Science

Human body temperature declined over the past two centuries

Human body temperature decline...
US medical records indicate that average human body temperatures have dropped over the last 200 years
US medical records indicate that average human body temperatures have dropped over the last 200 years
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US medical records indicate that average human body temperatures have dropped over the last 200 years
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US medical records indicate that average human body temperatures have dropped over the last 200 years

A new study by a team of Stanford University School of Medicine scientists suggests that the average human body temperature in the United States has decreased over the last century. By cross-checking a variety of health records, the researchers concluded that the average American's body temperature is about 0.58° F (0.03° C) and 1.06° F (0.6° C) lower for women and men, respectively, than it was in the 19th century.

The idea that the standard human body temperature is about 98.6° F (37° C) was first presented by the German physician Carl Reinhold August Wunderlich in 1851. Since then, it's become so widely accepted that it serves as a touchstone for health – a diagnostic tool used by physicians and parents as a basic indicator if someone is sick or well.

However, it turns out that this well-established fact isn't, in fact, correct – or, to put it more accurately, human beings have been getting cooler over the years.

Recent studies have shown that temperature records of groups of people have tended to run low compared to the accepted norm, so the Stanford team, led by Julie Parsonnet, MD, professor of medicine and of health research and policy, decided to do a more in-depth study to compare modern measurements with historical records to try to identify body temperature trends and, perhaps, uncover the reason why this cooling is happening.

For their research, the Stanford team looked at three distinct datasets from three historical periods. One was military service records, medical records, and pension records from Union Army veterans of the American Civil War that were compiled from 1862 to 1930. The second was from the US National Health and Nutrition Examination Survey I collected between 1971 and 1975, and third from adult patients visiting Stanford Health Care from 2007 to 2017.

In all, the team went through 677,423 temperature measurements, making sure that temperatures recorded were accurate rather than the result of poorly designed thermometers. They did this by looking at the change in temperature inside each group over time to ensure that the curves showing a decrease were consistent between the datasets.

At the end of the day, the team found that men born in the 21st century had an average body temperature of 1.06° F (0.6° C) lower than those born in the early 19th century, while modern women showed an average decrease of 0.58° F (0.03° C) compared to those born in the 1890s. Together, this means that human body temperatures have fallen by 0.05° F (0.03° C) per decade. However, because there are many factors that influence body temperatures, the team says that it isn't necessary to update the definition of the average body temperature for everyday purposes.

As to why this is happening, the researchers say that the most likely mechanism is a reduction in the human metabolic rate due to environmental factors. One possibility is that improvements in public health over 200 years have reduced the incidence of inflammations, which boosts metabolism. This, combined with people living more comfortable lives in more stable environments means that the body doesn't have to work as hard to stay warm, so the average temperature falls.

"Physiologically, we’re just different from what we were in the past," says Parsonnet. "The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically."

The study was published in eLife.

Source: Stanford University

4 comments
nameless minion
Given the proposed causes of this decline, it might have been useful to look at the body temps of various sub-populations. For example, the homeless, those with inflammatory disease, athletes practicing for endurance events and those high or low BMIs.
Nobody
One other factor that was briefly mentioned is that people keep their homes much warmer than they used to. When I was a child we slept in unheated bedrooms where the temperature often fell below 40 degrees in the winter. The first one up had to shake down the furnace grates and put more coal in the furnace. The house was never heated above 60 degrees and of course the outhouse was not heated at all. The older folks all wore long underwear but not us kids. We thought that was normal and a luxury since our grandparents only had a small wood stove for the entire house. I don't know what our average body temperature was but we were much more resistant to the cold. When we got a new home with gas heat and every room was 68 degrees, it was great. Today I am much older and must be warmly dressed to tolerate anything colder than 72 degrees. I don't know what my temperature was when I was a kid when 98.6 was the standard but today my average temperature is 96-97 degrees and I freeze when we go winter camping at temperatures that wouldn't have bothered me at all years ago. I also know that our diet was very different when I was a kid. We ate cracklins instead of potato chips and hot cereals instead of corn flakes along with sausage instead of hamburger. Hominy with bacon was a real treat for breakfast and of course bacon and eggs every day. Anyone in their 90s or older probably ate the same things we did.
Ralf Biernacki
Much of this data came from patients---people who were ill at the time of temperature measurement. Two hundred years ago, medicine did not have effective antipyretics, and a lot of these patients had fevers. In the modern day, people with fever are immediately treated, or self-treat, with antipyretics, which are common over-the-counter. That's why the average temperature of a patient today is lower than 200 years ago. This research is worthless---it is comparing apples with oranges.
ljaques
I wonder if this genetic change is responsible for the raft of sicknesses we are now experiencing. 1 in 2 will get cancer in their lifetimes today. It has already been discussed that we wash too much, losing too many beneficial colonies of bacteria. And we are staying much warmer indoors than our predecessors.