Posted on Jan 13, 2020, 8 a.m.
A recent analysis of temperature trends suggests the average human body temperature has dropped since the 19th century because of physiological changes and the researchers suggest potential causes in their paper published in the journal eLife.
Most people don’t even really think about temperature unless they are not feeling well, but body temperature can be influenced by other factors such as lifestyle habits, age, and ambient temperature. Body temperature is a marker of metabolic health, and according to the authors it indicates metabolic rate which has been linked to longevity and body size in some research.
In 1851 a survey conducted by Carl Reinhold August Wunderlich involving 25,000 people in one city established that 37 degrees Celsius or 98.6 degrees Fahrenheit is the standard temperature of the human body. But recent analyses and surveys suggest the average temperature is now lower, as a study of over 35,000 people in the UK and close to 250,000 temperature measurements found 36.6 degrees Celsius is now the average oral temperature.
Myroslava Protsiv formerly of Stanford University's Division of Infectious Disease and Geographic Medicine investigated if the study had discrepancies or if the findings reflect a higher life expectancy and better overall health; the team hypothesized that “the differences observed in temperature between the 19th century and today are real and that the change over time provides important physiologic clues to alterations in human health and longevity since the Industrial Revolution."
To test the hypothesis information was analyzed from three datasets: the first included data from 1862-1930 obtained from medical records of Union Army veterans of the Civil War; the second was from the United States Health and Nutrition Examination Survey I from 1971-1975; and the third was from the Stanford Translational Research Integrated Database Environment containing data from those who received healthcare at Stanford between 2007-2017. This study had access to 677,423 temperature measurements overall which were integrated to form a model of change over time.
According to the researchers, their findings suggest that the body temperature of men today on average is 0.59 degrees Celsius less than it was in the early 19th century; the average body temperature of women has also dropped by 0.32 degrees Celcius from 1890- 2017, and overall there was a 0.03 degree Celsius drop in average temperature with every decade.
The researchers looked at changes within the datasets to check whether the decreases stemmed from advances in thermometer technology, assuming that in each historical period the same types of thermometers were used. According to the research team their findings held true and the results of the analysis within datasets does reflect the changes in the combined data.
"Our temperature's not what people think it is," says Dr. Julie Parsonnet, a professor of medicine, health research, and policy, and the senior author of the study. "What everybody grew up learning, which is that our normal temperature [37°C/98.6°F], is wrong."
“Physiologically, we're just different from what we were in the past," Dr. Parsonnet says. "The environment that we're living in has changed, including the temperature in our homes, our contact with microorganisms, and the food that we have access to. All these things mean that, although we think of human beings as if we're monomorphic and have been the same for all of human evolution, we're not the same. We're actually changing physiologically."
Parsonnet believes the average metabolic rate has declined over time, and this decrease could result from a reduction in inflammation: "Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature," she says. Additionally heating and air conditioning have resulted in a more consistent ambient temperature that makes it unnecessary to expend as much energy as was once required to maintain the same body temperature.
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