Over 200 years, human temperature has dropped significantly – from 37°C to 36.4°C and among the Tsimane people in Bolivia, this happened in just 20 years.

A team of scientists at the University of California, led by Michael Gurven, have found that the human body temperature is gradually decreasing in populations around the world. For what reason this is happening is still a question for experts, writes Sciences Advances.

German physician Karl Wunderlich about 200 years ago found that the normal temperature of the human body is 37 degrees C. However, two centuries later this figure has changed.

In the UK, for example, three years ago, the average body temperature of a healthy person was 36.6 degrees Celsius, and in 2019 in the Californian city of Palo Alto – only 36.4 degrees Celsius.

In the 16 years since Gurven, co-director of the T’simanes Health and Life History Project, and his colleagues have been studying the population, they have observed a rapid decline in mean body temperature of 0.05 ° C per year.

Their analysis is based on a large sample of 18,000 observations from nearly 5,500 adults and is adjusted for multiple other factors that could affect body temperatures, such as room temperature and body mass.

The anthropologist assures that something in human physiology could have changed.

According to him, one of the main hypotheses is the lower number of infections due to improved hygiene, clean water, vaccinations and medical treatment. 

The researchers tested this idea in practice, as they had information on clinical diagnoses and biomarkers of infection and inflammation at the time each patient was seen.

“Although some infections were associated with higher body temperature, the adjustment for these did not account for the steep decline in body temperature over time,” Gurven said. 

“We used the same type of thermometer for most of the study, so it is not due to changes in instrumentation,” he added.

“No matter how we did the analysis, the decline was still there. Even when we restricted the analysis to 10% of adults who were diagnosed by doctors as completely healthy, we still saw the same drop in body temperature over time,” says Thomas Kraft, who was also involved in the research. 

The drop in body temperature could be due to the increase in modern medical care and lower rates of persistent mild infections today compared to the past. However, infections are still widespread in rural Bolivia.

“The results suggest that the reduction in infection alone cannot explain the observed drops in body temperature,” he said.

Another hypothesis consists of the best conditions in which people live today. Greater access to antibiotics and other treatments means that the duration of infections is shorter now than in the past. The bodies may be working less to fight infection, Gurven explained.

“We found that having a respiratory infection in the initial period of the study led to a higher body temperature than having the same respiratory infection in more recent times,” he noted.

In general, scientists cannot definitively explain the drop in body temperature. “It is likely to be a combination of factors, all pointing to improved conditions,” Gurven said.

As a vital sign, temperature is an indicator of what is happening physiologically in the body, much like a metabolic thermostat. 

“One thing we’ve known for a long time is that there is no universal normal body temperature for everyone at all times, so I doubt that our findings affect the way physicians use body temperature readings in practice.” Gurven clarified.