In the mid-19th century, German physician Carl Reinhold August Wunderlich set about establishing the average temperature of the human body. He believed (correctly) that fever is a symptom of a disease, not a disease in itself, and introduced temperature charts at the general hospital at Tübingen, of which he was head.
Building on the work of French doctors that showed inflamed parts of the body have a higher temperature than the rest, and that the average human body temperature was 36.9°C (98.5°F), he set out to prove and refine it. Using a foot-long thermometer (you'll be relieved to know it went under the armpit), he measured the temperature of over 25,000 patients, taking more than a million readings. In 1868 he published his findings, having established that the average normal human body temperature was 37°C (98.6°F).
That knowledge has been refined over time. We know temperatures vary throughout the day, being lower in the morning and peaking at around 6pm, and that there are differences in temperatures between men and women.
However, since then scientists have noticed something odd. The average body temperature appears to be dropping. A 2017 study that looked at over 250,000 temperature measurements of 35,000 British patients found that the mean oral temperature was 36.6°C (97.8°F). A study published earlier this year showed that the average body temperature in America has been steadily dropping about 0.02°C (0.05°F) a decade since the 1860s.
A new study published in Science Advances has found something stranger still. Looking at the Tsimane people, an indigenous population of forager-horticulturists in the Bolivian Amazon known for their excellent health, researchers found that their average temperature has been declining rapidly, roughly 0.05°C (0.09°F) per year since they began measuring it as part of the Tsimane Health and Life History Project in 2001.
Looking at medical records of 5,500 people between 2002 and 2018, the researchers analyzed 18,000 observations, adjusting for factors that might affect body temperature, including ambient temperature, infections, and body mass. In less than two decades, the Tsimane had seen the same level of temperature drop that took place in the US over two centuries.
"No matter how we did the analysis, the decline was still there," Thomas Kraft from UC Santa Barbara said in a statement. "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.”
The average Tsimane body temperature is now 36.5°C (97.7°F). So what is going on?
“Declines might be due to the rise of modern health care and lower rates of lingering mild infections now as compared to the past,” Michael Gurven, professor of anthropology at UC Santa Barabara, explained. “But while health has generally improved over the past two decades, infections are still widespread in rural Bolivia. Our results suggest that reduced infection alone can’t explain the observed body temperature declines.”
The team suggests that the change could be because people are in better condition, so their bodies work less to fight off infections, or that access to antibiotics and other medical treatments such as anti-inflammatory drugs means that infections last for less time than in the past, adding that respiratory infections caused a bigger increase in temperature during the early years of their study than recent years.
However, the team found no magic bullet that explained the drop, instead believing it could be a combination of factors, from better access to healthcare to an improvement in living standards.
“Another possibility is that our bodies don’t have to work as hard to regulate internal temperature because of air conditioning in the summer and heating in the winter,” Kraft said. “While Tsimane body temperatures do change with time of year and weather patterns, the Tsimane still do not use any advanced technology for helping to regulate their body temperature. They do, however, have more access to clothes and blankets.”
Though it's difficult to pin down the cause of changes in body temperature, the team believe that measuring the average temperature (and change thereof) of entire populations could be used as an indicator of health in the overall population, in the same way life expectancy is.