Thermometers Aren't the Problem
This isn't the first time scientists have noted body temperatures are falling. A 2002 research review indicated the 98.6 degrees standard was too high. And a 2017 study concluded 97.9 degrees is the current average in Britain. Scientists, however, thought the discrepancy had to do with false readings from the thermometers of the past — in other words, that we weren't really 98.6 in the first place.
But now, after reviewing three additional datasets, the Stanford researchers conclude that over the past 150 years a steady cooling trend in American body temperatures has, indeed, occurred. The researchers analyzed Civil War veterans' records, 1970s data from the U.S. National Health and Nutrition Examination Survey, and recent medical records from patients who visited Stanford Health Care.
"Without changing our genetic makeup, we are changing physiologically over time," Parsonnet said. "We've seen that people have grown taller, they've grown fatter, and their body temperature is declining ... ."
The researchers did not offer an updated average for all Americans because body temperatures change throughout our day as well as lifetime. Age and sex influence our internal thermostats; and factors like activity level, food consumption and what time it is affect how hot we run daily.
Modern Life Influences Evolution
The researchers surmise body temperature might be steadily declining because Americans suffer less inflammation due to improved water quality, sanitation, antibiotics, vaccines and dental hygiene.
"The standard of 98.6 was set back in the mid 19th century when people were suffering all of the time from infectious diseases," said Parsonnet. "It wouldn't surprise me if their temperature was a little bit higher because they were fighting tuberculosis, malaria and dysentery all the time."
Because we live and work in more consistent environments than previous generations, she says, our bodies expend less energy. Two hundred years ago, homes did not have central heating and air conditioning, for instance.
Why It Matters
Whether or not our temperature is dropping is not merely an interesting tidbit. Body temperature affects our basal metabolic rate, which is linked to longevity and body size. A high metabolism can shorten one's lifespan; a low metabolism can lead to weight gain.
Parsonnet says researchers' next step is to analyze temperature variability around the globe. For example, conditions in a resource-poor country may preclude a decline because chronic infections are still common. Or, perhaps people who live in harsher climates like the one in the Arctic may run hotter because their bodies must work harder to stay comfortable.