It's a fact everyone knows- a thermometer reading of 98.6°F (37°C) indicates "normal" body temperature.

But like me, you may be puzzled because your own body temperature is cooler. We're not abnormal. Like many things, the 98.6°F reading is no longer a valid standard.

Where does 98.6°F standard for body temperature come from?

In 1851, German physician Carl Reinhold August Wunderlich took millions of auxiliary (under-arm) temperatures from 25,000 patients in Leipzig, setting the standard for normal human body temperature of 98.6°F.

But Stanford researchers have found that over 150 years there has been a steady cooling in body temperatures in the U.S. This is also been observed in Britain.

97.9 degrees is the current average.

This is an actual physiological difference that has happened over time.

What this indicates is that basal metabolic rates have gone down. A high metabolism can shorten one's lifespan; a low metabolism can lead to weight gain.

Longer lifespans and increases in mean weight have happened over the past ~150 years.

So what is going on? We are probably seeing a population-level control of inflammation due to less infection, better sanitation, and standards of hygiene. Fewer people have chronic infections. Malaria is no longer the scourge it once was. TB is treatable. We have antibiotics.

Body temperature is such a fundamental aspect of normal health and of assessing illness. I am glad researchers are finally reviewing this.

Reference: "Decreasing human body temperature in the United States since the Industrial Revolution" https://elifesciences.org/articles/49555 

I can’t stop thinking about how this simple observation might be direct physiological evidence from our own bodies of how microbiology and modern medicine have improved human life. I learn something new every day.