According to a working group from WHO’s International Agency for Research on Cancer, coffee should not be classified as carcinogenic to humans, but very hot beverages should. The group’s report, “Carcinogenicity of drinking coffee, mate, and very hot beverages,” has been published in The Lancet Oncology.
Coffee is one of the world’s most consumed beverages, whose carcinogenicity was last assessed in 1991. At that time, coffee was considered “possibly carcinogenic to humans,” based on limited data from case-control studies linking coffee consumption with bladder cancer, even though there was no evidence of carcinogenicity in experimental animals. There was also evidence for a lack of carcinogenicity in breast and large intestine cancers.
Recently, a working group composed of 23 scientists from 10 countries met in Lyon, France, where they examined more than 1,000 observational and experimental studies evaluating coffee consumption and cancer development, where important confounders, such as tobacco and alcohol consumption, were adequately controlled.
“The working group concluded that positive associations reported in some studies could have been due to inadequate control for tobacco smoking, which can be strongly associated with heavy coffee drinking,” wrote Dana Loomis, Ph.D., deputy head of the section of IARC monographs at the International Agency for Research on Cancer.
From the analysis of 10 cohort studies, an evidence of an association with coffee drinking and bladder cancer was no longer found. Furthermore, some cancers, such as endometrial and liver cancer, were found to have an inverse association with coffee consumption, with a particular meta-analysis reporting a decrease of 15 percent in the risk of liver cancer per daily cup of coffee.
Similarly, “more than 40 cohort and case-control studies and a meta-analysis from nearly 1 million women indicated either no association or a modest inverse association for cancer of the female breast and coffee drinking,” Loomis and colleagues wrote. Pancreas and prostate cancer also consistently showed no association with coffee drinking.
The scientists also analyzed data on more than 20 other cancers, including lung, colorectal, ovarian, brain cancers, and childhood leukemia. The group reviewed the potential carcinogenicity of mate, a traditional South American caffeine-rich infused drink that is often consumed very hot (more than 65 degrees Centrigrade, or 149 degrees Farenheit). Drinking hot mate had previously been classified as “probably carcinogenic to humans” when it was evaluated in 1991. Now, the team found that cold mate drinking was not associated with oesophageal cancer, and that only hot or very hot mate consumption was significantly associated with a risk of oesophageal cancer.
Additionally, when the group examined the association of oesophageal cancer with the drinking temperature of other beverages, such as very hot tea, a similar increase in risk was found. In experimental animals, very hot water (65-70 degrees Centrigrade) was also found to induce the formation of oesophageal tumours.
“On the basis of these considerations and on the totality of the evidence, drinking very hot beverages at above 65°C was classified as probably carcinogenic to humans,” the authors concluded.
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