98.6. Why does that number ring a bell?

For years, the figure has held an important place in hospital rooms and physiology textbooks: 98.6 degrees Fahrenheit (37 degrees Celsius) is widely considered to be the "normal" average human body temperature.

But is this temperature still accurate? New research suggests the average American body temperature has dropped, and researchers think they know why.

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A German physician named Carl Reinhold August Wunderlich was the first to crunch the 98.6 degrees number in 1851 after collecting millions of temperatures from about 2,500 patients in the city of Leipzig. "He took temperatures of everybody he could find, whether they were healthy sick, and he wrote a large book on temperature variation with illness," said study senior researcher Dr. Julie Parsonnet, a professor of medicine and of health research and policy at Stanford University. Wunderlich's work also highlighted temperature variations between people of different sexes, ages, weights and heights.

"Almost everything he said was correct," Parsonnet told Live Science. "He must have been sitting there with a pen, paper and pencil for an awful long time with all those temperatures."

Since Wunderlich's pioneering efforts, doctors still use body temperature as a key vital sign to help determine a person's health status. We now know that body temperature fluctuates as much as 0.5 F (0.2 C) throughout the day; that young people generally stay warmer than elderly people; and that women tend to maintain a higher temperature than men, depending on where they are in their menstrual cycles, according to a 2019 report in the journal Open Forum Infectious Diseases. Our body temperature also varies with the weather, our level of physical activity and whether we've eaten recently.

But why is it that, in general, the human body tends to hover around 98.6 degrees?

Evidence suggests that the body maintains a relatively stable temperature in order to keep its many organs and chemical reactions running smoothly, and potentially keep fungal infections at bay. But, according to the new study, published Jan. 7 in the journal eLife, the ideal body temperature may no longer be 98.6 F.

Rather, the average body temperature among Americans has dropped about 0.05 F (0.02 C) every decade since the early 1800s, the researchers found. American men born in the 2000s measure an average 1.06 F (0.58 C) cooler than men born in the early 1800s. Women born in the 2000s measure about 0.58 F (0.32 C) cooler than women born in the 1890s. The big question is, why?

As an infectious disease researcher, Parsonnet has spent many years studying a bacterial disease caused by the microorganism Helicobacter. The bug causes open sores called ulcers in the esophagus, stomach and small intestine and raises affected people's risk of developing gastric cancers. Over the years, though, Helicobacter infections have become less common in the U.S.

"I became aware, because I worked on it for 30 years, that that organism is disappearing from populations in the United States," Parsonnet said. The change reflects a larger trend; compared with our 19-century relatives, modern humans catch far fewer infectious diseases. People who lived through the 1800s were plagued with recurrent malaria, chronic wounds, tuberculosis, never-ending dental disease and bouts of dysentery, Parsonnet said.

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Today, we don't have all these bugs swimming through our bodies and revving our immune systems into overdrive. Parsonnet wondered how the loss of these microorganisms has altered human physiology through time.

To find out, Parsonnet and her co-authors dug through the data, including data sets from the American Civil War, the 1970s and the early 2000s. With these data sets combined, the researchers accrued more than 677,000 temperature measurements to examine.

The team spotted a steady drop in average human body temperature through the years. To rule out the possibility that improved thermometer technology had skewed the data, the researchers also looked for trends within each individual data set. Sure enough, the cooling trend appeared in each, regardless of the thermometer used by each historical group.

"We as human beings have evolved over time physiologically changed," Parsonnet said. "We've changed from who we were in the 19th century, and who we were in the 1960s, to a different human today that's colder."

The findings echo the results of a 2017 study conducted in England that analyzed about 250,000 temperature measurements from more than 35,000 patients. The average temperature among the British patients measured about 97.88 F (36.6 C), down a significant fraction from the "normal" average temperature of 98.6 F (37 C). Although humankind seems to be growing cooler by the decade, what does this actually mean for our physiology?

It's still a mystery, Parsonnet said. "We don't really understand what this cooling means in humans, what it means to our health, what it means to our longevity," she said.

Perhaps our decreased body temperature likely reflects the historical decline in infectious disease rates a trend that reduced excess inflammation in the human body to a significant degree, the researchers wrote in the study. Inflammation produces proteins called cytokines that ramp up the body's metabolic rate, thus generating heat.

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Additionally, unlike our ancestors, many people now live in a largely temperature-controlled world. "We don't have to work very hard to maintain our body temperature; it's always 70 F (21.1 C) in our houses," Parsonnet said.

Of course, it may be that people living in regions beyond the U.K. and the U.S. maintain entirely different body temperatures. For example, a 2008 study determined that the average body temperature in Pakistan still hovers around 98.6 F. However, these slight temperature differences between populations likely don't alter how our bodies function, physiologically, Parsonnet said.

"It might affect how microbes function, [but] I don't think we know the answers to those questions at all," she said. On the level of individuals, only extreme temperature changes signal worrisome health issues, such as fever or hypothermia. On a grand scale, though, average body temperatures may continue to fall as medicine advances and life expectancy increases, Parsonnet added.

Body temperature is "a marker of inflammatory state. And if you can take the temperature of a population, you might be able to predict their life expectancy," she noted. Parsonnet added that, someday, both life expectancy and body temperature will likely level off and remain consistent into the future.

Originally published on Live Science.

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Has the Average Human Body Temperature Always Been the Same? - Livescience.com