Updated:01/15/2020 09: 51h
The medical books say it and everyone knows it: the average human body temperature It is 37 degrees. Now, a team of researchers from Stanford University, in California, has just shown that this is not true. And that in fact, at least in the United States, the average body temperature is almost half a degree colder than it was 150 years ago.
In the words of Julie Parsonnet, one of the authors of the study recently published in the eLife magazine, doctors who study body temperature have known for decades that 37 degrees is too high, "but they had always thought it was a mistake. of measurement in the past and not of a real drop in temperature in humans. "
To find out the truth about the issue, Parsonnet and his team decided to combine three sets of data. The first covers 23,710 veterans of the Army of the United States Civil War, whose temperatures were measured first in 1860 and then in 1940. "It took me a long time – Parsonnet says – to find a 19th-century database that included temperatures." . The other two datasets range from 1971 to 1975 and from 2007 to 2017. In total, the researchers analyzed 677,423 different temperature measurements.
0.03º per decade
The result was that the body temperature of Americans has been slowly but steadily decreasing, at a rate of 0.03 degrees per decade. For example, men born in the nineteenth century had a body temperature of 0.59 degrees higher than current men. As for women, the data does not go back as far back, but their body temperature has also dropped 0.32 degrees since the 1890s. This means that, at present, the average temperature of the human body is 36, 6 degrees, and not 37 degrees as recorded in all manuals.
In his study, Parsonnet provides two evidence that the decline is real and not simply the result that old thermometers were unreliable. First, the cooling trend is evident in the most modern data sets, in which the thermometers were presumably more reliable. "We observe the same decrease between the 1960s and the current situation between 1860 and 1960," Parsonnet says. "And I don't think there is much difference between the 1960 and current thermometers."
Second, the researchers found that older people had higher body temperatures than younger people measured in the same year, regardless of what that year was. If the old thermometers had been less accurate, variations in that relationship would be appreciated.
Why are we getting cold?
Once ruled out that the "cooling" of the human body in the last 150 years is a simple illusion attributable to measurement biases, the researchers tried to find out why our bodies are getting colder. "In my opinion – Parsonnet explains – the most likely reason is that, from the point of view of microbiology, we are very different from what we were." In fact, people today suffer fewer infections thanks to vaccines and antibiotics, so that Our immune system is less active and our body tissues less inflamed.
Is that the real reason? The truth is that, with the available data, we cannot be sure. The next step will be to check if body temperature has also decreased in other countries where, as in the US, people's health has improved in the last century.
Another intriguing issue is that it is unknown to what extent the human body will continue to cool in the future, although researchers point out that the tendency to cool shows no signs that indicate it will stop soon. "There will be a limit," Parsonnet concludes, "of course we will not reach zero … Although for now I don't know where that limit is."