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The World Health Organisation’s International Agency for Research on Cancer (IARC) issued its latest report that classified very hot drinks as group 2A carcinogens, meaning they are “probably carcinogenic to humans”, while coffee and maté (a herbal drink) served cold were in group three, which means there was insufficient evidence to suggest they cause cancer.
The SMC also issued comments on the background to this issue before the IARC released their report.
Prof. Andrew Sharrocks, Professor of Molecular Biology, University of Manchester, said:
“Results suggest a clear link between drinking very hot beverages and developing oesophageal cancer – however this is a particular subtype of oesophageal cancer that is more prevalent in Far Eastern countries rather than the Western world. This is in keeping with the observation that people in these countries often drink their beverages at a much higher temperature than in the Western world (>65 degrees).
“Instead, in the Western world, a different type of oesophageal cancer is more common, and this is not thought to be caused by consuming hot drinks but by diet and resulting gastroesophageal reflux (heartburn) amongst other factors. Therefore drinking hot beverages at the temperatures we typically consume them in the UK (typically <65 degrees) is very unlikely to be a major contributory factor in the onset of this disease.”
Dr Jennifer Rogers, NIHR Post-Doctoral Research Fellow, University of Oxford, said:
“The first thing to note from this is announcement is that the classification of very hot beverages as probably carcinogenic to humans is based on limited evidence from epidemiological studies. Meaning that a positive association has been observed for which a causal interpretation is considered to be credible, but chance, bias or confounding could not be ruled out with reasonable confidence.
“What the IARC Monographs also don’t take into account are the risks associated with each of the agents that are identified as carcinogenic. The lifetime risk of getting oesophageal cancer in a man is 1 in 55, meaning that for every 550 men, 10 would get oesophageal cancer anyway. This new research suggests that this increases to 20.6 out of 550 for men who drink very hot beverages. If we consider 550 men who are current smokers, however, we would expect 47.5 of them to get oesophageal cancer. So there are other factors that have a bigger effect on the risk of oesophageal cancer than hot beverages.
“It should also be noted that the studies carried out on people did not actually measure the temperature of hot drinks, rather individuals were just asked to self-report temperatures. Finally, based on studies in experimental animals, the definition of “very hot” that is being classified as probably carcinogenic to humans is for beverages at above 65°C. The typical drinking temperature for tea and coffee in most parts of the world is less than this and in comparison, the preferred tea temperature in the UK is 56-60°C.”
Prof. Alan Boobis, Professor of Biochemical Pharmacology, Imperial College London, said:
“IARC had down-graded the cancer classification of coffee from 2B to 3, which means that the data are such that it cannot be classified. However, to put this into context, amongst all of the substances and processes evaluated by IARC only one has ever been classified as not carcinogenic. This is because of the difficulty of proving a negative. If a compound is carcinogenic, this will be apparent from a significant elevation in cancer incidence in exposed people. However, if there is no significant difference, this may because it is not carcinogenic, but it could also be because there were not enough subjects to see an effect, or other factors interfered with the ability to detect a difference. Classification as category 3 does mean that there is no credible evidence for a link with cancer from drinking coffee.
“The category 2A classification for consumption of very hot drinks is consistent with knowledge of the effects of repeatedly damaging tissues over prolonged periods of time on the risk of cancer. The evidence reviewed by IARC strongly suggests that the temperatures necessary for this effect are specific to certain ways in which beverages are consumed, and are higher than those used by tea (and other beverage) drinkers in the UK. The temperature would have to be high enough to cause some prolonged irritation to the upper digestive system.”
Prof. Sir Colin Berry, Emeritus Professor of Pathology, QMUL, said:
“Hazard identification may be useful to those who set out to look for a carcinogen – but it does not inform us about the level of carcinogenicity and it certainly does not provide a basis for the estimation of risk. What is fundamentally wrong is the statement (from the IARC Q&A) that says “Identification of a cancer hazard in the IARC Monographs is an important alert that exposure can cause cancer in exposed people.” Without knowing what exposure and for what time, the information does not give you any useful information about which group might be at risk of what, and there is no consideration of balance (such as, in the case of sunlight for example, the risk of skin cancer and the benefit of Vitamin D synthesis).
“I’ve previously critiqued issues of the IARC process of hazard identification and public understanding, which can be seen here. One relevant section is: “It is important to emphasise that the monographs deal with hazards and estimates, not risks. The monographs are also intended to identify cancer hazards even when the putative risks are very low at current exposure levels, because new uses or unforeseen exposures could engender risks that are significantly higher. Thus the role of exposure in current use is bypassed.”
Prof. Tim Underwood, Associate Professor in Surgery at the University of Southampton, said:
“The bottom line here is that drinking very hot liquids is a cause of squamous cell cancer of the oesophagus, but the IARC classification can’t tell us anything about the size of the risk – so we shouldn’t take from this that there’s a high risk of developing oesophageal cancer after drinking very hot drinks.
“This finding comes from data in the Far East, Iran and South America. In the UK we predominantly see a different type of oesophageal cancer – adenocarcinoma, this is caused by acid reflux and obesity. Besides, most people in the UK drink tea with milk, which lowers the temperature of the drink to a safe level. For the coffee drinkers among us (me included) the evidence suggests that we can carry on.”
Prof. Sir David Spiegelhalter, Winton Professor of the Public Understanding of Risk, University of Cambridge, said:
“The IARC chooses to focus on the evidence of a ‘hazard’, which is the possibility of causing harm, rather than the ‘risk’, which is the actual magnitude of the harm. Being 6 miles up in a plane is a hazard, but not very risky. Last year the IARC said that bacon is carcinogenic, but it became clear that when eaten in moderation it is not very risky. In the case of very hot drinks, the IARC conclude they are probably hazardous, but can’t say how big the risk might be. This may be interesting science, but makes it difficult to construct a sensible response.”
“It’s interesting that coffee has been downgraded, and after years of debate the IARC now conclude there is not enough evidence to say it is hazardous.”
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 http://www.iarc.fr/en/media-centre/iarcnews/pdf/Monographs-Q&A.pdf. 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.”
* Lancet Oncology summary: http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045(16)30239-X/fulltext
IARC Monographs Q&A: http://www.iarc.fr/en/media-centre/iarcnews/pdf/Monographs-Q&A_Vol116.pdf
Declared interests
Prof. Sir Colin Berry: “I don’t have any relevant conflicts for this. I have been consulted by both Regulatory Authorities and the Agrochemical and Pharmaceutical industries over Risk vs Hazard.”
Prof. Tim Underwood: “Tim is funded by the MRC (Clinician Scientist Award) and he is a trustee of Heartburn Cancer UK.
Prof. Sir David Spiegelhalter: “No conflict of interest.”
Prof. Kevin McConway: “I have no conflicts of interest.”
Prof. Paul Pharoah: “I have no conflicts to declare.”