“We are cooling down”: the study conducted by Julie Parsonnet, a doctor specializing in infectious diseases at Stanford University, is formal.
An interval of 1.5°C
Today, 36.5°C should be considered the “norm” for a healthy human body, she says – and that any temperature between 35.7 and 37.3°C should be considered normalsay the researchers.
To function properly, the body must indeed remain within a temperature difference of approximately 1.5°C. Outside, neurons, organs, muscles work less well. The body therefore expends a lot of energy to stay at this temperature level.
All of this is done through signals emitted by the hypothalamus, which warn us that our blood is not at the right temperature.
Julie Parsonnet, researcher at Stanford
Many factors have an influence: age, morphology, activity, diet, time and method of measurement, etc. An infection, of course, can also cause the temperature to fluctuate, to help the immune system fight off pathogens.
Initially, Julie Parsonnet worked on the weight gain of Americans. By studying 200,000 measurements spread over 160 years, she found that the body temperature of an “average” American had dropped by 0.5°C (or 1° Fahrenheit) in the meantime. Data which largely justifies, according to her, the need to reassess the level considered “normal” of this temperature:
We got taller, fatter, colder and [nous vivons] Longer. These four elements go together, in a way
A thermometer error in question?
Confronted with the results of Julie Parsonnet, Philip Mackowiak defends this hypothesis. Professor emeritus at the University of Medicine of Maryland, he thinks that it is… the starting reference that is inaccurate: the famous “37°C” rises to an 1871 book, published by German physician Carl Rinhold August Wunderlich. Which would have averaged a million temperatures accumulated over his years of practice in a clinic.
Mackowiak points to two pitfalls: the fact that he was able to achieve an average, at the time, on such a large ensemble, without a computer. And the weakness of the mercury thermometers of the time. Pr Mackowiak examined the instrument used, exhibited in Philadelphia:
[Il] was calibrated 1.5 degrees [centigrade] more than modern or contemporary thermometers
What invalidate the theory of Julie Parsonnet? Refusing to decide, he says he is “not convinced” by his work, but does not close the door – and also contests the 37°C as a reference :
I have no way of being sure, one way or the other. But my intuition is no. [la température corporelle humaine] has not diminished over time
Other researchers, not necessarily convinced at the start, joined the Stanford scientist. Like University of California anthropologist Michael Gurven, who reanalyzed all the data. And succeeded to the same result:
We don’t quite understand why exactly, but it does seem that there has been a decline
He wanted to check whether the evolution of lifestyles could play a role. Working on the people of Chimanes, a population of Bolivia quite isolated from the outside world, he also noted a drop from 37 to 36.5… but in only 20 years! Without being able to explain it to this day – whether for this people or in general.
Among the possible causes, air conditioning, diet, chronic diseases, parasites, sleep habits, medication… The simple fact of better access to the health system or to certain basic goods, such as blankets, has could play.
Should we worry at 37°C?
The researchers insist on the need to evolve with regard to the immutable reference of 37°C, at least in the medical field. However, they remind us: at 36.5°C or 37°C, there is no fever.
37°C is a normal temperature, but it is not “the” normal temperature
Today, in the hospital, we generally qualify as a fever above 38.3°C (101°F). But this is never the only symptom observed by the nursing staff:
Fever is only one of the indications of the disease. But the fact is, if you feel sick, then you’re sick, no matter what your temperature.