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For years we have been thinking that 98.6 F (37 degrees Celsius) is the normal body temperature.

But new research suggests the average human body temperature has dropped. 97.5 F not 98.6 F, could be new normal body temperature.

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. Wunderlich's work also highlighted temperature variations between people of different sexes, ages, weights and heights. 

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. 

How the researchers found this? Like this ...

As an infectious disease researcher, Parsonnet, one of the scientists, has spent many years studying a bacterial disease caused by the microbe  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, TB, never-ending dental disease and bouts of dysentery, Parsonnet said. 

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. "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).

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. 

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.

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".

However, it should be mentioned that  a 2008 study determined that the average body temperature in Pakistan still hovers around 98.6 F. So there is a difference between developed countries and developing countries with regard to body temperatures!

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