It has been nearly two centuries since 37°C was established as the normal body temperature by the German physician Carl Wunderlich. This standard has been employed by doctors and researchers alike as the measure by which the severity of fevers and other illnesses are assessed. More recently however, lower body temperatures have been commonly reported in healthy adults.
The level of decline in the Bolivian Amazon in under two decades had been equivalent to that observed in the United States over approximately two centuries
In 2017, a study in the United Kingdom involving 35 000 healthy adults found the average body temperature to be approximately 36.6°C. Another study in 2019 found the normal body temperature of Americans in Palo Alto, California, to be about 36.4°C.
A research team led by UC Santa Barbara Professor, Michael Gurven, found a similar curtailed body temperature of 36.5°C among the Tsimane (an indigenous population within the Bolivian Amazon), with a rapid average decline of about 0.05°C per year. Gurven also stated that the level of decline in under two decades had been equivalent to that observed in the United States over approximately two centuries.
So, why could this be happening?
The initial leading hypothesis was in relation to humans experiencing fewer infections over time due to medical treatment, vaccinations and improved hygiene.
Accounting for infections did not rationalise the steep decline in average body temperature
This theory was tested directly as the team collected information regarding clinical diagnoses and biomarkers of infection or inflammation from each patient. Although some infections were linked to higher body temperatures, accounting for these did not rationalise the steep decline in average body temperature observed.
Temperature was still declining in tropical environments such as that of the Tsimane, where infections still contribute to a large proportion of morbidity and mortality in the population. The question still remained, why was body temperature rapidly falling?
“It’s likely a combination of factors – all pointing to improved conditions,” Gurven stated.
Body temperature is often an indicator of the underlying physiological processes occurring within the body, almost acting as a ‘metabolic thermostat’. It is possible that due to people being in better condition, their bodies might be working to a lesser extent to fight infections and inflammation, reducing spikes in temperature.
Thermoregulation of the body has become easier due to common use of heating and cooling appliances
A greater use of anti-inflammatory medication such as ibuprofen could also contribute to this. Another possibility could be that thermoregulation of the body has become easier due to common use of heating and cooling appliances, implying that the body does not have to work as hard to maintain a homeostatic body temperature by itself.
Professor Gurven emphasizes that there is no universal ‘normal’ body temperature that is consistent with everyone at all times. The study does, however, highlight the importance of including body temperature as a measure in broader epidemiological and socioeconomic studies to help provide insight to a population’s overall health.
Featured Image: Kelly Sikkema on Unsplash