Everybody knows that the average human body temperature is 37°C, but everybody is wrong. It turns out the bodies of people in the US have been cooling since the 1860s.
Physicians who have studied body temperature have known for decades that 37°C was too high, says Julie Parsonnet of Stanford University in California. “But they’ve always thought that it was just measurement error in the past, not because temperature had actually dropped.”
To find out what really happened, Parsonnet and her team combined three datasets. The first covered 23,710 Union Army veterans from the American Civil War, whose temperatures were measured between 1860 and 1940. “It took me a long time to find a database back to the 19th century that had temperature in it,” says Parsonnet. The other datasets spanned 1971-1975 and 2007-2017. In total, the team analysed 677,423 temperature measurements.
On average, American body temperature has declined by 0.03°C per decade. Men born in the early 19th century had body temperatures 0.59°C higher than men today. The data for women does not go as far back, but their body temperature has dropped 0.32°C since the 1890s. That means average body temperature today is about 36.6°C, not 37°C as widely thought
Parsonnet offers two pieces of evidence that the fall is real, and not simply because older thermometers were unreliable.
First, the cooling trend is visible within the more modern datasets, which are presumably more reliable. “The decline we saw from the 1860s to 1960s, we see the same decline from the 1960s to today,” says Parsonnet. “I don’t think there’s much difference in the thermometers between the 1960s and today.”
And second, if the measurements were wrong, we would expect the temperatures to be determined by when the measurements were taken, as thermometer technology changed over time.
That wasn’t the case, but grouping measurements by subjects’ dates of birth did line up with the changing temperatures. Older people showed higher temperatures than young people when measured at the same time. That means changing thermometers cannot account for the effect.
“The most likely explanation in my view is that microbiologically we’re very different people than we were,” says Parsonnet. Modern people have fewer infections, thanks to vaccines and antibiotics, so our immune systems are less active and our body tissues less inflamed. If that’s true, body temperatures should have fallen in other countries where people’s health has improved.
The cooling trend shows no sign of stopping, says Parsonnet. “There is going to be a limit, we’re not going to get down to zero,” she says. “But I just don’t know where that is.”
Journal reference: eLife, DOI: 10.7554/eLife.49555