“Very hot drinks may cause cancer, but coffee does not, says WHO,” The Guardian reports.
A review by the International Agency for Research on Cancer (IARC) concluded that only beverages consumed at higher than 65C posed a possible cancer risk.
The working group’s report re-evaluated the cancer-causing properties of drinking coffee, maté (a South American drink), and very hot beverages.
Coffee was classified as a possible cause of cancer in 1991, but the group has cleared the previous classification and suggested any suspected link was because of the hot temperature of the drink.
The researchers concluded there was limited evidence that drinking coffee and maté causes cancer, but say the risk of cancer of the oesophagus – the gullet – may increase with the temperature of the drink above 65C (149F).
Both the Daily Mirror and Daily Mail covered the story. The Mirror reports that leaving your cup of tea for around five minutes should cool it to a safe level.
The Mail reports that, not entirely surprisingly, store-bought black coffee is hot, at between 66 and 81C. So again, it is best left to cool for a while.
As it stands, smoking or alcohol consumption pose a bigger – and better documented – risk for oesophageal cancer.
Who produced the report?
The report was published by an international collaboration of researchers (working group) of the IARC, a specialised cancer agency of the World Health Organization (WHO).
The group came together in France as part of the IARC Monographs Programme, which seeks to evaluate and identify environmental factors that can increase the risk of human cancer.
The researchers reviewed epidemiological studies of exposure to carcinogens in human populations, and used the evidence to classify potential hazards as:
- group 1 – carcinogenic to humans
- group 2A – probably carcinogenic
- group 2B – possibly carcinogenic
- group 3 – not classifiable (no evidence to make a reliable judgement)
- group 4 – probably not carcinogenic
However, the classification does not indicate what level of risk is associated with the exposure to a classified hazard.
For example, smoking cigarettes and using a sunbed are both group 1 hazards. But the risk of cancer associated with smoking cigarettes is far higher than using a sunbed.
Overall, the exact method of how the authors identified and selected the research is unclear. As such, it’s not possible to say that this was a systematic review.
The monographs are published so they can be used by national health agencies to support their actions in preventing exposure to potential carcinogens.
What did the report find?
As part of their re-evaluation, the group assessed more than 1,000 observational and experimental studies.
- coffee drinking was “not classifiable as to its carcinogenicity to humans” (group 3)
- maté was “not classifiable as to its carcinogenicity to humans” (group 3)
- hot drinks above 65C were “probably carcinogenic to humans” (group 2A)
Coffee drinking was evaluated by the IARC in 1991, and at the time was classified as “possibly carcinogenic to humans” (group 2B).
However, this was based on “limited evidence” – defined on the basis that a positive association between hazard and outcome was observed, but bias could not be ruled out.
The current evaluation has been conducted on a much stronger and larger body of evidence, with nearly 500 relevant epidemiological studies identifying more than 20 different cancers.
The group assessed a collection of epidemiological evidence, and gave the greatest weight to prospective cohort and population-based case control studies that had controlled for other exposures, such as tobacco and alcohol consumption.
The studies followed cohorts of people who self-reported their coffee drinking habits to see how many individuals developed cancer and how it was related to their consumption of coffee.
During this re-evaluation, the majority of epidemiological studies showed no association between coffee drinking and cancers of the pancreas, female breast, and prostate. Reduced risks were observed for liver and endometrial cancers.
On judging the various studies, the group concluded the evidence for “coffee drinking causing cancer” was inadequate. Reasons included insufficient data, inconsistency of findings, inadequate control of potential confounders, and bias.
Maté is a hot drink consumed in South America, and is also the national drink of Argentina.
It’s a caffeine-rich infusion made from dried leaves of the yerba maté plant. In 1991 the IARC classified it as “probably carcinogenic to humans” (group 2A).
Since then, several epidemiological studies have been conducted evaluating the risk of oesophageal cancer and the consumption of hot maté.
With this new data, the IARC wanted to better understand whether the associations from previous studies were the result of maté itself or the hot temperatures at which it is usually consumed.
The studies found cancer of the oesophagus was associated with drinking hot maté, rather than maté at warm or cold temperatures.
The findings from the evaluations of maté led the researchers to assess the association between oesophageal cancer and other hot drinks.
Previous research from China, Iran, Japan and Turkey also found the risk of cancer may increase with the temperature of the drink.
The IARC conducted a combined analysis on several epidemiological studies that had assessed the effect of both temperature and the amount of maté consumed on 1,400 patients with oesophageal cancer.
The results showed that regardless of the amount consumed, the risk of cancer increased with an increase in temperature.
There were significant differences in the results from drinking very hot maté, but not with warm maté.
The studies suggested the carcinogenic effects occur when drinking at temperatures above 65C.
What are the implications?
The IARC monographs seek to identify potential cancer hazards to raise awareness that a certain exposure can cause cancer in exposed people. However, they don’t issue recommendations.
Their assessment of scientific evidence is produced so the World Health Organization, health agencies and governments can take it into consideration when developing health policies and guidelines. Whatever actions are taken as a result remain in the hands of the authorities concerned.
Professor 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.”
Professor Sir David Spiegelhalter, Winton professor of the public understanding of risk at the University of Cambridge, said: “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.”
Arguably, a commonsense approach would be to not drink anything hot enough to give you a serious burn if you spilt it on yourself, whether it’s maté, coffee or tea.