Historical body temperature records as a population-level 'thermometer' of physical activity in the United States.

Over the past two centuries profound technological and social changes have reduced overall levels of physical activity (PA). However, just how much population-level PA levels have declined since the Industrial Revolution is unknown because methods for measuring PA, such as accelerometry and the doubly labeled water technique, were developed only within the last few decades. Here, we show that historical records of resting body temperature (T) can serve as a 'thermometer' of population-level PA, enabling us to use the well-documented secular decline in T in the US to approximate PA decline in the US since 1820. Using cross-sectional data relating T to resting metabolic rate (RMR) and RMR to PA, we estimate that RMR has declined by ∼6% and moderate to vigorous PA by ∼27 minutes per day since 1820 in the US.