Coffee should no longer classified as a carcinogen to humans, according to World Health Organization (WHO) International Agency for Research on Cancer. The new assessment is summarized in a report titled “Carcinogenicity of drinking coffee, mate, and very hot beverages,” published in the journal The LANCET Oncology.
In 1991, the working group classified coffee as “possibly carcinogenic” to humans. Now, the team revised the data used to form a potential association between coffee consumption and cancer. They determined the link was “based on limited evidence of an association with cancer of the urinary bladder from case-control studies, and inadequate evidence of carcinogenicity in experimental animals.”
The 23 scientists from the 10 countries that composed WHO’s working group analyzed more than 1,000 relevant observational and experimental studies. The team gave the greatest weight to well-conducted cohort and population-based case-control studies that controlled for important potential confounders, including tobacco and alcohol consumption.
Analyzing 10 cohort studies and several population-based studies researchers concluded there is “no consistent evidence” linking coffee drinking and bladder cancer.
“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 study author 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.
“More than 40 cohort and case-control studies and a meta-analysis including nearly 1 million women consistently indicated either no association or a modest inverse association for cancer of the female breast and coffee drinking,” Loomis and colleagues wrote. “Similarly, numerous cohort and case-control studies of cancers of the pancreas and prostate consistently indicated no association between these cancers and coffee drinking.”
The team reviewed data related to more than 20 types of cancer, including colorectal, lung, ovarian, and brain cancers. They noted, “Although the volume of data for some of these cancers was substantial, the working group judged the evidence to be inadequate for all of the other cancers reviewed for reasons including inconsistency of findings across studies, inadequate control for potential confounding, potential for measurement error, selection bias or recall bias, or insufficient numbers of studies.”
The same pattern of inadequate evidence was also extended to the potential carcinogenicity of mate — caffeine-rich drink consumed primarily in South America.
Instead, the temperature at which people drink their beverages is important, highlighting that very hot beverages should be classified as “probably carcinogenic” to humans (with temperatures of at least 65°C or 149°F).
“The epidemiological evidence for very hot beverages and human cancer has strengthened over time, with positive associations and trends in studies that considered qualitative gradations of temperature. Although the mechanistic and other relevant evidence for very hot beverages is scant, biological plausibility exists for an association between very hot beverages and cell injury and the sequelae that might lead to cancer,” authors concluded.