The “normal” body temperature of 98.6°F (37°C) is actually not so normal. New research finds the average human body temperature of Americans has dropped.
“What everybody grew up learning, which is that our normal temperature is 98.6, is wrong,” said Dr. Julie Parsonnet, a professor of medicine as well as health research and policy at Stanford University School of Medicine.
The 98.6°F standard was established by a German doctor in 1851. Recent studies have indicated that’s too high; research on 35,000 British people found their average was 97.9°F.
Parsonnet’s study published this week in eLife. It found that temperature changes since 1851 reflect a historical pattern instead of an error. They contend the decrease is the result of environmental changes over the past 200 years that have affected human physiology.
Parsonnet looked at data from 1862 through 1930, 1971 through 1975, and 2007 through 2017. It included 677,423 temperature measurements.
The body temperature of men born in the 2000s is 1.06°F degrees lower, on average, than men born in the early 1800s. Women have temps about 0.58°F lower than those born in the 1890s. That means body temperatures declined 0.05°F every decade.
Parsonnet’s team also found a decline in temperature since the 1960s, not just since the Civil War.
Body temperature is complicated, Parsonnet says. It can vary not just from person to person but at different times of day and at different ages.
It can be almost half a degree higher in the afternoon than morning. It’s much lower in older adults. Your weight and height as well as hot or humid weather can also affect it, Parsonnet says.
In her experience, Parsonnet says at least 75 percent of normal temperatures are below 98.6°F.
“Even in younger adults at the end of the day, when temperatures are at their highest, the temperature still doesn’t get up to 98.6,” she said. “In the elderly, it would be quite unusual to have a temperature as high as 98.6.”
“The threshold for fever is generally held as 1°C above normal,” said Dr. Bradley Uren, assistant professor of emergency medicine at Michigan Medicine. “A fever is generally held as 38°C, or 100.4°F.”
“It is rare to see a patient at exactly 98.6,” he noted. “A temperature is just one piece of information that physicians must and do consider in determining a course of treatment for an individual.”
Even though our temperatures can fluctuate, doctors still know what’s generally normal. They also know to look for variations in temperature, which can indicate a medical condition, Uren says.
“Physicians must take into account the entirety of a patient’s condition in determining if a temperature, either higher or lower, or even within the normal range, is significant,” Uren said.
“For example, patients may be treated for infection even with a normal temperature if the other historical and physical findings, and the patient’s overall condition, is consistent with infection,” he said.
Dr. Amy Mullins, medical director of quality improvement for the American Academy of Family Physicians, notes that the study won’t change how patients or doctors should define fevers.
Why Temps Are Falling
It’s possible that some of these changes may be because thermometers provide more accurate readings, Parsonnet noted in her study.
However, that doesn’t explain all of it. Temperatures declined over time even when her team controlled for different devices.
Susan Yeargin, Ph.D., associate professor of exercise science at University of South Carolina, has tested all types of temperature devices in diagnosing hyper- and hypothermia. They were inaccurate compared to the rectal temperature.
“Each person’s unique normothermic body temperature should be considered by healthcare providers. Certain medications can influence a person’s regular ‘set point’ temperature, and obviously hot and cold outdoor temperatures can affect a person’s body temperature over time,” Yeargin said.
“When healthcare providers give advice to individuals about these medications and environments, their unique normothermic body temperature should be considered,” she said.
According to Parsonnet, other reasons for the decline in body temperature over time could be that we’re also using less energy and have a lower metabolic rate than in the past.
The reduction may be due to a population-wide decline in inflammation. Generally, inflammation increases our metabolism and raises temperature.
Because of improvements in public health, this could be why inflammation has decreased. The ambient temperatures we live in, thanks to heating and air conditioning, could be factors in lower metabolic rates.
“I think it’s most likely because we have much less inflammation in our bodies now than we did when the standard was developed in the mid-19th century,” Parsonnet said.
“We have less inflammation because we have far fewer chronic infectious diseases like tuberculosis and periodontal disease, far less recurrent infection, shifts in our microbiomes, and we also have learned how to combat inflammation directly through better diets, and also with things like nonsteroidal drugs and statins,” she explained.
In general, humans are physiologically different than we were in the past, Parsonnet says.
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