An extensive evaluation of coffee's cancer-causing risk, that resulted in a reclassification, but did not prove the beloved beverage's safety, was released by the World Health Organization (WHO) Wednesday.
"The classification does not show it is certainly safe, but there is less reason for concern today than before," said Dana Loomis, deputy head of the International Agency for Research on Cancer (IARC), the WHO's cancer agency.
Some studies showed coffee had no effect on pancreatic, breast and prostate cancers. However, for 20 other cancers, the evidence was inconclusive.
"Overall coffee drinking was evaluated as unclassifiable as to its carcinogenicity to humans," the report said.
Coffee was last reviewed in 1991 and classified as Group 2B: possibly carcinogenic to humans. Coffee now joins 501 other substances in Group 3: not classifiable as to its carcinogenicity to humans.
Misleading media reports have stated coffee is safe to drink; however, a Group 3 designation means that "existing scientific data do not enable a conclusion to be made about whether it causes cancer."
There are 5 groupings from 1 to 4; smoking tobacco is under group 1: carcinogenic to humans.
The reason for the reclassification is that evidence suggesting drinking coffee caused bladder cancer has weakened in the last 25 years. The scientists also found evidence that coffee reduces the risk of both liver and endometrial cancers.
However, before coffee can be designated non carcinogenic, there needs to be enough substantial evidence to dismiss previous research showing an association, Loomis explained.
The 23 scientists on the IARC team looked at over 1,000 human and animal studies dating back to the 1970s, Loomis said.
Many studies looked at research participants' coffee drinking habits over a number of years and whether they developed cancer. The majority of the studies did not distinguish between different ways of preparing or different kinds of coffee.
Coffee is comprised of 100 different chemicals, Loomis said. Research at this time is insufficient to determine whether these observations can be attributed to caffeine.
The experts also looked at mate and very hot beverages overall. They found that "drinking very hot beverages probably causes cancer of the oesophagus in humans."
Worldwide oesophageal cancer is the eighth most common cause of cancer.
"It is the temperature, rather than the drinks themselves, that appears to be responsible." said IARC Director Christopher Wild. Very hot beverages are defined as 65 degrees celsius or hotter.
Mate is served at almost 70 degrees in many Latin American countries. Whereas coffee drinks in Europe and North America are typically served at 60 degrees or below.
"If you spill 60 degree coffee in your lap, it hurts," Loomis said.
A summary of the evaluations, which includes carcinogenicity classifications, was published in the Lacet Oncology.