Some studies say red and processed meat cause cancer - is that correct?

The correlation of food, meats and cancer is very difficult to study because there are many elements, real or perceived, that may favour the onset and the development of cancer. National authorities have based recommendations on are the studies developed by the International Agency for Research Studies on Cancer (IARC) that highlight and classify the considered agents, certainly or presumably, responsible for cancer onset.

“Carcinogenic” is the term given to something that can cause cancer. The problem, in terms of communication to the public, is in the verb “to cause”. It is not possible to give a determined cause-effect interpretation in this instance. In other words, it is not possible to say “if you eat processed meat THEN you will surely get colorectal cancer“. In the same way it is not possible to say that if someone is exposed to a carcinogenic agent he/she will certainly get cancer. Scientists hold to the premise that “carcinogenic” is something that, taken in certain doses and for a certain period, can increase the risk to develop a certain type of cancer throughout life. However, when such information is shared with a general public, the interpretation is often that if a substance or a food is carcinogenic, this most certainly causes cancer. 

Everyone has an opinion when it comes to risks and probabilities, and anecdotal evidence can be used to prove or disprove certain beliefs. And so, some people will believe that if we do not eat a specific food or something with a carcinogenic substance, then surely we are safe from cancer. Unfortunately, this is not true. We may get (and statistically it happens!) lung cancer even if we do not smoke, and a colon cancer even if we are strictly vegan. No one will ever be able to say with certainty whether, even eating processed meat every single day, we will get a colorectal cancer or not. But this does not mean that eating a certain food or not eating it would expose someone to the same risk. 

Going back to the IARC report, the various agents are not classified based on how carcinogenic they are, nor does the report deal with the estimation of the risk, individual or collective, of an exposure to a given agent, once established to be carcinogenic. This means that it is not correct to treat all carcinogenic agents in the same way. Stating that “processed meat is like smoking or asbestos” is deeply wrong and certainly it pays no service to the public opinion. Carcinogenic agents are different, but it is not the IARC’s task to classify this aspect. There is also an interesting point to note regarding the consumption amounts investigated by the IARC, which are 50 grams of processed meat or 100 of red meat per day. This level of consumption is much higher than the level of European consumers and, in general, of the rest of the world.

Consumers, like most people confronted with decisions, have different reactions, so the information provided by sources of authority are very important and should be advised over non-science-based sources. Some may can decide to continue eating a food because the increased risk is small. Others may decide to reduce their consumption. The most important thing for authorities to keep in mind is to communicate any potential risk in a clear and proper way.

Cancer in British vegetarians – Keys T et al. (2014) Am J Clin Nutr 100(suppl): 378S–85S.