While the normal human body temperature has long been considered to be 37 degrees Celsius, scientists have found evidence that it may go down to even about 36.5 degrees Celsius, based on a long-term study of Bolivian forager-farmers.
According to the study, published in the journal Science Advances, the Tsimane — an indigenous population of forager-horticulturists in the Bolivian Amazon – exhibited higher body temperature (BT) when first examined in the early 21st century which subsequently declined by nearly 0.05 units annually over 16 years of socioeconomic change to about 36.5 degrees Celsius by 2018.
“In less than two decades we’re seeing about the same level of decline as that observed in the US over approximately two centuries,” said Michael Gurven, a co-author of the study from the University of California Santa Barbara in the US.
In the research, the scientists assessed a large sample of 18,000 observations of almost 5,500 adults and adjusted for multiple other factors that might affect body temperatures, such as ambient temperature and body mass.
While previous research, such as ones conducted in Palo Alto in the US and among 35,000 adults in the UK had found the average body temperature to be lower than observed in the previous century, the scientists said these studies could not explain the reason for the decline.
“The provocative study showing declines in normal body temperature in the US since the time of the Civil War was conducted in a single population and couldn’t explain why the decline happened,” said Gurven.
“One leading hypothesis is that we’ve experienced fewer infections over time due to improved hygiene, clean water, vaccinations and medical treatment. In our study, we were able to test that idea directly. We have information on clinical diagnoses and biomarkers of infection and inflammation at the time each patient was seen,” he added.
While some infections were associated with higher body temperature, Gruven said adjusting for these did not account for the steep decline in body temperature over time.
“And we used the same type of thermometer for most of the study, so it’s not due to changes in instrumentation,” he said.
The scientists believe the declines might be due to the rise of modern health care and lower rates of lingering mild infections now as compared to the past.
“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,” Gruven said.
“It could be that people are in better condition, so their bodies might be working less to fight infection,” he added.
Another possibility for the BT decline could be due to greater access to antibiotics and other treatments which reduce the duration of infection than in the past.
“We found that having a respiratory infection in the early period of the study led to having a higher body temperature than having the same respiratory infection more recently,” Gurven said.
The use of anti-inflammatory drugs like ibuprofen may also be acting to reduce BT, according to the study, but the researchers added that the temporal decline in body temperature remained even after their analyses accounted for biomarkers of inflammation.
“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,” said Thomas Kraft, another co-author of the study.
“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,” he added.
The scientists believe no single “magic bullet” could explain the decline in body temperature.
“It’s likely a combination of factors — all pointing to improved conditions,” Gurven said.
But the researchers added that there is no universal ‘normal’ body temperature for everyone at all times.
“I doubt our findings will affect how clinicians use body temperature readings in practice” said Gurven.
Despite the fixation on 37 degrees Celsius, the researchers said most clinicians recognise that ‘normal’ temperatures have a range.
But by linking improvements in the broader epidemiological and socioeconomic landscape to changes in body temperature, the scientists believe information on body temperature might provide clues to a population’s overall health, as do other common indicators such as life expectancy.