Analyses and surveys suggest that the average human body temperature is now lower than the 37°C standard established more than 150 years ago.
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, says a Medical News Today report, 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 a 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.
To test their hypothesis, the researchers analysed 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.
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; and 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 at Stanford University School of Medicine, and the senior author of the study. “What everybody grew up learning, which is that our normal temperature is (37°C), is wrong.” However, because gender, time of day, and age can each change our body temperature, the researchers do not advise updating the standard for all US adults.
So why has the average body temperature changed? “Physiologically, we’re just different from what we were in the past,” 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, 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.
In the US, the normal, oral temperature of adults is, on average, lower than the canonical 37°C established in the 19th century. We postulated that body temperature has decreased over time. Using measurements from three cohorts—the Union Army Veterans of the Civil War (N = 23,710; measurement years 1860–1940), the National Health and Nutrition Examination Survey I (N = 15,301; 1971–1975), and the Stanford Translational Research Integrated Database Environment (N = 150,280; 2007–2017)—we determined that mean body temperature in men and women, after adjusting for age, height, weight and, in some models date and time of day, has decreased monotonically by 0.03°C per birth decade. A similar decline within the Union Army cohort as between cohorts, makes measurement error an unlikely explanation. This substantive and continuing shift in body temperature—a marker for metabolic rate—provides a framework for understanding changes in human health and longevity over 157 years.
Myroslava Protsiv, Catherine Ley, Joanna Lankester, Trevor Hastie, Julie Parsonnet