Forget about that old touchstone of health — a thermometer reading of 98.6°F. After poring over more than 650,000 temperature measurements taken during the past century and a half, the authors of the new study have concluded that the average normal body temperature has dropped almost a full degree, to 97.9°F.
“Our temperature’s not what people think it is,” says Dr. Julie Parsonnet, the study’s senior author and a professor of medicine and of health research and policy at Stanford University, in a released statement. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”
That standard was established back in the 1850s, when a German physician reported 98.6°F to be the average temperature collected from 25,000 patients in Leipzig. It soon became accepted worldwide.
But studies in recent years have found the average body temperature to be lower. A 2017 study involving 35,000 British people reported, for example, that it was 97.9°F.
Does the drop in average body temperature over the past 150 years represent a true change or simply differences in the quality of thermometers used then and now? Parsonnet and her colleagues set out to answer that question.
How the study was done
The researchers analyzed three sets of body temperature data collected in the United States during three distinct historical periods spanning 157 years. One of the datasets was amassed from the service, medical and pension records of Union Army veterans of the Civil War. Spanning the years 1862 through 1930, it included people born in the early 1800s. The second dataset came from adult men and women who participated in the U.S. National Health and Nutrition Examination Survey (NHANES) between 1971 and 1975. It included individuals born as early as the 1890s. The third dataset was collected from adult patients visiting Stanford Health Care from 2007 to 2017.
Parsonnet and her colleagues ended up with 677,423 temperature readings from these three datasets to analyze. When they plotted the temperatures on a linear timeline, they found that the body temperatures of men born at the turn of the 21st century averaged 1.06 °F lower than those of men born 200 years earlier, in the early 1800s. The trend was similar among women. The average body temperature of women born around the turn of the 21stcentury was 0.58°F lower than that of women born 100 years earlier.
Overall, the analysis uncovered a drop in average body temperature of about 0.05°F per decade.
To make sure those were real changes, the researchers examined the temperature trend within each historical group — people who, presumably, had their temperature taken with similar types of thermometers. They found a downward slope in average temperatures within each group, evidence that the study’s findings can’t be explained by faulty thermometers.
The most likely cause
Parsonnet and her colleagues say the steady decline in average body temperature over the past 150 years is likely the result of people having a lower resting metabolic rate (the rate at which the body expends energy while resting) than in the past. Several environmental factors may have contributed to that lower rate, but the researchers believe the most plausible one has to do with changes in inflammation levels.
Inflammation is the body’s response to fighting injury and infection. It requires extra expenditures of energy, which, in turn, leads to a spike in body temperature.
“Economic development, improved standards of living and sanitation, decreased chronic infections from war injuries, improved dental hygiene, the waning of tuberculosis and malaria infections, and the dawn of the antibiotic age together are likely to have decreased chronic inflammation since the 19th century,” Parsonnet and her colleagues write in their paper. “For example, in the mid-19th century, 2 to 3% of the population would have been living with active tuberculosis.”
Changes in inflammation levels may also explain the continued drop in average body temperatures between the two more recent groups in the study — the NHANES participants and the Stanford Health Care patients.
“Although many chronic infections had been conquered before the NHANES study, some — periodontitis as one example — continued to decrease over this short period,” the researchers write. “Moreover, the use of anti-inflammatory drugs including aspirin, statins and non-steroidal anti-inflammatory drugs (NSAIDs) increased over this interval, potentially reducing inflammation.”
But less inflammation isn’t the only environmental factor that could have played a role in reducing people’s metabolic rates — and thus their body temperatures — over the past two centuries. There’s also the advent of central heating and central air-conditioning. The more time the human body spends in “thermoneutral zones,” the less energy it needs to maintain a constant body temperature, the researchers explain.
Parsonnet and her colleagues are not advocating for an updated definition of average body temperature. Given that body temperature varies by such factors as age, time of day, weight, height and outside temperatures, any strict definition of a “normal” temperature is kind of meaningless, they say.
But they are suggesting we ditch our antiquated fixation with 98.6°F.
“Physiologically, we’re just different from what we were in the past,” says Parsonnet. “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,” she adds. “We’re actually changing physiologically.”
FMI: You can read the study in full on the eLife website.