Nov. 2. 2021 -- Hard workouts make us feel hot and sweaty because they increase our body temperature and metabolism. A new study tracking declines in both temperature and metabolism puts a number on how much daily physical activity has dwindled in the U.S. since the 1800s.

We’re all taught that a normal human body temperature is about 37 C, or 98.6 F, a benchmark that traces to studies from the 1800s. Back then, many U.S. adults had physically demanding jobs as carpenters, farmers, bricklayers, and blacksmiths and did many household chores by hand. Now, that’s not the case, especially when a large proportion of U.S. workers sit for hours at a time and machines do many household tasks.

Researchers publishing in Current Biology relied on two studies tracking historic body temperatures to calculate declines in daily activity. The first study included more than a half million temperature measurements for U.S. adults. It suggested that during each decade since the early 1800s, resting body temperature has dropped by 0.03 C. The second study they used, of white men in Minnesota, showed that with every degree Celsius increase in resting temperature, there was a 10% increase in resting metabolic rate.

The investigators using these studies assumed that the opposite would be true -- that a single degree drop in resting temperature could lead to a 10% decline in resting metabolic rate. Using this assumption and the temperature change study, they calculated that among U.S. adults, resting metabolic rate has decreased by 0.3% per decade, or about 6% in the last two centuries.

They then estimated how much physical activity levels would have to drop for a resting metabolic rate to fall that far. The answer: 27 fewer minutes of physical activity each day.

Their estimate is rough at best and not based on studies using objective measurement tools for physical activity levels. Still, the results suggest another way that modern habits may take their toll on our health.

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Current Biology: “Historical body temperature records as a population-level ‘thermometer’ of physical activity in the United States.”

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