The average human body temperature has steadily declined since the 19th century, according to a study published earlier this month in eLife, raising questions about whether the "normal" human body temperature is actually lower than 98.6 degrees Fahrenheit, Nicholas Bakalar reports for the New York Times' "Well."

Study details

According to researchers, the common claim that human body temperature averages 98.6 degrees Fahrenheit originated with a study by the German doctor Carl Reinhold August Wunderlich, who repeatedly measured the temperatures of 25,000 people in Leipzig in 1851. But researchers questioned whether that data truly represented average body temperature in the modern age.

To find out, they examined 677,423 human body temperature measurements from three databases to determine how body temperatures have changed over time. Human body temperatures serve as "a crude surrogate for basal metabolic rate which, in turn, has been linked to both longevity (higher metabolic rate, shorter life span) and body size (lower metabolism, greater body mass)," the researchers noted.

The databases spanned 157 years of measurement. The first database contained temperature readings obtained from 23,710 Civil War veterans between 1862 and 1930. The second database contained temperatures readings for 15,301 individuals collected by CDC's National Health and Nutrition Examination Survey from 1971 to 1975. The third database contained temperature readings for 150,280 individuals collected by the Stanford University from 2007 to 2017.

Findings

Overall, the researchers found the average human body temperature has decreased by 0.03 degrees centigrade, or about 0.05 degrees Fahrenheit, per birth decade. Pointing to the findings, Bakalar in the writes, "Today, a temperature of 97.5 may be closer to 'normal' than the traditional 98.6."

According to the researchers, "men born in the early 19th century had temperatures 0.59°C higher than men today, with a monotonic decrease of −0.03°C per birth decade." Meanwhile, women's average body temperatures have decreased by −0.32°C since the 1890s, at a similar rate of −0.029°C per birth decade.

The researchers said the decline in the average human body temperature could not be explained by differences in measurement techniques. They explained that the decrease in average body temperature occurred annually within each of the three databases and that they found identical declines between the two modern databases, which presumably involved the same equipment and measurement techniques.

While it's unclear what drove the decline in body temperatures, the researchers did offer a few possible explanations. Namely, the researchers pointed to advancements in heating and air conditions, which help maintain constant temperatures; reductions in chronic inflammation; and improvements in dental care, medical care, and sanitation.

Julie Parsonnet, a professor of medicine at Stanford who was involved in the study, said, "We've looked at the U.S and we have to see if this holds true elsewhere. We're evolving physiologically. But what does it really mean? I don't know. I haven't figured out exactly how to look at that" (Bakalar, "Well," New York Times, 1/9; Protsiv, eLife, 1/7).