‘Very hot’ drinks probably cause cancer, coffee redeemed – Expert reaction

The World Health Organization’s cancer agency has found no conclusive evidence that coffee causes cancer but that drinking very hot drinks (65 °C or above) probably causes cancer of the oesophagus.

Coffee - waferboard
Credit: Flickr / Waferboard

The International Agency for Research on Cancer (IARC) said the finding suggests that it is the temperature of drinks rather than the drinks themselves that is important when it comes to cancer.

IARC have classified the consumption of coffee as “not classifiable as to its carcinogenicity to humans” (Group 3). A Group 3 evaluation does not mean coffee drinking has been proved safe when it comes to cancer, but it means that there is not enough evidence to say it is hazardous.

Coffee drinking was previously categorised as Group 2B or “possibly carcinogenic to humans” on the basis of a link to bladder cancer, the experts say this link has become weaker and it is no longer possible to determine if coffee causes bladder cancer.

Tea was not evaluated as part of this review.

Read more about the IARC classification on Scimex.

The SMC gathered the following reaction from New Zealand experts. Contact the SMC for a follow up interview.

Thomas Lumley, Professor of Biostatistics, University of Auckland comments:

“The main finding relevant to New Zealanders in the IARC report is that coffee is safe, to the extent that years of research have been able to tell. Accumulating data has shown that previous concerns about possible risks were unfounded. There are types of cancer where it hasn’t been studied enough to say, but there is no specific reason to be concerned.

“Very hot drinks were classified as ‘probably carcinogenic’, increasing the risk of cancer of the oesophagus. “Very hot” means over 65°C at a minimum, hot enough to cause serious burns if spilled.

“Drinking tea or maté at these temperatures is common in parts of China, South America, and the Middle East. New Zealanders following tea drinking traditions from those parts of the world might consider letting their tea cool for a few more minutes, but alcohol and smoking are much more important risk factors for oesophageal cancer in this country.”

Prof Lynette Ferguson, Auckland Cancer Society Research Centre, University of Auckland, comments:

“IARC classifications are very carefully worked out with an international team. So the news that drinking coffee appears safe is good.

“My interpretation of Hot drinks is that this means scalding hot drinks, which may cause damage to the oral cavity.

“No, it will not be generally warm drinks, just going too far with VERY hot drinks on a regular basis.

“I do agree that it is important that the public do not panic over this one.  “

Associate Professor Andrea ’t Mannetje, Centre for Public Health Research, Massey University comments:

“Coffee: Although more than 1000 studies have been done on this topic, there is not enough evidence to suggest coffee drinking increases the risk of cancer.

“Very hot beverages: Collectively, the studies done to date suggest that drinking very hot beverages probably causes cancer of the oesophagus in humans. This is mainly relevant for those countries where tea or mate (a traditional South-American beverage) is traditionally drunk very hot, and the scalding hot drink may reach the oesophagus.

“Should we stop drinking all hot drinks? No. The studies done to date do not suggest that drinking hot drinks in general increases the risk of cancer. However, this IARC evaluation does suggest that drinking these drinks at scalding hot temperatures should be avoided, as this probably increases the risk of cancer of the oesophagus.”

From the Australian SMC

Professor Sanchia Aranda is CEO of  Cancer Council Australia, comments:

Coffee drinkers should be comforted to know they are not increasing their cancer risk – as long as their coffee isn’t too hot. The risk applies to beverages at 65 degrees Celsius or hotter. As a guide, a beverage at that temperature is likely to be uncomfortably hot for some people to drink. So let the drink cool a little and enjoy it.

“This IARC analysis should help dispel the myth that everything causes cancer – and help get the focus back on things we can all do to reduce Australia’s cancer burden.

“People worry too much about exposure to things that pose no cancer risk. Right now we’ve got good evidence on how the next Australian government could save tens of thousands of lives by investing more in bowel cancer screening and anti-smoking and skin cancer awareness programs – that’s where Cancer Council Australia would like the focus to be.”

Dr Christina Pollard is a Research Associate in the School of Public Health at Curtin University, comments:

“The cancer causing potential of drinking coffee has been re-evaluated by the International Agency for Research on Cancer (IARC) after a 25 years and has been down-graded to ‘no conclusive evidence’ as carcinogenic to humans.

“The available evidence has grown substantially over the time and enabled a more extensive analysis with a variety of cancers studied. The IARC committee suggest that other factors such as smoking may have accounted for the original classification as possible carcinogenic in 1991.  At that time it was common for people to both drink coffee and smoke.

“Coffee consumption varies by country in type and amount.  Australia has seen an increase in consumption over the last decade and it is important to continue to monitor foods or beverages that are commonly consumed in large amounts. Australians drink more coffee than tea, the 2011-2012 Australian Nutrition Survey shows that coffee was consumed by nearly half of the population with an average intake of 300 mls (equivalent to a large mug, but most were from instant coffee powder.”

Professor Ian Olver AM is Professor of Translational Cancer Research and Director of the Sansom Institute for Health Research at the University of South Australia, comments:

“The International Agency for Research on Cancer scans the world literature to determine how likely it is that an agent causes cancer. In this case after reviewing 1000 animal and human studies they have found no evidence that drinking coffee causes cancers of the breast, pancreas and prostate and found reduced risk of liver and endometrial cancer.

“This is a very large number of studies which gives confidence in this result. There is no evidence that any particular type of coffee is worse than any other. Coffee had previously been thought to be possibly associated with bladder cancer. This shows how just having a small number of studies can cause uncertain results which are clarified by considering a very large number of studies. It also shows that if other known causal factors are not controlled for, in this case smoking, the cause of the cancer can be attributed to an agent that is merely associated with the cancer but does not cause it.

“The current IARC report makes this point in relation to very hot drinks (over 65C) which have been found to probably cause cancer of the oesophagus, where it is the temperature of the drink, not the type of drink that is the factor causing the cancer.”

From the US-Based Genetic Expert News Service

Dr. Thomas G. Sherman, Associate Professor, Department of Pharmacology and Physiology, Georgetown University Medical Center (webpage):

“Studies over the past 4–10 years have narrowed the list of suspected dietary habits associated with the incidence of esophageal cancer. These studies originally focused on specific ingredients within beverages, such caffeine in coffee. Several of these studies, including a recent meta-analysis, however, focused on a common factor for all such beverages: they can be very hot, and the question was asked whether recurring thermal injury to the esophagus may be the real risk factor for esophageal cancer.

“Explanations for how thermal injury can increase the risk of esophageal cancer vary and include both potential genetic and metabolic processes. For example, a series of studies have independently identified a specific unusual pattern of mutations in the TP53 gene associated with esophageal cancers in countries where the link to drinking very hot beverages is the strongest, such as in Lower Normandy, France, Northern Iran, and Southern Brazil.

“An alternative (and attractive) explanation focuses on the important barrier function that the cells lining the surface of the esophagus play in protecting tissues from exposure to potential carcinogens in foods and the air. Repeated thermal injury to this barrier might permit otherwise insignificant levels of food-borne carcinogens, such as alcohol or air-borne components of tobacco, that would normally pass through the digestive system or airways undetected, to impact exposed sensitive tissues.

“The observation that the consumption of very hot beverages is a risk factor for esophageal cancer is quite relevant, but the possible explanations for this increased risk await further research. The International Agency for Research on Cancer (IARC) has classified very hot beverages as a Group 2A carcinogen, meaning that very hot beverages are one probable cause of esophageal cancer.

“This classification conveys no information on risk, however. According to a thorough 2015 meta-analysis in BMC Cancer, the odds ratio (OR) of hot beverages and esophageal cancer for men or women was 2.36 and 2.45, respectively. This means that the risks for esophageal cancer are roughly 2.40 times higher if you drink very hot beverages compared to those who do not drink very hot beverages. Fortunately, this is 2.40 times a very small number; as an absolute cancer risk, very hot beverage consumption is very low compared to known carcinogens, including alcohol.”

From the UK SMC

Prof Andrew Sharrocks, Professor of Molecular Biology, University of Manchester, comments:

“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, comments:

“The first thing to note from this is announcement is that the classification of very hot beverages asprobably 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 asprobably 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, comments:

“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, comments:

“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, comments:

“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, comments:

“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, comments:

“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, comments:

“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. 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.”

Declared interests

Prof Andrew Sharrocks: “No interests.  I work on the molecular aspects of oesophageal cancer.”

Dr Jennifer Rogers: “Dr Jennifer Rogers is a paid employee of the University of Oxford and she is currently on a NIHR Post-Doctoral Research Fellowship. She has previously held consultancy contracts with Novartis, Abbott, Celladon and Amgen. She is a member of the Royal Statistical Society, currently sitting on Council and holding the position of Honorary Officer for Meetings and Conferences.”

Prof Alan Boobis: “Public sector Chair of the Board of Trustees of the International Life Sciences Institute; public sector member of the Board of Trustees of the ILSI Health and Environmental Sciences Institute; public sector vice-president of the European Branch of ILSI. Member of a number of expert groups of ILSI HESI and ILSI Europe developing generic methodology in the field of chemical risk assessment (none of these positions is remunerated).”

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.”