Even in the Bolivian Amazon, Average Human Body Temperature Is Getting Cooler

A new study finds the average body temperature among Bolivia’s Tsimane people dropped by nearly a full degree in just 16 years

Thermometer on red background
Most people will tell you that the average temperature for the human body is 98.6 degrees. But a growing body of research is challenging that idea, suggesting peoples' bodies now run a bit cooler on average. Arizona State University / Unsplash

If you’ve ever taken your temperature and wondered why your body wasn’t hovering at the supposedly normal 98.6 degrees Fahrenheit, a new study offers the latest in a growing body of evidence suggesting that oft-repeated figure might no longer be the norm.

Published last month in the journal Science Advances, the study finds the average body temperature among the Tsimane people, who live in the Bolivian Amazon rainforest, has dropped by almost a full degree over the last 16 years.

The dogma of 98.6 degrees Fahrenheitstarted in 1867 when a German doctor named Carl Wunderlich took the temperature of some 25,000 people in Leipzig and arrived at the figure. But several recent studies have suggested that people have cooled off over the last 150 years.

A study published earlier this year compiled hundreds of thousands of temperature readings in Palo Alto, California, and found the average body temperature among the study participants was around 97.5 degrees, reports Sujata Gupta for Science News. In 2017, yet another study of 35,000 healthy adults in the United Kingdom found the average body temperature was 97.9 degrees.

One of the main hypotheses to explain this drop in average body temperature is that improvements in hygiene and medical treatment might have reduced the number of infections experienced by the general population and thus also cut down on fevers which might bend the average higher, write Michael Gurven and Thomas Kraft, University of California, Santa Barbara anthropologists and co-authors of the new research, in the Conversation.

Gurven and Kraft designed their study to probe this idea by focusing on the Tsimane, who experience frequent infections and have limited access to modern medicine and other amenities. The Tsimane Health and Life History Project, which started in 2002, sends Bolivian physicians and researchers to Tsimane villages to treat patients and record health data. This database allowed Gurven and Kraft to control for underlying health conditions and other medical factors that could influence the data, they write in the Conversation.

After 17,958 temperature measurements across 5,481 Tsimane adults and teenagers between 2002 and 2018, the researchers saw a startlingly fast drop in average body temperature—0.9 degrees in less than two decades.

When the researchers started to dig into what might be behind this finding, they found little evidence to support the ideas that ambient air temperature or health—incidence of infections or other ailments—are behind the cooler body temperatures. “No matter how we did the analysis, the decline was still there,” says Kraft in a statement. “Even when we restricted analysis to the <10% of adults who were diagnosed by physicians as completely healthy, we still observed the same decline in body temperature over time.”

However, according to Science News, Gurven and Kraft still suspect that increased access to medications such as painkillers and antibiotics, or better nutrition might be driving the phenomenon.

“One thing we’ve known for a while is that there is no universal ‘normal’ body temperature for everyone at all times, so I doubt our findings will affect how clinicians use body temperature readings in practice,” says Gurven in the statement. But keeping a closer eye on average human body temperature might still provide important information about the health of the general population. “Body temperature is simple to measure, and so could easily be added to routine large-scale surveys that monitor population health.”

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