The thermometer reading of 98.6°F has been a gold standard for a century and a half, ever since a German doctor laid it down as the “normal” human body temperature. If you suspect you have a fever, a reading of 98.6 tells you that you are not. Over the last few decades, the benchmark has often been questioned. Different studies have found the human body temperature averaging out differently, including at 97.7°, 97.9° and 98.2°F.
Now, new research has found that body temperatures have, in fact, been declining over the last two centuries. This was determined from an analysis of records of Americans dating between the 19th century and 2017.
Why we follow 98.6°F
In 1851, Carl Reinhold August Wunderlich pioneered the use of the clinical thermometer. It was a rod a foot long, which he would stick under the armpits of patients at the hospital attached with Leipzig University, and then wait for 15 minutes (some accounts say 20 minutes) for the temperature to register. He took over a million measurements of 25,000 patients, and published his findings in a book in 1868, in which he concluded that the average human body temperature is 98.6°F.
Most modern scientists feel Wunderlich’s experiments were flawed, and his equipment inaccurate. In 1992, a study by the University of Maryland made 700 temperature measurements of 148 individuals over various times of the day, concluded that the average human body temperature is closer to 98.2°F, and suggested that the 98.6°F benchmark be discarded.
In 2017, a study on 35,000 British individuals published in The BMJ found their average body temperature to be 97.9°F. And in 2018, Boston rheumatologist Jonathan Hausmann used an iPhone app, Feverprints, to collect 11,458 temperatures crowd-sourced from 329 healthy adults, and published findings that put the average normal temperature in adults at 97.7°F, measured orally.
Last month came the new study, published in the journal eLife, that concluded the average human body temperature has never been constant in the first place.
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The body is cooler
Stanford University researchers recorded temperatures from three datasets covering distinct historical periods. One set was from 1862-1930, with records of Union Army veterans of the Civil War and including people born in the early 1800s. Another set was from 1971-75, from the US National Health and Nutrition Examination Survey. The newest set was from adult patients who visited Stanford Health Care between 2007 and 2017.
From 6.77 lakh measurements and statistical modelling, the researchers reconfirmed some known trends — body temperature is higher in younger people, in women, in larger bodies and at later times of the day. Additionally, they found that the bodies of men born in the early to mid-1990s is on average 1.06°F cooler than those of men born in the early 1800s. And the body temperature of women born in the early to mid-1990s is on average 0.58°F lower than that of women born in the 1890s.
The calculations from the research correspond to a decrease in body temperature of 0.05°F every decade, Stanford University said in a statement.
Explaining the trend
The researchers have proposed that the decrease in body temperature is the result of changes in the environment over the past 200 years, which have in turn driven physiological changes.
The decrease in average body temperature in the US, they said, could be explained by a reduction in metabolic rate, or the amount of energy being used. This reduction, in turn, may be due to a nationwide decline in inflammation as a result of better healthcare in the US. Inflammation would have revved up metabolism and raised body temperature.
“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. We’re actually changing physiologically,” senior author Julie Parsonnet said in the university statement.
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So, what is normal?
While the authors are confident of a cooling trend, they do not offer an updated definition of “average body temperature” to cover all Americans today. The strong influences of age, time of day, and genders on body temperature preclude such a definition, they said.
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