What is a 'Normal' Body Temperature?

New research from the University of California - Santa Barbara has challenged the long established ‘normal’ body temperature of 37.0 degrees Celsius and suggests a lower figure could be healthier.

Pioneered by a team of physicians and anthropologists, the 16-year-long study analysed the body temperatures of indigenous Tsimane people living in the Bolivia’s Amazon region. Over the study period the team noticed a significant drop in the average body temperatures of healthy Tsimane adults, with the average now 36.5 degrees Celsius.

"In less than two decades we're seeing about the same level of decline as that observed in the U.S. over approximately two centuries," says Michael Gurven, a professor of anthropology at the University of California - Santa Barbara.

A shift in human physiology

The study was published in the journal Sciences Advances, with Gurven theorising the drop could be caused by a change in human physiology. "The provocative study showing declines in normal body temperature in the US since the time of the Civil War was conducted in a single population and couldn't explain why the decline happened," says Gurven. "But it was clear that something about human physiology could have changed. One leading hypothesis is that we've experienced fewer infections over time due to improved hygiene, clean water, vaccinations and medical treatment. In our study, we were able to test that idea directly. We have information on clinical diagnoses and biomarkers of infection and inflammation at the time each patient was seen.”

“Improved conditions” ease physiological stress

The study builds on a growing body of research suggesting average body temperatures are on the decline around the world, including in the United Kingdom and United States. In 2017 a British study involving more than 35,000 adults suggested the average body temperature was just over 36.6 degrees Celsius. A second study conducted in California recorded averages of around 36.4 degrees Celsius.

For Gurven, factors like lower rates of infectious diseases could explain why average body temperatures have dropped in both remote Amazonian regions and global cities. Even the Tsimane people are enjoying what Gurven refers to as “improved conditions” and have better access to antibiotics and simple comforts such as blankets and warm clothing. This could reduce pressure on the body to regulate internal temperature and result in an overall decline.

While the findings are interesting, Gurven admits they are unlikely to affect the standardised 37 degrees Celsius used to detect fevers and irregularities. "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.