A normal human body temperature, as every anxious parent will have checked a million times, is 37 degrees Celsius. It’s 98.6º Fahrenheit in old money.
Normal temperature seems like the average number of fingers and toes – a reliable kind of thing that never changes. But our body temperatures have changed over time.
You might not be shocked to learn that the average body temperature of an Ice Age human was different from ours – but even people from 150 years ago ran hotter than we do today.
For example, the body temperature of men in the US today is, on average, 0.59°C lower than that of men born in the early 19th century.
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Women’s body temperatures may have changed slightly less, but the average temperature of a woman’s body has still dropped by 0.32°C since the 1890s.
A 2017 study revealed a similar decline in the UK. The average temperature among the British patients now measures about 36.6º C (97.88º F), notably lower than those ‘traditional’ normal temperatures.
And nobody is quite sure why.
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Dr Julie Parsonnet, a specialist in health research and policy at Stanford University, conducted a study into the causes of our body’s falling temperatures. She has a few theories on why the change might be occurring.
“Physiologically,” she says, “we're just different from what we were in the past.
”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.”
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Evolution – driven by our changing environment rather than survival of the fittest – is still working away: “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,” says Dr Parsonnet. “We’re actually changing physiologically.”
She theorises that another cause may be a general slow-down in our metabolisms, because of changes in diet and lifestyle.
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It’s also possible, she says, that the widespread use of central heating may have created generally warmer conditions for most of us, meaning that our bodies just aren’t working as hard to keep us warm.
One of Dr Parsonnet’s specialities is the study of gastrointestinal infections – food poisoning in everyday language.
She looked at bacterial disease caused by the microorganism Helicobacter . It causes a number of nasty symptoms including stomach ulcers and can increase the risk of developing certain cancers.
"I became aware, because I worked on it for 30 years, that that organism is disappearing from populations in the United States," says Dr Parsonnet. The lower levels of inflammation, across the entire population, may be one of the causes of the lower temperatures.
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She notes that the old, higher, average body temperatures still prevail in countries like – for example – Pakistan.
There may be other factors at play. As a population, we are living longer than people from every 50 or 60 years ago. Your body temperature naturally declines as you get older, so an ageing population will tend to drag down the average.
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No two people are precisely the same temperature all the time, and every individual changes in core temperature throughout the day, generally recording temperatures about 1º Fahrenheit higher in the evening than in the morning.
But overall, it’s not just in your mind. You really are cooler than your grandparents.
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