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Body temperature declining over time

Date: 01 November 2020 Tags: Miscellaneous


For several years now, doctors and researchers have known that 98.6°F is not really the gold-standard “normal” body temperature it was once considered to be. Studies in the US and Europe have found average body temperatures declining over time.



Indeed, body temperatures have declined in an indigenous rural population in Bolivia, a 16-year study has fund. The study also looks at possible reasons that may have caused this decline among people in general.



  • The German doctor Carl Reinhold August Wunderlich, took over a million measurements of 25,000 patients, and published his findings in a book in 1868, in which he concluded that the average human body temperature is 98.6°F.

  • In recent years, however, different studies have found the human body temperature averaging out differently, including at 97.7°, 97.9°, and 98.2°F.

  • One of the largest such studies, published last year, found that body temperatures among Americans have been declining over the last two centuries.

  • In previous studies, the reasons for declining body temperatures were not clear, nor was it known whether a temperature below 98.6°F is “normal” outside of high-income countries.

  • The new study made 18,000 observations of body temperature in 5,500 individuals among the Tsimane, an indigenous population in the Bolivian Amazon.

  • Greater exposure to infection can lead to higher inflammation, which is turn can lead to a higher body temperature.

  • The study found, average body temperatures among the Tsimane have fallen by 0.09°F per year; they average roughly 97.7°F today. This decline in less than two decades, was about the same as that observed in the US over two centuries.

  • The study looked at a number of hypotheses about factors that may be causing the decline of body temperature among people in general, and tested these against their findings among the Tsimane.

  • One hypothesis is that improved hygiene and healthcare in high-income population groups have led to fewer infections over time and, in turn, to lower body temperature. 

  • People use anti-inflammatory drugs such as ibuprofen more frequently than earlier. Again, even after accounting for biomarkers of inflammation, body temperature declines over time remained among the Tsimane.

  • Another hypothesis is that people are healthier, so their bodies might be working less to fight infection. 

  • Together, the findings underline that there is no single cause that could explain the decline. The researchers said it’s likely a combination of factors, all pointing to improved conditions.

  • Among its limitations, the study used the same type of thermometer, but not the same thermometer over the entire 16 years. In the earliest study years, the sample size was smaller.

  • The study did not account for pregnancy or lactation, or the time of day when body temperatures were recorded.

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