For several years now, doctors and researchers have known that 98.6°F is not really the gold-standard “normal” body temperature it was once considered to be. Studies in the US and Europe have found average body temperatures declining over time. But does this trend also hold good outside of high-income countries?
Indeed, body temperatures have declined in an indigenous rural population in Bolivia, a 16-year study has fund. Published in Science Advances, the study also looks at possible reasons that may have caused this decline among people in general.
The German doctor Carl Reinhold August Wunderlich, who in 1851 pioneered the use of the clinical thermometer, took over a million measurements of 25,000 patients, and published his findings in a book in 1868, in which he concluded that the average human body temperature is 98.6°F.
In recent years, however, different studies have found the human body temperature averaging out differently, including at 97.7°, 97.9° and 98.2°F. One of the largest such studies, published last year, found that body temperatures among Americans have been declining over the last two centuries.
Also Read: Explained: Why 98.6°F is no longer ‘normal’ for the body
So, what does the new study add?
In previous studies, the reasons for declining body temperatures were not clear, nor was it known whether a temperature below 98.6°F is “normal” outside of high-income countries. The new study made 18,000 observations of body temperature in 5,500 individuals among the Tsimane, an indigenous population in the Bolivian Amazon.
“The Tsimane are indigenous forager-horticulturalists who inhabit a tropical environment rife with diverse pathogens — from familiar ones like a cold or pneumonia, to less familiar, like hookworm and tuberculosis,” lead author Michael Gurven, an anthropologist at University of California, Santa Barbara, said by email.
Greater exposure to infection can lead to higher inflammation, which is turn can lead to a higher body temperature. “From earlier studies, we also know that Tsimane experience higher inflammation due to this high infectious burden. And so we expected to find that body temperatures would be higher among Tsimane than they are in the US, UK and Germany,” Gurven said.
Instead, the study found, average body temperatures among the Tsimane have fallen by 0.09°F per year; they average roughly 97.7°F today. This decline in less than two decades, the researchers noted, was about the same as that observed in the US over two centuries. 📣 Express Explained is now on Telegram
What could be the reasons for this?
The study looked at a number of hypotheses about factors that may be causing the decline of body temperature among people in general, and tested these against their findings among the Tsimane.
BETTER HEALTHCARE: One hypothesis is that improved hygiene and healthcare in high-income population groups have led to fewer infections over time and, in turn, to lower body temperature. While the Tsimane live a rural lifestyle with a relatively low access to healthcare, they do have better access than they did two decades ago.
Indeed, some infections were found to be associated with higher body temperature. But when the statistical model adjusted the temperature findings for infection, it found that reduced infection alone could not explain the declines. “This is to say that the decline in body temperature over the duration of the study is not altered by considering patient characteristics, including their medical diagnoses,” Gurven said.
LOWER INFLAMMATION: People use anti-inflammatory drugs such as ibuprofen more frequently than earlier. Again, even after accounting for biomarkers of inflammation, body temperature declines over time remained among the Tsimane.
BRIEFER ILLNESS: Since people have greater access to treatment, has it reduced the duration of infection? That was another hypothesis the study tested. The findings among the Tsimane, indeed, were consistent with this argument. If a study participant had a respiratory infection in the early stages of the 2002-18 study, it led to having a higher body temperature than the temperature if they had the same infection more recently.
BODIES WORKING LESS: Another hypothesis is that people are healthier, so their bodies might be working less to fight infection. Also, our bodies may not have to work as hard as before in order to regulate internal temperature, because of air-conditioning and winter heating. The Tsimane do not use such advanced technology, but do have more access to clothes and blankets.
So, what are the implications?
Together, the findings underline that there is no single cause that could explain the decline. The researchers said it’s likely a combination of factors — all pointing to improved conditions.
The researchers do not expect their findings to influence how doctors use body temperature readings in practice. Doctors already acknowledge there is no universal ‘normal’ body temperature for all people at all times.
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Among its limitations, the study used the same type of thermometer, but not the same thermometer over the entire 16 years. In the earliest study years, the sample size was smaller. The study did not account for pregnancy or lactation, or the time of day when body temperatures were recorded.
In recent years, different studies have found the human body temperature averaging out differently, including at 97.7°, 97.9° and 98.2°F. Image: leo2014/Pixabay 
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