Average Human Body Temperature in the U.S. Has Dropped Since the 1800s

The average body temperature of people in the U.S. has dropped over the past two centuries, scientists have said.

Body temperatures dropped by 0.05 Fahrenheit (0.03 degrees Celsius) every decade since the 1800s, moving away from the average of 98.6 degrees Fahrenheit (37 degrees Celsius) in adults estimated in the 19th century. The authors of a paper published in the journal eLife said this shift reflects how the lives of Americans have changed in that period.

Compared with men alive today, the body temperatures of men born in the early 19th century were 1.06 degrees Fahrenheit (0.59 degrees Celsius) higher. For women, temperature decreased by 0.58 Fahrenheit (−0.32 degrees Celsius) since the 1890s.

Researchers crunched numbers from three studies that provided 677,423 temperature measurements over 157 years.

The Union Army Veterans of the Civil War cohort included 23,710 people followed between 1860 and 1940; the National Health and Nutrition Examination Survey conducted between 1971 and 1975 involving 15,301 people; and the Stanford Translational Research Integrated Database Environment was carried out between 2007 to 2017, 150,280 participants. The team accounted for changes in methods of collecting temperatures which could skew the results.

The team explained German doctor Carl Reinhold August Wunderlich calculated the average human body temperature as 37 degrees Celsius or 98.6 degrees Fahrenheit in 1851, after tests on 25,000 patients. But in recent decades, evidence has emerged suggesting the number could now be lower.

Previous research suggested a large proportion of the population Reinhold August Wunderlich tested would have had untreated chronic infections like tuberculosis, syphilis, and gum inflammation, which could have made the average temperature at that time higher.

In the eLife study, the team argued the drop they noted could be explained by falling metabolic rates, or how the body uses energy, linked to reductions in inflammation in the body. That is down to huge leaps in medicine, as well as improvements in sanitation, hygiene, and living standards over the past two centuries.

Warmer homes, for instance, mean our bodies spend less energy trying to keep a constant temperature than they previously did.
"Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature," study co-author Julie Parsonnet, professor of medicine and of health research and policy at Stanford University School of Medicine, said in a statement.

"Physiologically, we're just different from what we were in the past. 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," she said.

"Our temperature's not what people think it is. What everybody grew up learning, which is that our normal temperature is 98.6, is wrong," she added.