First, the good news: The International Agency for Research on Cancer (IARC), headquartered in Lyon, France, is letting coffee off the hook after having classified it as “possibly carcinogenic” in 1991. It is now officially in the category “not classifiable as to its carcinogenicity.”
But here’s the bad news: IARC experts have also concluded that consuming very hot beverages can probably cause cancer of the esophagus. How big a risk that poses, however, is unclear.
Confused? Join the crowd. The two verdicts, announced today by an expert panel at an agency that is part of the World Health Organization, show the limits of such blanket yes-or-no decisions on whether something causes cancer. Such an announcement is “interesting for science but does not provide the information for making decisions, either policy or individuals,” says David Spiegelhalter, a statistician at the University of Cambridge in the United Kingdom.
Here’s what is going on. The 1991 verdict was based on much less data than are available today. Since then, hundreds of new studies on the effects of coffee have been published, says Rudolf Kaaks, an epidemiologist at the German Cancer Research Center in Heidelberg. Many of these were cohort studies, in which healthy people are followed over years until some of them develop a disease. Such studies are generally considered to provide better evidence than case-control studies, which formed the basis for the 1991 classification.
The review panel also took great care this time to exclude any results that might be explained by confounding factors, says Kaaks, for example, the fact that people who drink a lot of coffee are also more likely to smoke. Overall, the studies showed that drinking coffee does not increase the risk for pancreatic, prostate, or female breast cancer, explained the panel’s leader, Dana Loomis, at a press conference today in Lyon. It may even reduce the risk for liver cancer, Loomis said, adding that the evidence for most other cancer types is inconclusive. “In other words, we should not be worried about drinking coffee because of any possible risk of cancer,” says Paul Pharoah, a cancer epidemiologist at Cambridge.
Much less is known about the effects of drinks consumed at temperatures above 65°C. But what data there are suggests they can cause cancer of the esophagus. The link is plausible, says Kaaks, who has worked at IARC in the past, because scalding hot water can cause inflammation, which is known to increase cancer risk. These hot drinks now join red meat, shift work, and dozens of chemicals in category 2A (probably carcinogenic) of the IARC classification system.
But there are important caveats. “This finding is of limited relevance to people in the U.K. or USA, as it is very uncommon for people here to drink tea [or coffee] at temperatures defined by IARC as very hot,” Pharoah says. Even for those living in China, Iran, and parts of South America, where tea is traditionally drunk very hot, it is unclear how big a risk hot drinks pose. Asked to quantify the risk, Loomis said, “we cannot put a number on this at the moment.”
Despite sending those mixed messages, scientists still have something of value to share, Kaaks says. “I think it’s okay to tell the public: Don’t drink your tea too hot,” he counsels. He figures that advice could benefit some people, and it’s unlikely to cause any harm.