In the United States, body temperature has decreased since the industrial revolution, according to a study by researchers from Stanford University (California) published in the journal eLife January 7. “What everyone has learned, that our normal temperature is 37 degrees, is wrong”said Julie Parsonnet, who led the study, in a statement released by Stanford. The time comparison of three cohorts indicates that men born in the early 19th centurye century had a temperature 0.59 ° C higher than that of men today; for women, the more recent data follow the same slope, with a drop of 0.32 ° C since the 1890s.
Experts already knew that the totemic value of 37 ° C was no longer valid. In recent years, several cohort studies have revealed lower temperature averages: the most recent, in 2017, which involved 35,000 British patients and 250,000 oral measurements, had given a value of 36.6 ° C. The average of 37 ° C was established in 1851 by the German doctor Carl Reinhold August Wunderlich (1815-1877), who had placed over the years a thermometer under the armpits of 25,000 patients in Leipzig.
But the latest observations “Did not answer the question of whether the difference observed between Wunderlich’s measurements and modern averages was due to a real change or to biases in the method to obtain the measurement (armpit for Wunderlich, against mouth today) or the quality of the thermometers and their calibration ”, say Myroslava Protsiv and colleagues in eLife.
Physiologically counterintuitive decline
To decide, the Stanford researchers therefore compared three large cohorts: 23,710 veterans of the American Civil War, followed between 1862 and 1930; 15,301 people enrolled in a nutrition study between 1971 and 1975; and 150,280 people integrated into a cohort followed by Stanford between 2007 and 2017. A total of 677,423 temperature measurements to be analyzed. The result ? In men, the temperature dropped on average by 0.03 ° C per decade of birth, and by 0.029 ° C in women.
How to explain this drop, which is physiologically counter-intuitive? Indeed, the weight of Americans has increased since the middle of the XIXe century, which should have resulted in an increase in temperature, itself reflecting an increase in basal metabolism (the minimum energy expended to maintain bodily functions). The opposite seems to have happened.