The Average Human Body Temperature Is Going Down. Here's Why
Thu, April 22, 2021

The Average Human Body Temperature Is Going Down. Here's Why

A new study argues that the human body is getting colder after having dropped by a fraction of a degree since the 19th century / Photo by: Antonio Guillem via 123RF

 

General knowledge tells us that the normal body temperature is 37 °C, and any drops or increases may indicate a fatal condition. However, that figure seems to be changing over time.

A new study argues that the human body is getting colder after having dropped by a fraction of a degree since the 19th century. Researchers from Stanford University in California believe the dropping body temperatures could be due to the falling rates of chronic infections.

They add that the differences between body temperatures in the 19th century and today may provide important clues to the changes in human health and longevity since the Industrial Revolution.

Measurement Errors or Changing Temperatures?

No one questioned the standard 37 °C figure for the average body temperature, even if it was determined in 1851 after German physician Carl Reinhold August Wunderlich took millions of measurements from about 25,000 people to conclude that the normal temperature is between 36.2 to 37.5 °C. It was common knowledge taught and learned by many.

It was only in 1992 that a team from the University of Maryland challenged the idea and claimed that the average temperature is at 36.8 °C. Another study conducted 25 years later in the UK revealed similar figures of 36.6 °C.

Physician Philip Mackowiak, the lead author of the 1992 study, speculated that the discrepancy between their estimates was because Wunderlich used old-fashioned rudimentary thermometers (a thermometer made of a sealed glass cylinder containing a clear liquid and several glass vessels of varying density).

According to the Nature journal, Mackowiak tested one of the German physician's thermometers and found that temperature readings were over 1°C higher. The physician then concluded that the 37.5 °C figure was due to measurement error.

However, the new study from Stanford indicates that the human body is cooling down. Looking at three data sets, with the earliest being a database of 83,900 temperatures collected between 1862 and 1930 from veterans of the American Civil War, the results show people born earlier have higher body temperatures compared to those who were born later.

Their findings were the same after measuring the temperatures in the same period—possibly using the same technology—and after accounting for other factors like age, height, weight, and gender.

Cooling Bodies

The new study analyzed 677,423 human body temperatures from three different databases—civil war veterans, the National Health and Nutrition Examination Survey, the Stanford Translational Research Integrated Database Environment—that span a total of 157 years of measurement. They found that men born in the early 19th century have body temperatures of 0.59°C higher compared to today's male population, indicating a decrease of -0.03°C per birth decade. 

This downward trend is also evident among women, in which a -0.32°C decrease occurred since the 1890s following a similar decline rate of −0.029°C per birth decade.

Some may argue that the differences could be due to the different thermometers and methods used to gather the temperature measurements. However, the researchers said such an explanation is "unlikely."

"If it’s just the thermometers changing, the year that the temperature was taken should make the difference," lead author Julie Parsonnet said in a statement. They concluded that the changes in body temperatures are due to the physiologic differences instead of discrepancies in the measurement methods.

"We’ve changed in height, weight—and we’re colder. I don’t really know what [the new measurements] mean in terms of health, but they’re telling us something," Parsonnet told the science news site Scientific American.

"They’re telling us that we are changing and that what we’ve done in the last 150 years has made us change in ways we haven’t before."

While they did not determine the cause of the body's dropping temperatures, the lead author speculated that a combination of factors like warmer clothing, indoor temperature controls, and a decline in infectious diseases are driving the cooldown.

The Most Plausible Explanation

Among the number of possible causes of the drop in body temperatures, the researchers said lower rates of infections are the best explanation since inflammatory immune responses to infections cool down the body.

"If you looked at the great majority of people back in the 19th century, I’m sure literally all of them had a chronic inflammatory condition," Parsonnet said. "They lived to be 40 years old or less. They all had terrible dentition."

A 2011 study supports this claim, with results suggesting 2-3% of the population in the mid-19th century would've contracted active tuberculosis at the time. This figure is consistent with surgeons' certificates of the civil war veterans that reported 737 cases of active tuberculosis among 3.1% of the cohort.

"Veterans who reported either current tuberculosis or pneumonia had a higher temperature (0.19°C and 0.03°C respectively) than those without infectious conditions supports this theory," the researchers said, adding they would have compared these results to temperature measurements taken from a location with a high risk of chronic infection.

While no such data was available, they did find a small study from Pakistan—a nation with a high incidence of tuberculosis and other chronic infection—that reported average body temperatures of 36.9 °C.

Such an explanation is "intriguing and plausible," according to epidemiologist Jill Waalen, who also reviewed the study for online journal eLife. None of the temperature measurements of war veterans used in the study spanned at the beginning of the 1940s, the time when antibiotics were introduced.

Waalen said the significant drop in body temperatures during this period upholds the idea that infections are the cause of the cooling body temperatures.

Another Mark of Change

Makowiak remains cynical of the study's findings, saying "there are so many variables that are unaccounted for" such as whether the veterans were healthy when they were tested, where the thermometer was placed, and the kind of instrument used.

"That’s not to say that what [the new study] found is not valid," Makowiak told the Scientific American. "It could be, but you just don’t know."

The researchers acknowledge the limitations of their study, specifically the data set of civil war veterans, but Porsennet said those concerns were addressed with their finding of a similar annual drop in temperature between the 1970s group and the modern cohort.

“We’ve also grown taller, we’ve grown fatter. We’ve changed since the 1850s. Temperature is just another marker of that change,” the lead researcher concluded.