The average human body temperature is dropping, study suggests

The average temperature of the human body is slightly lower than the standard 98.6 degrees Fahrenheit established more than 150 years ago, according to research from the Stanford University School of Medicine.

According to the new study, the human body is getting colder. Researchers found that the average body temperature of men and women born today were approximately 1.06 degrees and 0.58 degrees Fahrenheit lower, respectively, compared to those alive in the 1800s.

The average human body temperature was determined by German physicist Carl Reinhold August Wunderlich in 1851. To do so, he collected millions of temperature readings for 25,000 patients in Leipzig and calculated the average of these readings to be 37 degrees Celsius, or 98.6 degrees Fahrenheit.

A 2002 analysis of 27 modern studies found the mean temperature of the human body to be uniformly lower than Wunderlich’s standard, and a 2017 analysis of more than 35,000 British patients with nearly 250,000 temperature measurements confirmed the lower value.

The Stanford research team set out to determine whether this observed lower average human body temperature was representative of a true change, or if it was the result of bias from either method of collecting temperature measurements or the quality of thermometers and their calibration. Wunderlich collected temperature data via the armpit, whereas modern studies collected temperature data orally.

“We speculated that the differences observed in temperature between the 19th century and today are real and that the change over time provides important physiologic clues to alterations in human health and longevity since the Industrial Revolution,” researchers wrote.

When Wunderlich was gathering temperatures in Leipzig in 1851, the average life expectancy was just 38 years. Researchers note that large portions of the population at this point in time had untreated chronic infections such as tuberculosis and syphilis, which cause chronic inflammation and therefore could have accounted for a higher average body temperature.

To better understand this trend over time, researchers looked at three data sets: one that looked at nearly 84,000 measurements from a cohort of Civil War veterans obtained between 1862 and 1930; one that looked at nearly 6,000 measurements from the National Health and Nutrition Examination Survey cohort (NHANES) obtained between 1971 and 1975; and one that looked at approximately 230,000 measurements from the Stanford’s STRIDE cohort obtained between 2007 and 2017.

Researchers found that average body temperature was highest for the cohort of Civil War veterans and lowest for the STRIDE cohort, which showed a clear temperature decrease over time.

Across all three cohorts, researchers noted a steady decrease of about -0.03 degrees Celsius per birth decade. Because this trend held steady across the two modern cohorts — which used similarly calibrated thermometers — researchers said it’s unlikely that the change in average body temperature is due to calibration bias.

The researchers behind the study believe that the change can be explained by metabolic changes and overall changes to human physiology. In large part, they point to the decrease in prevalence of chronic inflammation over time, which can be explained by many factors, including economic development, improved standards of living and sanitation, improved dental hygiene and the creation of antibiotics.

“Our investigation indicates that humans in high-income countries have changed physiologically over the last 200 birth years with a mean body temperature 1.6% lower than in the pre-industrial era,” the study’s authors conclude.

This story was reported from Los Angeles.