No conclusive evidence was found to suggest that drinking coffee has carcinogenic effect on humans, however a pooled data analysis showed that drinking ‘very hot’ beverages may increase the relative risk for esophageal cancer.

Those are the findings of a new assessment by the International Agency for Research on Cancer (IARC) – the WHO’s cancer agency – on the carcinogenicity of drinking coffee, maté, and very hot beverages. A previous IARC report in 1991 classified coffee as, “possibly carcinogenic to humans.”

For this evaluation the researchers used a database of over 1,000 observational and experimental studies. The greatest weight was given to population-based, case-controlled studies with a prospective cohort. 

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The link between esophageal cancer and the consumption of hot beverages came from hospital-based case-control studies on cancer in South America. Risk of esophageal cancer increased with the quantity of maté that was consumed, but this trend was only significant for those who consumed maté ‘hot’ or ‘very hot’. This trend persisted when drinking temperature were assessed independent of amount consumed. Maté is an infusion made from dried leaves of Ilex paraguariensis and is consumed mainly in South America.

Similarly, increased relative risks for drinking very hot beverages other than maté was identified by the researchers in another pooled analysis of South American case-control studies on esophageal cancer. The authors conclude that drinking any beverage hotter than 149 degrees Fahrenheit is “probably carcinogenic to humans.” The average coffee drinking temperature in the U.S. is around 140 degrees Fahrenheit.

“We were now able to evaluate more carefully the effect of maté itself from the effect of temperature, and concluded that the observed links between maté drinking and cancer of the esophagus seem to be largely driven by drinking maté very hot,” said Marian Stern, professor at USC and co-author of the study.

Despite the findings for hot beverages, coffee consumption was shown to have an inverse association with many types of cancer. The five largest cohort studies concerning endometrial cancer found inverse associations with coffee, and another meta-analysis estimated that the risk of liver cancer decreases 15% for each 1 cup per day increment. Meta-analysis of more than 40 studies consisting of over 1 million women, consistently found no association, or a modest inverse association for breast cancer and drinking coffee. Additionally, coffee consumption was tied to robust antioxidant effects; apoptosis was found to be promoted in cancer cell lines and moderate evidence of a link between coffee consumption and reduced risk of colorectal adenoma was identified. However, for more than 20 other cancers, the evidence was inconclusive.

The full findings of the report will be published as Volume 116 of the IARC Monographs.

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