this is not helped by how IARC describe their categories. Ed Yong has suggested their choice of language is unhelpful. I agree.
It’s also not helpful that many media sources seem to not understand what makes up a risk estimate – possibly one of the most important things for a journalist or editor to understand. While we can certainly say IARC could try to be clearer—and they could—we could also have wished those reporting risk understood what to look for and know how to present it. The Science Media Centre Desktop Guide for Covering Science touches on presenting risk responsibly. They also offer some expert reaction to the announcement.
I’ve deliberately left out specific types of cancer in places as I think at this stage it adds to the confusion. There are links with several types of cancer (e.g. colorectal, bowel, pancreatic, stomach): each of these has different risk profiles and different biological aspects to consider.
1. They do sort of, kind of get it right in their final paragraphs — but not before making a complete hash of it earlier.
2. I’m taking this from Lancet: different sources are reporting different cancers, which I imagine is adding to the confusion. I say ‘modest’ rather than ‘small’ level of cancer risk as ~6% isn’t that small.
More articles on Code for life: