Medical News Today: Body temperature: What is the new normal?

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Most of us only take our temperatures when we are worried that we have a fever, as a result of an infection or a cold, for example.

But body temperature can indicate and be influenced by many other factors; lifestyle habits, age, and ambient temperature can all influence how our body disperses heat.

Body temperature is also a marker of metabolic health. Specifically, the authors of the new study explain, human body temperature indicates metabolic rate, which some have linked with longevity and body size.

So what is our normal body temperature? In 1851, a German physician called Carl Reinhold August Wunderlich surveyed 25,000 people in one city and established that 37°C is the standard temperature of the human body.

However, recent analyses and surveys suggest that the average body temperature is now lower.

For instance, a study of more than 35,000 people in the United Kingdom and nearly 250,000 temperature measurements found that 36.6°C is the average oral temperature. Could this discrepancy be a result of changes in measurement tools? Or, do the new findings reflect higher life expectancy and better overall health?

Myroslava Protsiv, then at Stanford University’s Division of Infectious Diseases and Geographic Medicine, in California, and colleagues set out to investigate.

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.”

Their paper appears in the journal eLife.

Studying historic trends in body temperature

To test their hypothesis, the researchers analyzed information from three datasets:

The first included data from 1862–1930 obtained from Union Army veterans of the Civil War.

The second dataset was from the United States National Health and Nutrition Examination Survey I, which took place from 1971–1975.

The third dataset was from the Stanford Translational Research Integrated Database Environment, which contains data from people who received healthcare through Stanford between 2007 and 2017.

Overall, the scientists had access to 677,423 temperature measurements, which they integrated, forming a model of change over time.

New normal body temperature is cooler

Some of the researchers’ findings include:

• The body temperature of men today is, on average, 0.59°C lower than that of men born in the early 19th century.
• Similarly, women’s body temperature dropped by 0.32°C from the 1890s to today.
• Overall, the analysis found a 0.03°C decrease in average temperature with every decade.

To check whether the decreases stemmed from advances in thermometer technology, Protsiv and the team looked at changes within datasets, assuming that doctors in each historical period were generally using the same types of thermometers.

The results of the analysis within datasets reflected 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 is [37°C], is wrong.”

Dr. Julie Parsonnet

However, because gender, time of day, and age can each change our body temperature, the researchers do not advise updating the standard for all U.S. adults.

What has caused our temperature to drop?

So why has the average body temperature changed? “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.”

Furthermore, Dr. Parsonnet believes, the average metabolic rate, which indicates how much energy our bodies use, has declined over time. This decrease could result from a decrease in inflammation.

“Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” she says.

Finally, air conditioning and heating have resulted in a more consistent ambient temperature, making it unnecessary to expend energy to maintain the same body temperature.