The first thing to note is none of the information that the IARC have based their decision on is new. Processed meat being a possible factor for increasing the risk of cancer is old news and has hit the headlines many times in the past. It is just that the IARC have now collated existing information and used that as a body of evidence to determine whether processed meat should be a Group 1 classification. So what does a particular agent having IARC Group 1 classification mean? Well that means that there is sufficient evidence that the agent is carcinogenic to humans, but there are 118 agents in Group 1. One of the agents on the list is cyclophosphamide, which is actually itself a chemotherapy drug. Other, more well-known, agents on the list include: alcoholic beverages, outdoor air pollution, occupational exposure as a painter, tobacco smoking and salted-fish, Chinese style.

Well, the lifetime risk for bowel cancer is a little over 6% (in fact it is approximately 6.06%). This means that if we were to consider 100 people who didn’t eat bacon and other processed meat, we would expect six of them to get bowel cancer anyway. We know that eating bacon increases this risk by 18%, which means that if we were to take 100 people who ate 50g of bacon daily, we would expect 7.15 of them to be diagnosed with bowel cancer. So that’s pretty much just one extra person per 100 people.

It is important to mention here that receiving a Group 1 classification is based on statistical significance, and we must note that some of these agents will increase the risk of cancer a lot and some will increase it only a little. In addition, the underlying risks associated with the different cancers will also vary, so let’s take a look at the important numbers. A meta-analysis that formed an important part of the IARC’s decision looked at 9 studies that considered the effect of processed meat on colorectal (bowel) cancer [5]. The authors found that processed meat intake is associated with an increased risk of bowel cancer, with the risk amongst those eating 50g of processed meat daily being 18% higher than those who ate none. But what does this mean in terms of absolute risk?

Well, the lifetime risk for bowel cancer is a little over 6% (in fact it is approximately 6.06%). This means that if we were to consider 100 people who didn’t eat bacon and other processed meat, we would expect six of them to get bowel cancer anyway. We know that eating bacon increases this risk by 18%, which means that if we were to take 100 people who ate 50g of bacon daily, we would expect 7.15 of them to be diagnosed with bowel cancer. So that’s pretty much just one extra person per 100 people. If we consider something like lung cancer, however, well for non-smokers the lifetime risk of getting lung cancer is around 1%, meaning that for every 100 people who don’t smoke, only 1 of them would be diagnosed with lung cancer. Now consider those individuals who smoke around 20 cigarettes a day, these individuals have 26 times the lung cancer risk of non-smokers, so that’s 26 people being diagnosed from 100 smokers. That’s an increase of 25 people! So even though both tobacco smoking and processed meat both appear in Group 1 of the IARC list of classifications, they both have very different effects on the associated cancer risks, something that is extremely important to note.

Finally, this 18% increase in the risk of bowel cancer was associated with people who ate 50g of bacon or other processed meat daily and not all of those studies included in the meta-analysis controlled for other factors such as BMI and physical activity. What is 50g of bacon? Well it’s about 2 rashers, which I guess would be a bacon sandwich daily. One might reasonably suspect that those individuals who eat a bacon sandwich every single day may also have an unhealthy lifestyle in general, which would be contributing to an increased cancer risk. The biggest two risk factors for bowel cancer are in fact two things that we can’t do anything about: increasing age and family history. So what does this all mean for my Saturday morning bacon sandwich? Well, if eating a bacon sandwich daily only corresponds to an extra 1 person in every 100 being diagnosed with bowel cancer, I think my once-a-week treat can stay!

References

[1] IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Press Release. Available from: http://www.iarc.fr/en/media-centre/pr/2015/pdfs/pr240_E.pdf
[2] http://www.telegraph.co.uk/news/health/news/11950018/Bacon-ham-and-sausages-as-big-a-cancer-threat-as-smoking-WHO-to-warn.html
[3] http://www.dailymail.co.uk/wires/pa/article-3286072/Processed-meats-big-cancer-threat-cigarettes-health-experts-say.html
[4] http://www.thegrocer.co.uk/buying-and-supplying/health/bacon-and-sausage-sales-10-down-after-who-cancer-link/527973.article?rtnurl=http://www.google.co.uk/url?sa=t#  (behind a paywall)
[5] Chan DSM, Lau R, Aune D, Vieira R, Greenwood DC, et al. (2011) Red and Processed Meat and Colorectal Cancer Incidence: Meta-Analysis of Prospective Studies. PLoS ONE 6(6): e20456. DOI: 10.1371/journal.pone.0020456

Link to image used: https://commons.wikimedia.org/wiki/File:Bacon_butty_sandwich_roll.JPG