Plenty of people like to start their day with a cup of tea or coffee, but the temperature of your preferred beverage could matter a lot more than you realize.
A decade ago, the International Agency for Research on Cancer (IARC) made headlines when it classified very hot beverages at temperatures above 65 °C (149 °F) as "probably carcinogenic to humans".
That 2016 finding followed evidence that mate, a herbal drink popular in South America and often drunk at very hot temperatures, was associated with an increased risk of esophageal cancer.
Mate is typically consumed at temperatures as high as 70 °C (158 °F), but until now, the evidence for hot beverages consumed at cooler temperatures was inconclusive.
"International research has shown for some time that drinking beverages like mate at very high temperatures, as is typical in South America, is linked to increased risk of certain esophageal cancers," says nutritional epidemiologist Keren Papier from the University of Oxford.
"What was less clear was whether this also applied to the temperatures at which hot drinks are usually consumed in Western populations."
To examine the issue, Papier and fellow researchers analyzed data from two large UK cohorts, the Million Women Study and UK Biobank.
In total, the researchers incorporated data from 977,282 participants, in what they claim is the largest study to date examining the associations between hot drinks and esophageal cancer.
In both studies, participants were asked similar questions about hot drink consumption, including how hot they liked their beverages to be ('very hot', 'hot', 'warm', etc.), and how many cups of tea and coffee they consumed each day.
The researchers found that participants who preferred 'very hot' drinks had a 3.17-fold higher relative risk of developing esophageal squamous cell carcinoma (SCC) during the study period compared to people who took their drinks 'warm', after accounting for drinking frequency.
People who indicated they preferred 'hot' drinks had a 69 percent higher SCC risk than those who drank 'warm' beverages, the researchers found.
Further, the data showed that the amount of hot drinks participants consumed was also associated with cancer risk, with those consuming six or more hot drinks a day having a 71 percent increase in SCC risk compared to people who drank fewer than six hot beverages daily, after accounting for drink temperature.
The association was similar regardless of whether participants primarily drank tea or coffee, and adding milk (which was recorded in the Million Women Study) didn't appear to significantly alter the temperature-associated risks identified.
While the observational findings don't establish that hot drinks cause cancer, the researchers suggest that reducing the temperature of hot beverages even slightly could potentially prevent a substantial number of SCC cases.
"Assuming a causal association between preferred drink temperature and the risk of esophageal SCC, the estimated proportion of esophageal SCC cancers that would be prevented if individuals who consumed drinks at a very hot temperature instead consumed their drinks at a hot temperature is 14 percent," the study authors write.
"Based on national incidence rates for SCC esophageal cancers in the United Kingdom, these attributable risks translate into absolute numbers of cancers that could be prevented each year of 440."
A significant limitation with the study, however, is that the researchers didn't collect absolute temperature measurements of the beverages the participants were drinking – only their self-reported preferences of 'very hot', 'hot', 'warm', etc.
The researchers say future studies might be able to clear that up, but for now, they suggest their findings could indicate that temperature ranges below those cited in the IARC report (above 65°C) may increase cancer risk.
While their own study did not examine the biological mechanisms involved, the researchers say their findings – with similar risks seen across tea and coffee, and with milk or without – suggest that beverage temperature is the key risk factor, rather than what's in the drink.
"Animal studies support this, showing that very hot water (70° C) promotes premalignant lesions and esophageal necrosis, whereas lower-temperature beverages do not," the researchers write.
"The most likely mechanism is thermal injury, which may impair mucosal integrity and promote carcinogenic processes."
The findings are reported in the International Journal of Cancer.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.