The warning, issued Wednesday, follows an exhaustive review of studies on coffee, tea and cancer by the WHO’s International Agency for Research on Cancer. A working group of 23 scientists declared that drinking beverages hotter than 149 degrees Fahrenheit is “probably carcinogenic to humans” — a category that also includes red meat, the pesticide DDT and the human papillomavirus.
However, the report makes clear that it is temperature that makes an espresso, cappuccino or plain old cup of joe risky — not the coffee itself. The panel downgraded coffee from its former status as “possibly carcinogenic to humans” to a category of items whose cancer risk is “unclassifiable.”
America’s 130 million daily coffee drinkers are likely to feel vindicated by the report. The expert panel said that coffee may actually reduce the risk of liver and endometrial cancers. They also cleared it of any role in causing cancers of the breast, uterus and prostate.
But some coffee lovers may want to change the way they drink the beverage. Until last week, the National Coffee Association recommended that hot coffee be served between 180 and 185 degrees Fahrenheit. Now it advises people to allow the beverage to reach “a comfortable temperature before enjoying.”
The experts on the WHO panel traced their concern to “limited evidence” from animal studies that drinking beverages at temperatures above 149 degrees Fahrenheit, or 65 degrees Celsius, may promote the growth of tumors. In addition, they noted that epidemiological evidence connecting beverage temperature and esophageal cancer risk showing that the hotter the beverage consumed, the greater the incidence of esophageal cancer “has strengthened over time.”
The IARC initially declared that consumption of coffee and yerba mate — a highly caffeinated tea widely drunk in South America — might cause cancer in 1991. After sorting through more than 1,000 well-conducted studies 25 years later, however, the drinks got a virtual all-clear.
The working group suggested that their predecessors might have missed a key signal: that heavy smoking and alcohol use often are more common among people who consume lots of coffee and mate. In 1991, researchers did not adjust their findings to take this into account.
Smoking and heavy drinking are known to be key factors in esophageal cancer risk. But heat, too, may have been an unseen factor, the working group suggested.
In drafting its warning on the consumption of hot beverages, the expert panel cited a 2000 analysis of esophageal cancer and hot-beverage consumption in South America that showed “significantly increased relative risks” for drinking very hot tea and other beverages. They also cited the findings of a 2009 study that tracked a large population of tea-drinking Iranians that showed increased esophageal cancer risk among those who drank their tea hot or very hot.
In laboratory experiments, drinking plain water heated to about 150 degrees Fahrenheit increased the incidence of esophageal cancers in mice and rats. By contrast, cold mate had the opposite effect, reducing the incidence of esophageal and liver tumors.
The IARC report was published in the journal Lancet Oncology.
The goal of the working group was only to assess whether hot beverages were associated with cancer risk, not to determine the magnitude of the risk. As such, the report offers no guidance on how many hot drinks it would take to significantly increase one’s likelihood of developing cancer.
Other experts cited a 2015 analysis published in the journal BMC Cancer that concluded that men who drank very hot beverages had an average esophageal cancer risk 2.36 times as high as those who did not. Women who drank very hot drinks were 2.45 times more likely to develop esophageal cancer, the study found.
“Fortunately, this is 2.40 times a very small number,” said Dr. Thomas G. Sherman of Georgetown University Medical Center.
The WHO’s experts did not recommend a specific temperature that made coffee safe to drink. Luckily, a pair of researchers from the University of Texas in Tyler have analyzed this very question.
Using mathematical models, they weighed the sensory benefits of drinking hot coffee against the danger posed by scalding, according to their report in the journal Burns. In the end, they determined that the “optimal drinking temperature” was “approximately 136 degrees.”
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