A World Health Organization panel of experts on cancer risks just declared that drinking coffee may protect against some cancers. But if the coffee is too hot, it actually slightly increases the risk of other cancers.
The International Agency for Research on Cancer, an arm of the WHO, periodically makes declarations about the strength of evidence of a link between cancer and things like bacon, cellphones, or exposure to certain chemicals.
In the case of coffee, the IARC reviewed more than 1,000 studies in humans and animals and found there was "inadequate evidence" that coffee raises the risk of cancer. So the agency opted to categorize the beverage as "unclassifiable" when it comes to its carcinogenicity.
That's a pretty confusing classification, but it means the evidence went in so many directions, depending on the type of cancer, that there was no way to say with certainty that coffee is generally safe.
There was some good news, however. Coffee didn’t seem to raise the risk for pancreatic, breast, or prostate cancers, the experts wrote in an announcement, which was published along with a statement in the Lancet Oncology. Coffee drinking was also associated with a reduced risk of liver and endometrial cancer.
The data on more than 20 other cancers — including lung, colorectal, stomach, esophageal, oral cavity, ovarian, and brain cancers, and childhood leukemia — were too limited or inconsistent to draw any grand conclusions.
Hence the categorization of coffee as "unclassifiable" when it comes to its potential cancer-causing properties.
Still, this declaration on coffee was a slight improvement over the IARC’s 1991 finding that coffee was "possibly carcinogenic." (The IARC classifies carcinogens in five categories ranging from carcinogenic to humans to probably not carcinogenic to humans.)
That concern was based on some evidence that suggested coffee drinking might be linked with urinary bladder cancer. This time, with much more evidence at their disposal, the researchers found that there was "no consistent evidence" of an association between drinking coffee and bladder cancer, according to the group’s write-up in the Lancet.
Also note: The IARC is only looking at cancer risks. And coffee is also associated with other non-cancer health effects, including lowering the risk of cardiovascular disease and Type 2 diabetes.
That’s why the most recent US dietary guidelines suggested that drinking three to five cups per day could actually be part of a healthy diet.
Hot drinks raise the risk of esophageal cancer — by a little more than half a percentage point
The IARC had a much more declarative statement on very hot beverages: They are "probably carcinogenic" for humans.
This was based on evidence from hot beverage–loving countries like China, Iran, Turkey, and Argentina. There, researchers uncovered positive associations between people who drink hot teas or maté (a traditional South American caffeine drink) and esophageal cancer specifically.
The risk here has to do with how very hot beverages damage the lining of the esophagus, potentially triggering cancer-causing genetic mutations or making it easier for carcinogens — like cigarette smoke — to flow into the cells.
As Mariana Stern, an associate professor at the Keck School of Medicine at USC who worked with the IARC, explained in a statement, "There is physical evidence that very hot beverages can contribute to cell injury in the esophagus and thus contribute to cancer formation."
"Enjoy your coffee or maté, but make sure it’s not very hot," she added.
These risks need to be put into perspective, though. The overall lifetime risk of developing esophageal cancer is very low, and hot beverages would only raise it by a tiny amount.
Thomas Sherman, an associate professor in the department of pharmacology and physiology at Georgetown University Medical Center, explained in a statement from the Genetic Expert News Service that very hot drinks raise a person's absolute risk of cancer by about 2.4 times compared with someone who doesn't drink very hot beverages. Since the lifetime risk of developing esophageal cancer is about 0.5 percent, hot drinks would bring that risk up to a little more than 1 percent.
"As an absolute cancer risk, very hot beverage consumption is very low compared to known carcinogens, including alcohol," he added.
There's another thing to keep in mind here: In the countries where the cancer association was discovered, people typically drink their hot teas or matés at temperatures that exceed 150 degrees Fahrenheit, which many of us would consider to be scalding.
Though chains like Starbucks and McDonald’s brew their coffee at comparable temperatures, researchers have found the sweet spot for drinking is about 136 degrees Fahrenheit.
So as long as you let your drink cool down a bit from the scalding point, there is little to fear in your cup of joe (or maté).