Study: Average Human Body Temperature in U.S. Has Decreased Since 1800s

Scientists at the Stanford University School of Medicine have found that men born in the United States in the early 19th century had temperatures 0.59 degrees Celsius higher than men today, with a decrease of 0.03 degrees Celsius per birth...

Study: Average Human Body Temperature in U.S. Has Decreased Since 1800s

“Our temperature’s not what people think it is. What everybody grew up learning, which is that our normal temperature is 37 degrees Celsius (98.6 degrees Fahrenheit), is wrong,” said Professor Julie Parsonnet.

In 1851, the German physician Carl Reinhold August Wunderlich obtained millions of temperatures from 25,000 patients in Leipzig, Germany, thereby establishing the standard for normal human body temperature of 37 degrees Celsius (range: 36.2-37.5 degrees Celsius).

A series of modern studies, however, reported mean temperature to be lower than Wunderlich’s estimate.

In the new study, Professor Parsonnet, Myroslava Protsiv and their colleagues analyzed temperatures from three datasets covering distinct historical periods:

(i) the earliest set, compiled from military service records, medical records and pension records from Union Army veterans of the Civil War, captures data between 1862 and 1930 and includes people born in the early 1800s;

(ii) a set from the U.S. National Health and Nutrition Examination Survey I contains data from 1971 to 1975;

(iii) the Stanford Translational Research Integrated Database Environment comprises data from adult patients who visited Stanford Health Care between 2007 and 2017.

The scientists used the 677,423 temperature measurements from these datasets to develop a linear model that interpolated temperature over time.

The model confirmed body temperature trends that were known from previous studies, including increased body temperature in younger people, in women, in larger bodies and at later times of the day.

The researchers observed that the body temperature of men born in the early to mid-1990s is on average 0.59 degrees Celsius lower than that of men born in the first decade of the 1800s.

Similarly, they observed that the body temperature of women born in the early to mid-1990s is on average 0.32 degrees Celsius lower than that of women born in the 1890s.

According to the study author, the decrease in average body temperature in the United States could be explained by a reduction in metabolic rate, or the amount of energy being used.

They hypothesize that this reduction may be due to a population-wide decline in inflammation.

“Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” Professor Parsonnet said.

“Public health has improved dramatically in the past 200 years due to advances in medical treatments, better hygiene, greater availability of food and improved standards of living.”

“Comfortable lives at constant ambient temperature contribute to a lower metabolic rate. Homes in the 19th century had irregular heating and no cooling; today, central heating and air conditioning are commonplace. A more constant environment removes a need to expend energy to maintain a constant body temperature.”