Since the 19th century, the average human body temperature in the United States has dropped, according to researchers at the Stanford University School of Medicine

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

That standard of 98.6 degrees Fahrenheit was made famous by German physician Carl Reinhold August Wunderlich, who published the figure in a book in 1868. Modern studies, however, have called that number into question, suggesting that it’s too high. A recent study, for example, found the average temperature of 25,000 British patients to be 97.9 F. 

In a study published today in eLife, Parsonnet and her colleagues explore body temperature trends and conclude that temperature changes since the time of Wunderlich reflect a true historical pattern, rather than measurement errors or biases. Parsonnet, who holds the George DeForest Barnett Professorship, is the senior author. Myroslava Protsiv, a former Stanford research scientist who is now at the Karolinska Institute, is the lead author.

The researchers propose that the decrease in body temperature is the result of changes in our environment over the past 200 years, which have in turn driven physiological changes.

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The researchers used the 677,423 temperature measurements from these datasets to develop a linear model that interpolated temperature over time. The model confirmed body temperature trends that were known from previous studies, including increased body temperature in younger people, in women, in larger bodies and at later times of the day. 

The researchers determined that the body temperature of men born in the early to mid-1990s is on average 1.06 F lower than that of men born in the early1800s. Similarly, they determined that the body temperature of women born in the early to mid-1990s is on average 0.58 F lower than that of women born in the 1890s. These calculations correspond to a decrease in body temperature of 0.05 F every decade.

The decrease in average body temperature in the United States could be explained by a reduction in metabolic rate, or the amount of energy being used. The authors hypothesize that this reduction may be due to a population-wide decline in inflammation: “Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” Parsonnet said. Public health has improved dramatically in the past 200 years due to advances in medical treatments, better hygiene, greater availability of food and improved standards of living. The authors also hypothesize that comfortable lives at constant ambient temperature contribute to a lower metabolic rate. Homes in the 19th century had irregular heating and no cooling; today, central heating and air conditioning are commonplace. A more constant environment removes a need to expend energy to maintain a constant body temperature.

Information sourced from the Stanford Medicine News Center.

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