Human body temperature declined over the past two centuries
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