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Human body temperature has been falling since the 19th century

Researchers have determined that the average human body temperature in the United States has been decreasing since the 1800s.

Human beings are homeothermic organisms, which means that their body temperature is almost constant. However, it is also able to evolve. The value of 37°C was originally established by the German physician Carl Reinhold August Wunderlich, in 1851. In other words, it was more than 150 years ago. In a study published in eLife , researchers at Stanford University explored trends in this average over several decades to assess how much it may have changed.

For this work, the researchers analyzed the trends of three data sets covering three time periods. The first concerned the medical records of several thousand veterans of the Union Army (the United States Army during the Civil War) covering the period 1862 – 1930 . The second data set included records from a national health survey conducted in the early 1970s . Finally, the last included data on all adult patients who visited Stanford Health Care between 2007 and 2017 .

In total, the researchers had access to 677,423 body temperature measurements . They were then able to develop a linear model to analyze the evolution of temperature over time.

Our bodies gradually cooled

This analysis first confirmed that body temperature was generally higher in young people, in women or in people with large builds.

However, more generally, it has mainly emerged that the body temperature of men born in the 2000s is on average 0.59°C lower compared to that of men born in the early 1800s. This represents a constant drop of 0.03°C per decade . The decline was similar in women with a 0.32°C drop per decade since the 1890s .

With temperatures first recorded 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 checked for body temperature trends in each data set. They started from the idea that in each of these groups, the measurements had normally been taken with similar thermometers. Nevertheless, again, in the veteran dataset that spanned several decades, they observed a decrease in temperature over time.

Our environment has changed a lot

For researchers, the human body has therefore been well cooled for at least 150 years . The evolution of our society during this period could explain the trend. In particular, living conditions have improved significantly, which has helped to reduce our metabolic rate.

Inflammation produces all kinds of proteins and cytokines that speed up your metabolism and raise your temperature “, explains the researcher. “But public health has improved dramatically over the past 200 years, thanks to advances in medical treatment, better hygiene, greater availability of food. Houses are also warmer in winter and cooler in summer “.

Physiologically, we are therefore different than 150 years ago because our environment has changed.

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