Image source: BBC/ Serenity Strull
150 years ago, it appears that the average human body temperature was higher than it is today. Scientists are still searching for the exact reason behind this.
In the 1860s, German physician Carl von Wunderlich faced a pressing question: Is fever a disease in itself, or is it a symptom of an underlying disease?
At a time when thermometers were not routinely used, Wunderlich embarked on an ambitious mission: Over the course of 15 years, he collected body temperature measurements (under the armpit) for 25,000 patients of both sexes of different ages, and the result was more than a million readings.
From this huge data set, Wunderlich derived an average temperature for healthy adults of 37 degrees Celsius. This number has become the reference standard for determining normal human body temperature. Wunderlich stated that a temperature exceeding 38 degrees Celsius is an indication of fever.
Philip McQuack, a medical historian and infectious disease expert at the University of Maryland, points out that although others have reached the same conclusion, the sheer magnitude of the data Wunderlich collected gave his number a “high status” to become a global diagnostic tool.
But more recent research suggests that the average normal body temperature has actually changed over the past century and a half.
Was the original standard wrong? Or is our rapidly changing world affecting our biology?
Low temperatures
Wunderlich "established much of what we know today as facts about body temperature variation," says Julie Parsonnet, an infectious diseases physician at Stanford University.
Most read end
An in-depth explanation of the most prominent events and topics, to help you understand the most important variables around you and their impact on your life
Episodes
End podcast
Through his data set - drawn mainly from working-class patients at the Leipzig Hospital where he was medical director - Wunderlich discovered that the normal temperature is not constant; He noticed that women's temperatures are higher than men's, that they decrease with age, and that they change depending on the time of day.
Moreover, studies have shown that the average human body temperature today is lower than it was 150 years ago; The temperature of men in the United States is currently about 0.59 degrees Celsius lower than it was in the early nineteenth century.
A decade ago, a research team led by Parsonette analyzed three large data sets, which included multiple temperature readings for about 190,000 people in the United States, collected between 1862 and 2017. “The new normal for adults appears to be closer to 97.5 degrees Fahrenheit (36.39 degrees Celsius),” Parsonette explains.
Numerous studies conducted before or after that time have confirmed Parsonet's findings.
This downward trend is not limited to those living in developed countries; Adrian Yaegi, a biological anthropologist at the University of Zurich in Switzerland, points out that the average body temperature of the Tsimane tribe - the semi-nomadic indigenous people who live in the Amazon rainforest - was hovering around 37 degrees Celsius in the early 2000s.
But by 2018, after two decades of rapid social and economic change, their average temperature had fallen to 36.5 degrees Celsius.
Why did average body temperatures decrease?
There are several possible explanations for the drop in average temperature we are seeing today.
For example, Parsonette believes that this is because the population enjoys better health in general; In Wunderlich's era, life expectancy was 38 years, but today that number has almost doubled.
“People did not live as healthy lives as we do today,” says Parsonette. “They often suffered from numerous health problems, such as tuberculosis, poor dental care, dysentery, rheumatic fever, and osteomyelitis.”
“When your immune system is trying to defend you against all these life-threatening threats, it consumes calories, and calories are synonymous with thermal energy,” Parsonnet adds.
Likewise, Yaigi points out that vaccinations and improved sanitation have helped improve the health of the Tsimane. It is also noted that using toilets and wearing shoes has become more common now compared to what was the case 20 years ago, which has led to a reduction in the incidence of hookworm infection that was transmitted as a result of a person walking barefoot on contaminated soil.
Moreover, controlling indoor temperatures has become easier; At least 88 percent of homes in the United States have air conditioning, while a similar percentage of homes in the United Kingdom rely on central heating systems.
“We spend most of our lives in an environment with an ideal climate, in what is known as the ‘thermal neutral zone,’ where the body does not have to make an effort to maintain its temperature,” Parsonette says. “If the body does not make an effort, it will not generate energy, and therefore it will not produce heat.”
Factors that determine the average body temperature
It must be noted here that there are four factors that basically determine the average human body temperature:
1. Age - the body temperature of older people decreases:
As we age, our metabolic rate slows and our body is less active, which leads to us expending less energy, explains Walid Javed, MD, an infectious diseases physician at West Virginia University.
2. Gender - Women's body temperatures tend to be higher compared to men.
Parsonnet points out that women's body temperatures - even apart from the fluctuations associated with the menstrual cycle - are higher, which is also observed in older women. She adds that scientists do not know the exact reason behind this; “We simply have different biological settings,” she says.
3. Time: Decrease in the early morning and rise during the evening.
“Body temperature follows a circadian rhythm (circadian rhythm),” explains Jonathan Hausman, MD, a rheumatologist at Boston Children's Hospital and Massachusetts General Hospital. A recent study he conducted observed a decrease in temperature between 3:00 and 5:00 in the morning, and a peak between 16:00 and 18:00 in the evening, which likely reflects increased physical activity during the day.
4. Height and weight: one lowers the temperature, the other raises it.
Parsonette explains that body temperature decreases as height increases; Because the larger body surface area loses more heat, while the temperature rises slightly as weight increases.
Was Wunderlich wrong?
Mackowiak suggests, however, that the lower average body temperature may have a completely different cause.
Mackowiak says that when he had a chance to examine one of Wunderlich's glass thermometers, he noticed that it was set (calibrated) to a temperature about 2 degrees Celsius higher than modern thermometers. This means that any readings recorded using these instruments were 2 degrees Celsius higher than they should have been.
Mackowiak explains that measuring temperatures at that time was a laborious process; The glass mercury thermometers, which are about a foot (about 30 centimeters) long, had to be calibrated against fixed points, such as the boiling point of water and the melting point of ice. Patients then had to remain motionless for 15 to 20 minutes; Because “those thermometers took a long time to record the temperature,” he said.
In other words, Wunderlich's numbers may have been inaccurate from the beginning.
Furthermore, Wunderlich was taking the temperature under the armpit, rather than orally—the most common method today—which makes comparing his measurements to modern measurements complicated.
However, Parsonette does not believe a flaw in the calibration of the devices is the reason for Wunderlich's high estimate.
When she and her team analyzed body temperature data collected from Union Army veterans between 1862 and 1930, they found that those born earlier actually had higher average temperatures than those born later.
“Because we compared people born in different years and were measured at the same time, and those born in the same year and measured at different times, we were able to separate the effect of the instrument used from the effect of age and year of birth,” she explains.
The Parsonette team's conviction was further strengthened when they examined data from two other recent groups, where they observed a similar decline. This data included temperatures recorded between 1971 and 2017, “using thermometers that were assumed to have undergone similar calibration.”
New temperature guide
Does all this mean it's time to reconsider the special significance attributed to 37°C (98.6°F)?
Some have already begun to do this, and there are two references that doctors widely rely on: They are the book Harrison's Principles of Internal Medicine and the electronic database UpToDate.
However, while having a reference number for your body temperature can be a useful guide, most experts caution against focusing too much on one specific value.
“It is important not to take these thresholds too seriously,” says Yayghi. “They do not represent sharp dividing lines; no one can claim that 36.99 degrees Celsius is radically different from 37.01 degrees Celsius.”
However, when temperatures get too low or too high – usually below 35°C or above 40°C in adults – it is an indicator that there may be something more serious going on inside the body.
*All content in this article is provided for general information purposes only and should not be considered a substitute for medical advice from healthcare professionals.
Find out more
Short clips
End of short clips
