The World Health Organisation’s cancer arm made two announcements this week: one welcome and one not so welcome.
First, it announced there was no conclusive evidence to show coffee increases cancer risk. This is a reversal of the 1991 conclusion, when the carcinogenicity of coffee was first tested, that classified the drink as being “possibly carcinogenic to humans”.
But the International Agency for Research on Cancer’s (IARC) committee that exonerated coffee also found drinking beverages at very high temperatures – as is the cultural norm in some parts of South America, the Middle East and China – probably accounts for oesophageal cancer in those communities.
Not enough evidence
IARC evaluates the weight of evidence that an agent can increase the risk of cancer by getting together working groups of expert scientists to review published studies.
The data used to conclude in 1991 that drinking coffee may increase the risk of bladder cancer was based on case-control studies. These studies were commonly used to test the verdict of almost all the studies of environmental agents suspected to cause cancer in humans back then.
Case-control studies involve asking a group of, for instance, bladder cancer patients and an equal number of healthy people how often they drank coffee ten, 20 or more years ago. The patients and healthy people would be matched for age, sex and socioeconomic status.
The data from these questionnaires then implicated, but certainly didn’t prove, coffee drinking as a cause of bladder cancer. Such studies are now recognised as relatively unreliable by comparison with prospective studies.
Prospective studies involve obtaining data about, for instance, smoking, drinking and dietary practices of typically half to one million people. When monitored over a decade or more, a few hundred may be diagnosed with bladder cancer. Their coffee drinking and other data may be compared with that data for others in the larger group not diagnosed with bladder cancer.
In general, prospective studies sometimes confirm case-control studies about agents considered carcinogenic. But in the case of coffee drinking, recently conducted prospective studies failed to indicate increased risk of bladder cancer or cancer at any other site correlated with coffee intake.
Some findings even suggest coffee reduced risk of some cancers, such as liver cancer. To use the formal IARC language, coffee drinking is unclassifiable as a human carcinogen.
Very hot drinks
Also dating back to 1991 was a determination that drinking hot mate – a tea-like infusion common in South America – was probably carcinogenic to humans. It was linked to oesophageal cancer.