A new, large-scale study of body temperatures has found that “normal” isn’t one size fits all — it varies by age, sex, weight, time of day and more.
September 5, 2023 - By Nina Bai
Most of us can recite 98.6 degrees Fahrenheit as the normal human body temperature, but it turns out there is no such thing. A new study by Stanford Medicine scientists found that normal body temperature varies from person to person, depending on their age, sex, height and weight, among other factors — and it fluctuates throughout the day.
The normal body temperature of a tall, underweight, 80-year-old man in the morning, for instance, could be nearly 1 degree lower than that of a short, obese 20-year-old woman in the afternoon.
“Most people, including many doctors, still think that everyone’s normal temperature is 98.6 F. In fact what’s normal depends on the person and the situation, and it’s rarely as high as 98.6 F,” said Julie Parsonnet, MD, professor of medicine and of epidemiology and population health as well as the George DeForest Barnett Professor in Medicine. She is the senior author of the study that was published Sept. 5 in JAMA Internal Medicine.
The researchers are hopeful that knowing when a person’s temperature is outside their normal range could be helpful in signaling disease and getting a jump on treatment.
A body temperature drop
In recent years, Parsonnet’s team has found that the average body temperature in the U.S. has dropped from 98.6 F by about 0.05 F every decade since the 19th century, likely due to better health and living conditions that reduce inflammation. They found that today’s normal body temperature hovers closer to 97.9 F.
The often-cited standard of 98.6 F stems from data published in 1868. German physician Carl Reinhold August Wunderlich took more than a million temperature measurements from some 25,000 patients. In fact, Wunderlich reported a range of temperatures in this group — noting that men and the elderly had lower temperatures than women and young adults, and that temperatures were higher in the afternoon. But it was the overall average, 98.6 F, that stuck in the general consciousness.
“Instead of thinking about a distribution in temperatures, which is what the initial study showed, we’ve taken a mean of 98.6 F and used it as a cutoff value,” said Catherine Ley, PhD, senior research scientist and lead author of the study.
“We’ve used an average value to create a false dichotomy of what’s normal and what’s not.”
The researchers analyzed 618,306 oral temperature measurements from adult outpatients seen at Stanford Health Care from 2008 to 2017. They also tracked the time of day it was taken, along with each patient’s age, sex, weight, height, body mass index, medications and health conditions.
First, to ensure that the temperatures were not skewed by illness, the researchers applied a machine learning algorithm to identify diagnoses and medications that were disproportionately associated with extremely high or extremely low temperatures. All patient visits with these diagnoses, about a third of the total, were excluded from the analysis.
The excluded diagnoses included those related to infectious diseases, which were linked to high temperatures; surprisingly, Type 2 diabetes was linked to low temperatures, an association that had not been established previously.
From the remaining data, they found that adults have normal temperatures ranging from 97.3 F to 98.2 F, with an overall average of 97.9 F.
Next, the researchers used a statistical model to relate patient characteristics and time of day to temperature. Men tended to have lower temperatures than women. Temperatures decreased with age, decreased slightly with height and increased with weight. The factor with the most influence, however, was time of day, with temperatures coolest in the early morning and warmest around 4 in the afternoon.
A website using the study’s statistical model can estimate a person’s expected temperature throughout the day based on their personal characteristics.
The combination of age, sex, height, weight and time of day accounted for 25% of the variability in normal temperatures within an individual and 7% of the variability from person to person. That means other factors not included in this study, such as clothing, physical activity, menstrual cycle, measurement error, weather — even drinking a hot or cold beverage — are likely also at play.
How hot is a fever?
Individualized benchmarks could make body temperature a more accurate and useful vital sign, Parsonnet said. She recounted the experience of her elderly mother-in-law who had a serious heart infection, but was not diagnosed for weeks because her temperature never reached a conventional fever, usually defined as higher than 100.0 F or 100.4 F. She wonders whether an individualized temperature evaluation would have detected the illness earlier.
Future studies could look at personalized definitions of fever and whether having a consistently higher or lower normal temperature affects life expectancy, Parsonnet said.
“There’s a lot of temperature data in the world, so there’s a lot of opportunity to actually learn something about it,” she said.
About Stanford Medicine
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