Coffee no longer classified as a carcinogen

Coffee no longer should be classified as carcinogenic to humans, according to a working group from WHO’s International Agency for Research on Cancer.

The panel instead determined coffee drinking should be considered “unclassifiable as to its carcinogenicity in humans.”

Researchers from the International Agency for Research on Cancer announced in 1991 that coffee was “possibly carcinogenic” to humans.

However, the working group formed to reevaluate the potential association between coffee consumption and cancer determined the classification announced 25 years ago 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.”

In their new review, the working group — composed of 23 scientists from 10 countries — identified more than 1,000 relevant observational and experimental studies.

“[The group] gave the greatest weight to well-conducted prospective cohort and population-based case–control studies that controlled adequately for important potential confounders, including tobacco and alcohol consumption,” Dana Loomis, PhD, deputy head of the section of IARC monographs at the International Agency for Research on Cancer, and colleagues wrote in a paper published online this morning in The Lancet Oncology.

An analysis of 10 cohort studies and several population-based studies revealed “no consistent evidence” of an association between coffee drinking and bladder cancer, Loomis and colleagues wrote.

“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,” they wrote.

The five largest cohort studies showed primarily inverse associations between coffee drinking and endometrial cancer. Several case–control studies and a meta-analysis supported these results, Loomis and colleagues wrote.

Case–control and cohort studies also showed inverse associations between coffee consumption and liver cancer, with one meta-analysis suggesting liver cancer risk decreased by 15% per cup consumed each day.

“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 investigators also reviewed data related to more than 20 other cancer types, including colorectal, lung, ovarian and brain cancers.

“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,” Loomis and colleagues wrote.

The working group also reviewed the potential carcinogenicity of mate — a caffeine-rich drink consumed primarily in South America — as well as possible risks associated with drinking very hot beverages, defined as those at least 65°C or 149°F.

The scientists determined there is inadequate evidence in humans for the consumption of mate that is “not very hot.”

However, the scientists concluded consumption of very hot beverages should be classified as “probably carcinogenic” to humans.

“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,” Loomis and colleagues wrote. “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.” – by Mark Leiser