Normal human body temperature has declined over the past two centuries in the United States. A trend that coincides with better access to health care, which implies a reduction in infections. But can we really establish a link? To find out, researchers looked at the temperatures of Bolivian grower-gatherers.
The so-called "normal" value of 37°C was first established by the German physician Carl Reinhold August Wunderlich in 1851, after studying the cases of 25,000 people at Leipzig. In fact, we know that there really isn't a single "normal" body temperature. This varies throughout the day. It can drop in the morning, and with age, or increase in the afternoon, when we are sick, or during and after exercise. It also varies during the menstrual cycle, and between individuals.
The fact remains that if this body temperature remains almost constant, it is also capable of changing.
In a study published a few months ago in eLife , researchers at Stanford University explored trends in the US population over several decades. They found that the body temperature of men born in the 2000s was on average 0.59°C lower compared to those born in the early 1800s. This represented a constant drop of 0.03°C per decade . The drop was similar in women with a 0.32°C drop per decade since the 1890s.
With the first recorded temperatures in the 1850s, researchers naturally wondered if this decrease might reflect improvements in thermometer technology. "In the 19th century, thermometry was just beginning" , indeed recalls Julie Parsonnet, main author of the study.
To find out, the researchers then checked for body temperature trends in each data set, assuming that in each of these groups, the measurements had normally been taken with similar thermometers. Nevertheless, again they found a decrease in temperature over time.
For researchers, therefore, the human body has cooled well among Americans for at least 150 years. The question is:why?
One of the main assumptions points to improved hygiene, sanitation and medical treatment. The result:Americans today suffer from fewer infections that can trigger higher body temperatures. But is this really the only explanation? To test this idea, researchers from the Department of Anthropology at UC Santa Barbara (California) analyzed the body temperatures of Tsimanes horticulturists-gatherers, originally from the Bolivian Amazon.
“The Tsimani live in a remote area with limited access to modern amenities. And we know from experience that infections are common here – from colds to intestinal worms to tuberculosis” , explains Michael Gurven, lead author of this study.
By analyzing the case reports of thousands of patients (about 18,000 observations on nearly 5,500 adults), recorded over the past two decades, the researchers found that the body temperatures of the Tsimanes in 2002 were similar to those observed in Germany and the United States two centuries ago, averaging 37°C . However, over a relatively short period of sixteen years, they then observed a rapid drop in their average temperature (almost -0.018°C per year). Today it is around 36.5°C.
In other words, in less than two decades, researchers have seen about the same level of decline among the Tsimani as seen in the United States over about two centuries.
So why has body temperature dropped over time for both Americans and Gypsies? The researchers point out that the latter have, in previous years, also benefited from better health care, which has had the effect of reducing the risk of infections. For Michael Gurven, on the other hand, the reduced incidence of the infection cannot on its own explain the drops in body temperature observed.
“Another possible explanation is that bodies no longer need to work as much to regulate internal body temperature because of air conditioners in summer and heaters in winter , advances the researcher. The Tsimani do not use any of these technologies to regulate their body temperature, however, they have more access to clothing and blankets than before” .
These are of course only guesses. Understanding why body temperature decreases over time therefore remains an open question.
