Comfortable lives at constant ambient temperature contribute to a lower metabolic rate

Monday, January 20th, 2020

Since the early 19th century, the average human body temperature in the United States has dropped, according to a study by researchers at the Stanford University School of Medicine:

That standard of 98.6 F was established by German physician Carl Reinhold August Wunderlich in 1851. Modern studies, however, have called that number into question, suggesting that it’s too high. A recent study, for example, found the average temperature of 25,000 British patients to be 97.9 F.


Parsonnet and her colleagues analyzed temperatures from three datasets covering distinct historical periods. 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. A set from the U.S. National Health and Nutrition Examination Survey I contains data from 1971 to 1975. Finally, the Stanford Translational Research Integrated Database Environment comprises data from adult patients who visited Stanford Health Care between 2007 and 2017.

The researchers 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 2000s is on average 1.06 F lower than that of men born in the early 1800s. Similarly, they observed that the body temperature of women born in the 2000s is on average 0.58 F lower than that of women born in the 1890s. These calculations correspond to a decrease in body temperature of 0.05 F every decade.


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. The authors 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,” 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.

The authors also hypothesize that 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.


  1. Kirk says:

    Does make you wonder, though… Are the differences due to improved thermometers, different technique, or something else that the data gatherers are doing?

    Second issue is this: What, precisely, is the decrease in body temperature doing for our resistance to disease? Are there diseases tuned that closely to our baseline body temperatures, and how will they be affected by this “global human cooling” phenomenon? Will there be diseases that become less virulent, or more? Does it even make a difference?

  2. Felix says:

    They pick “inflammation” as a first choice to explain the temperature drop. People back in the day had a lot of tuberculosis, pneumonia, dental problems, etc.

    I wondered whether clothing could do the trick. But they found a down-slope inside what seems like a shorter, older time period — a period probably unaffected by modern, scant clothing.

    They figured they handled thermometer differences and such-like.

    One of the blog comments points out that 98.6°F is 37°C, exactly, i.e. presumably a rounded number.

  3. TRX says:

    Feynman talked about how Milliken’s figure for the speed of light was found to be wrong, but instead of correcting it, textbooks “fudged” it a little at a time over decades, until it became the modern commonly-accepted number.

    I also get a distinct whiff of Anthopogenic Global Warming…

    An obvious thing would be to measure the average temperatures of demographics who are unlikely to have been affected by “modern living.” Rural India, Russia, and China would have medical records going back far enough to be useful, and they’d be on paper, which has to be “interpreted” instead of outright falsified…

    Yes, I *do* have major problems with “reproducibility crisis” BS…

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