The news that coffee is being downgraded from “possible carcinogen” to “unclassifiable” sounds surprising, since we’re used to seeing trends go the other way, from safe to potentially cancer-causing. But this is how the scientific process works. The data on coffee consumption and risk of coffee has grown considerably since 1991 when the International Agency for Research on Cancer (IARC) put coffee on its list of possible carcinogens.
In 1991, a Working Group convened by IARC reviewed the scientific evidence available at that time, and concluded that coffee was a ‘possible’ carcinogen in humans. This was based on limited evidence for an association with bladder cancer. Notably, there was evidence at that time suggesting lack of carcinogenicity for female breast and colon cancer.
Last month, another expert working group was convened by IARC to re-evaluate the available evidence base, which now includes more than a thousand studies.
The new Working Group found that the evidence suggests that coffee drinking may in fact have some qualities that protect against cancer, including that the antioxidants in coffee may reduce the risk. Indeed, the report indicates there is some evidence coffee drinking is associated with reduced risk of endometrial and liver cancer. The group also found coffee drinking is not associated with female breast, pancreatic, or prostate cancer. And for 20 cancer sites, including lung and colorectal cancer, the evidence was inadequate and conclusions regarding carcinogenicity could not be drawn.
Therefore, the Working Group concluded that, overall, coffee drinking was unclassifiable as to its carcinogenicity in humans (Group3).
In its review for bladder cancer, the group could find no consistent evidence for an association with coffee drinking. As to why that changes, one possibility is that results from earlier studies may have been confounded by smoking, which is more common among coffee drinkers, especially heavy coffee drinkers, and is known to be associated with a higher risk of bladder cancer. Newer studies better corrected for that confounding.
The other major subject the report is the consumption of hot beverages, described as those over 65 degrees Celsius, 149 degrees Fahrenheit. The Working Group noted that the epidemiological evidence for very hot beverages and human cancer has strengthened over time. On the basis of that evidence, drinking very hot beverages at above 65°C (149 degrees Fahrenheit) was classified as “probably carcinogenic to humans” (Group 2A). Coffee is usually served at about 140 degrees Fahrenheit.
The evaluation included drinking of very hot mate, a tea-like drink consumed mainly in South America, and to a lesser extent in the Middle East, Europe, and North America. Mate is traditionally drunk very hot. Drinking mate that is not very hot was evaluated as “not classifiable” (Group 3), the same as coffee.
Some may see this change as a sign that “science can’t make up its mind.” In fact, this is what makes science powerful: its willingness to seek and consider the totality of the evidence and interpret evidence that challenges what we previously believed.