Thursday 30 March 2023

Expert reaction to IARC evaluation of the carcinogenicity of coffee, maté and very hot beverages

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The World Health Organization’s International Agency for Research on Cancer (IARC) has issued its latest report which classifies hot drinks as group 2A carcinogens, meaning they are “probably carcinogenic to humans”, while coffee and maté (a herbal drink) served cold are in group three, which means there is insufficient evidence to believe that they cause cancer.

The SMC also issued comments on the background to this issue before the IARC released their report.

Prof. Paul Pharoah, Professor of Cancer Epidemiology, University of Cambridge, said:
“This report is one of a long series of reports published by IARC, each of which summarizes the scientific evidence linking a wide range of possible risk factors to a risk of cancer.

“The primary target audience for these reports is policy makers – many of the early reports published over twenty years ago were about specific occupational or environmental exposures of limited interest to the general public.

However, as the remit for these reports became broader, the potential risk factors evaluated have been of interest to a wider general public.

The terminology used in the reports is very specific and clearly defined, but the meaning of these terms may be confusing for the public to understand.

“The primary aim of each report is to classify each potential factor (also called an exposure) according to how likely it is that the factor is truly associated with a risk of cancer.

A factor that is associated with any increased risk of cancer (however small that increase might be) is called a cancer hazard. The magnitude of that risk is not relevant to the classification.

However, it is the magnitude of the risk that is of primary importance in deciding the importance of the risk factor to the individual or to the public health.

“In this report “drinking very hot beverages” have been classified as a group 2A cancer hazard, specifically cancer of the oesophagus. This means it is probable that drinking very hot beverages is causally associated with cancer of the oesophgus, but it does not have any implication for what the magnitude of that risk is.

“This finding is of limited relevance to people in the UK as it is very uncommon for people here to drink tea (or coffee) at temperatures defined by IARC as very hot (>65 deg C).

“In the past (in 1991) IARC had classified coffee drinking as a Group 2B cancer hazard, but with a large body of new evidence available since then, coffee drinking has been downgraded to a Group 3 meaning there was inadequate evidence to link coffee drinking to a risk of cancer. In other words we should not be worried about drinking coffee because of any possible risk of cancer.”

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“These IARC conclusions are published in a series called “Monographs on the Evaluation of Carcinogenic Risks in Humans”, but technically that’s a misnomer, as I’ll explain later.

The key thing about this classification is that it is based solely on strength of evidence that there is some increase in the chance of cancer in people exposed to the thing in question, and not on how much the chance increases.

“For instance, there are well over 100 different agents in IARC’s “Group 1” in its classification. These are things where IARC thinks there is sufficient evidence that they really do increase the chance of cancer.

But some may increase it by a lot, some by a small amount. Tobacco smoking and eating processed meat are both in Group 1. 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 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.

OK, an increase, but only one more death and that’s nothing like the effect of cigarette smoking. So why are smoking and processed meat eating both in the same IARC group? Just because in both cases IARC is convinced that the evidence of some kind of effect, big or small, is quite strong.

There are plenty of other agents in IARC Group 1 besides smoking and processed meat eating. Many of them are chemicals that most people (including me) won’t have heard of, but others are more familiar, such as burning coal at home, or working as a painter. Some of these have a big effect on the chance of cancer, others don’t.

“In the new announcement, IARC have put drinking very hot beverages into their Group 2A, which means that they consider it “Probably carcinogenic to humans”.

But all that means is that the evidence that it causes cancer was a bit weaker than for the things in Group 1. Again Group 2A contains a huge range of things, for which, if they do actually cause cancer, the increase in the chance of cancer might be big or it might be quite small.

The group contains some scary-sounding things like glyphosate weedkiller, but plenty of others, including some kinds of shiftworking, or working as a hairdresser or barber, are familiar.

Again you have to remember that the increased chance of cancer may be small – the press release doesn’t give details on how big it is, though more details may emerge later – and in fact IARC did not consider that there is decisive evidence that there is any increased chance at all (or they’d have put it in Group 1).

“Before the new announcement, IARC had drinking coffee (in relation to bladder cancer) in their Group 2B, “Possibly carcinogenic to humans”.

This means that the evidence that it causes cancer was considered even weaker than for group 2A, so the removal of coffee from this group doesn’t mean that IARC were wrong before, or that they’ve made a U-turn.

They’ve reviewed the evidence, that they previously thought of as not particularly strong, and decided it isn’t even strong enough to put coffee in Group 2B, provided it’s not scalding hot.

Drinking coffee or Maté that’s not hot is in Group 3, which means “Not classifiable as to its carcinogenicity to humans”. IARC have investigated them, but have not found enough evidence that they did (or did not) cause cancer. (It’s very difficult to say with any degree of certainty that something does not, ever, cause cancer in humans.)

“IARC use the two words ‘hazard’ and ‘risk’ to explain what they do in these classifications, but they are using both words in a technical sense.

They explain it in a Q&A document at

They use the word ‘hazard’ to refer to the amount or weight of evidence that an agent is capable of causing human cancers, and it’s this ‘hazard’ that their classification is based on.

They use ‘risk’ to refer to the chance that cancer will occur in a person exposed to the agent, and they say explicitly that their classification does not measure ‘risk’ in this sense, despite the title of the monograph series.

“It’s important to realise, though, that no new evidence has been collected – instead IARC have followed their usual procedure of collating existing evidence and coming to a decision on how strong that evidence is.

And they have not yet published the detailed monograph that describes the basis for their findings – that will come later, and will allow scientists to evaluate the IARC classification thoroughly.

“Also IARC do not themselves make specific recommendations on what we should eat and drink. The press release does, however, draw attention to a report from another expert group convened by WHO and by FAO, the Food and Agriculture Organization of the UN.

This says, as one of its recommendations for reducing the risk of developing cancer, “Do not consume foods or drinks when they are at a very hot (scalding hot) temperature.” WHO published that recommendation back in 2003.”

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