Human Body Temperature In The U.S. Has Decreased Over Time, Study Finds

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Doctor: ‘What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.’


STANFORD, Calif. — Way back in 1851 German physician Carl Reinhold August Wunderlich declared that the standard human body temperature was 98.6 degrees Fahrenheit. Ever since, that reading has been universally considered the optimal body temperature of a healthy, “normal” human being. A whole lot has changed since the 19th century, and according to a new study conducted at Stanford University, that includes the average human body temperature.

Researchers say that the body temperature (Fahrenheit) of men born in the 2000s is on average 1.06 degrees lower than men born in the early 1800s. Similarly, women born in the 2000s have an average body temperature that is 0.58 degrees lower than women born in the 1890s. All in all, these conclusions point to a decrease of 0.05 degrees among the U.S. population each and every decade.

“Our temperature’s not what people think it is,” says Dr. Julie Parsonnet, a professor of medicine and of health research and policy, in a release. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”

While one could probably produce a number of theories or ideas as to why this is happening, the study’s authors say their temperature observations likely represent a “true historical pattern.” Essentially, researchers believe that our bodies are cooling off in response to changes in our living environment over the past 200 years.

Three distinct data sets from different time periods were analyzed for this study. The first consisted of U.S. military service, medical, and pension records for Union Army veterans from the Civil War. This dataset encompassed body temperature readings between 1862-1930, and included individuals born as far back as the early 1800s. The second was taken from a U.S. nutrition survey from 1971-75, and the third consisted of patient data from Stanford Health Care between 2007-2017.

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All three of these datasets amounted to a total of 677,423 temperature measurements for researchers to study. Using all of that information, they constructed a linear model of U.S. body temperatures over time.

Now, thermometry has obviously improved over the past 200 years, and the study’s authors wanted to be sure that their findings didn’t simply represent improvements in thermometer technology. So, they looked into fluctuating body temperature trends within each individual dataset, working off the assumption that measurements within each set would have been taken using similar thermometers. Even then, they noted incremental decreases in body temperature across individual decades, effectively confirming their findings regarding the dataset as a whole.

As far as identifying a new ideal human body temperature, the research team say there are a number of influential factors on body temperature, such as age, gender, and time of day, that make if very difficult to settle upon one set temperature for everyone at any given point in time.

Expanding on the possible reasons for this nation-wide decline in body temperature, researchers say it could be connected to a drop in metabolic rate, which basically means we’re all using less energy. This drop in energy usage may be due a decline in inflammation among Americans.

“Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” Parsonnet explains.

It shouldn’t come as much of a surprise that inflammation is largely down among the population, standard of living has drastically improved over the past 200 years, as well as health care, hygiene, and food availability. The American population largely live in comfortable settings, and in most cases, in homes complete with central heating and air conditioning. These wonderful temperature stabilizing tools make it much easier for our bodies to maintain a stable temperature, meaning we’re expending less energy.

“Physiologically, we’re just different from what we were in the past,” Parsonnet concludes. “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 is published in eLife.

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