In 1851, a German doctor named Carl Reinhold August Wunderlich took the temperatures of some 25,000 patients in the city of Leipzig and concluded that the average human body temperature sits at 37 degrees Celsius, or 98.6 degrees Fahrenheit. Though variations are known to exist from person to person, Wunderlich's standard remains a benchmark for medical professionals today. But a new study published in eLife suggests that in the United States, at least, average temperatures are going down—a trend that can be observed in medical records spanning more than 150 years.
The team looked at three large temperature datasets from three distinct periods. The first was compiled from medical records, military records and pension records of Union Army veterans; the data was obtained between 1862 and 1930. The researchers also consulted measurements from the U.S. National Health and Nutrition Examination Survey I, spanning from 1971 to 1975, and the Stanford Translational Research Integrated Database Environment, which consists of data from adult patients who visited Stanford Health Care between 2007 and 2017.
In total, the researchers studied 677,423 temperature measurements, collected over the course of 157 years and covering 197 birth years. They found that men born in the early 19th century display temperatures 0.59 degrees Celsius higher than men today, representing a decrease of 0.03 degrees Celsius per birth decade. Women's temperatures have gone down 0.32 degrees Celsius since the 1890s, representing a 0.029 degree Celsius decline per birth decade—a rate similar to the one observed among male patients.
According to the researchers, there are several reasons why our bodies might be cooling down. One is improvements in temperature regulation. "We have air conditioning and heating, so we live more comfortable lives at a consistent 68°F to 72°F in our homes," Parsonnet explains. "t's not a struggle to keep the body warm."
A more significant factor might be the reduction of inflammation-causing conditions like tuberculosis, malaria and dental diseases, thanks to improvements in medical treatments, hygiene standards and food availability. "In the mid-19th century," the study authors note as an example, "2–3 percent of the population would have been living with active tuberculosis." And the prevalence of such diseases may have had a population-level impact on average temperatures.
Stanford's study: https://elifesciences.org/articles/49555
Fascinating to think how our bodies might be changing. Also, since all these temperatures were taken Post-industrial revolution, I do wonder about the steady creep of climate change.