Our average body temperature has dropped over the last two centuries – scientists wanted to know why

  • The average temperature for healthy persons used to be around 37°C
  • Over the last two centuries, it seems there has been a drop in temperature
  • Researchers are puzzled as to why this may be the case

In 1868, German physician Carl Wunderlich reported that a healthy body temperature reading was 37°C. Now, almost two centuries later, researchers have noticed that the average body temperature in healthy individuals is declining. 

According to a paper compiled by researchers from Stanford University, temperatures of adults in the United States has decreased 0.03°C per decade, which led the authors to believe that people in higher-income countries now have an average body temperature that is 1.6°C lower than in 1868. Researchers attributed this drop in body temperature to reductions in infectious diseases.

Lower temperatures in high-income countries

A more recent study conducted by a group of physicians, anthropologists and local researchers found similar results when studying the Tsimane Amerindians of the Bolivian Amazon.

The existing hypothesis – that people have lower temperatures in high-income countries – led to researchers further hypothesizing that people in lower-income areas would have higher temperatures. The researchers, led by Michael Gurven (professor of anthropology at University of California Santa Barbara), wanted to test the hypothesis in this population.

Why choose the Tsimane?

The Tsimane are indigenous people of lowland Bolivia who have a minimalistic lifestyle, without access to running water and sanitation.

Researchers found that the Tsimane were exposed to many disease-causing agents, "indicated by elevated immune activation biomarkers throughout life".

The majority of deaths among the Tsimane was due to infection, and they also exhibit endemic polyparasitism (natural infestation by two or more parasites).

They also have a high resting metabolism, which means when they are not moving their bodies they still burn a high number of calories, leading to an increase in temperature. These reasons further led researchers to believe that they would have higher body temperatures.

Predictions contradicted

Body temperatures collected by the Tsimane Health and Life History Project (THLHP) since 2002 were used in the study. Researchers found that contrary to their predictions, there has also been a rapid decrease in average body temperature in the Tsimane population.

According to researchers, during the earliest study period from 2002 to 2006, unadjusted mean BTs for adult women and men were 37.02°C. However, between 2012 to 2018, body temperature dropped by 0.45°C for women and 0.49°C for men. 

This drop in temperature happened over two decades, whereas the drop in body temperature in the US population was observed over almost two centuries. Surprisingly, researchers could not find the reasons behind this drop in temperature. 

Image credit: Unsplash

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