There is no conclusive evidence that drinking coffee causes cancer, the World Health Organization's cancer agency said last week in a reverse of its previous warning. However, the agency also said all "very hot" drinks are probably carcinogenic.
The International Agency for Research on Cancer (IARC), based in Lyon, France, had previously rated coffee as "possibly carcinogenic" but has changed its mind.
IARC, an arm of the WHO, periodically makes declarations about the strength of evidence of a link between cancer and things like red meat, processed meat or exposure to certain chemicals.
In the case of coffee, IARC says its latest review found "no conclusive evidence for a carcinogenic effect" of coffee drinking and pointed to some studies showing coffee may actually reduce the risk of developing liver and endometerial cancers.
At the same time, however, IARC presented other scientific evidence which suggests that drinking anything very hot - around 65 degrees Celsius or above - including water, coffee, tea and other beverages, probably does cause cancer of the esophagus. (The boiling point of water is 100 degrees Celsius.) The risk has to do with how very hot beverages damage the lining of the esophagus, potentially triggering cancer-causing genetic mutations.
The IARC, which last year prompted headlines worldwide by saying processed meat can cause cancer, reached its conclusions after reviewing more than 1,000 scientific studies in humans and animals. There was inadequate evidence for coffee to be classified as either carcinogenic or not carcinogenic.
IARC had previously put coffee as a "possible carcinogen" in its 2B category alongside chloroform, lead and many other substances.
Let it cool down, says WHO
In its evaluation of very hot drinks, IARC said animal studies suggest carcinogenic effects probably occur with drinking temperatures of 65 Celsius or above. Some experiments with rats and mice found "very hot" liquids, including water, could promote the development of tumours, it said.
The agency said studies of hot drinks such as maté, an infusion consumed mainly in South America, tea and other drinks in several countries including China, Iran, Japan and Turkey, found the risk of esophageal cancer "may increase with the temperature of the drink" above 65 Celsius.
The IARC said the results suggest that drinking very hot beverages is one probable cause of esophageal cancer and that it is the temperature, rather than the drinks themselves, that appears to be responsible.
The agency stressed that smoking and drinking alcohol were among the most serious risk factors for esophageal cancer and urged people to focus on reducing these as a priority. The IARC's evaluation of hot drinks was based on limited available evidence in humans and animals and more research is needed.
The scientists recommend being prudent and letting hot drinks cool down and not to consume foods or drinks when they are at a very hot - scalding hot - temperature.
Drinking very hot beverages is now classified as probably carcinogenic in IARC's group 2A category, alongside red meat and nitrogen mustard. This rating does not, though, say how big the risk is.
No need to panic
Just because something raises the risk of cancer doesn’t mean it will cause cancer. Dose matters – how many hot beverages you drink, how often you drink them and for how long you’ve been doing so. And importantly, other dietary and lifestyle choices will affect the risk too.
Smoking and drinking alcohol are among the most serious risk factors for esophageal cancer and people should to focus on reducing these as a priority. Compared to these risk factors, the risk from drinking hot beverages is very low. The IARC's evaluation of hot drinks was based on limited available evidence in humans and animals and more research is needed.
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