A time a go media made a great deal of the IARC classification of processed meat as IARC group 1 carcinogens. One could see alarming headlines like this:

Vegans were sharing this all over the place ecstatically. Well, it is kind of a big deal that food is put on this list, since it’s very rare or even never happened.

Hold your horses

International agency for cancer research (IARC) is part of WHO. They are maybe most famous for classifying chemicals, environmental factors and mixtures as carcinogenic in different levels. The levels work accordingly:

“Group 1: The agent is carcinogenic to humans.

Group 2A: The agent is probably carcinogenic to humans.

Group 2B: The agent is possibly carcinogenic to humans.

Group 3: The agent is not classifiable as to its carcinogenicity to humans.

Group 4: The agent is probably not carcinogenic to humans” (IARC, 2006)

The news was that IARC classified processed meat as Group 1 and red meat as Group 2A (IARC, 2015)

So what is the problem? Well, it’s true that in Group 1 we find things like arsenic, asbestos, alcoholic beverages, tobacco and various radioactive elements. That made the press print those headlines shown above. But let’s have a look at what the criteria for being on these lists are:

“These categories refer only to the strength of the evidence that an exposure is carcinogenic and not to the extent of its carcinogenic activity (potency).” (IARC, 2006)

That means that in principle, whatever is in group 1 could have an extremely small effect size. The relative risk could be just above the detectable limit, just that we can be very certain that the effect is real. The effect size of course makes a huge difference in addition to the degree of evidence. What if agent X is increasing your cancer risk by 0.000001% but we are 99.99% sure that link is correct (group 1). Agent Y on the other hand increases your cancer risk by 80%, but we are only 10% sure that this link is real (group 3). Which is worse?

Of course this is just an extreme example, but I hope you see my point. But what are the effect sizes? Well, for processed meat the relative risk (RR) is actually quite low (according to IARC), 1.18 if eating 50g processed meat daily (IARC, 2015). Compare that to smoking where the effect size is between 1.2 and 20(IARC, 2004).

So how do we interpret these numbers? Well, it’s the relative risk increase from the basal probability of developing the disease. A RR of 1 means that there is equal probability of developing disease being exposed as not being exposed, a RR of 2 means that it is twice the probability of developing the disease when exposed. How bad something is, is both dependent on the level of exposure, but regarding the relative risk, risk is without exposure matters quite a lot.

So, yes, processed meat is causing cancer, probably red meat too. But not to the extent that the newspapers seem to want to claim, and I think we should avoid to compare meat consumption to smoking and arsenic.

References

IARC. (2004). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans.

IARC. (2006). B. SCIENTIFIC REVIEW AND EVALUATION. Retrieved January 5, 2016, from http://monographs.iarc.fr/ENG/Preamble/currentb6evalrationale0706.php

IARC. (2015). IARC Monographs evaluate consumption of red meat and processed meat.