- In 1868, German Physician, Carl Wunderlich established the average normal body temperature in humans as 37°C or 98.6°F.
- Over the years, scientists have noticed a steady decline in the average.
- A recently published study in the journal Science Advances found a rapidly declining body temperature among the indigenous population in the Bolivian Amazon.
Since its discovery in the 19th century, physicians have used the average human body temperature for assessing fevers and severity of illnesses among patients. However, over the past two centuries scientists have noticed a strange drop in the body temperature of healthy adults.
In 2017, British researchers conducted a study among 35,000 adults and found that the average body temperature was 36.6°C (97.8°F), much lower than the normal. Later on, in early 2019, a study conducted by American researchers discovered the average temperature among Americans had dropped by 0.02°C (0.05°F) since the 19th century.
Now, a study led by researchers at UC Santa Barbara has found a similar decline among the Tsimane, an indigenous population in the Bolivian Amazon.
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
Michael Gurven, professor of anthropology at UC Santa Barabara
A Rapid Decline in the Average Body Temperature
For their study, the researchers looked at medical records of over 5,000 adults. They adjusted for factors that can affect body temperatures such as body mass, infections, and ambient temperatures. Over 18,000 observations were analyzed between 2002 and 2018.
The analysis found a rapid decline of roughly 0.05°C (0.09°F) per year. This has now resulted in the Tsimane having an average body temperature of 36.5°C (97.7°F).
No matter how we did the analysis, the decline was still there. 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.
Thomas Kraft, study author
So, what’s causing the drop?
According to lead researcher Michael Gurven, the decline could be as a result of an overall improvement in healthcare and lower rates of infections as compared to the past. However, infections are still common among the rural population of Bolivia. Therefore, reduced infection rates cannot be held responsible for the observed decline.
There is no single explanation for the decline. The authors of the study believe it could be due to a combination of factors.
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
Michael Gurven, professor of anthropology at UC Santa Barabara
The team of researchers suggests that body temperature can serve as an indicator of a population’s overall health, similar to common indicators such as life expectancy.
References:
Gurven, Michael, et al. “Rapidly Declining Body Temperature in a Tropical Human Population.” Science Advances, vol. 6, no. 44, 2020, doi:10.1126/sciadv.abc6599.
Obermeyer Ziad, Samra Jasmeet K, Mullainathan Sendhil. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ 2017; 359 :j5468. doi: https://doi.org/10.1136/bmj.j5468
Protsiv, Myroslava, et al. “Decreasing Human Body Temperature in the United States since the Industrial Revolution.” ELife, vol. 9, 2020, doi:10.7554/elife.49555.
