For decades, people have considered the average human body temperature to be 98.6 F, or 37 degrees Celsius. However, a new study published January 7, 2020, in the journal eLife indicates this may no longer be the norm – and the researchers suggest a reason for their observed drop in average temperature.
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From where did 98.6 F come?
In 1851, a German physician named Carl Reinhold August Wunderlich, who had an amazing eye for detail and a passion for research collected literally millions of temperature readings from about 2,500 patients in Leipzig, Germany. He screened healthy and sick patients alike, and observed how temperatures fluctuated in health and sickness, as well as with body weight, height, age and between the sexes. Astonishingly accurate, his work led to the establishment of 98.6 F as the official human body temperature in a healthy individual. Even now, the temperature is among the basic parameters recorded by physicians as part of a fundamental health assessment.
Currently it is established that the body temperature can change by 0.5 F (0.2 degrees Celsius) over a single day. Older people generally have a lower temperature than younger, and men than women, though female temperatures vary with the time of month as well. Of course, physical exertion, the weather conditions, and the food eaten, all have significant influences on the body temperature.
However, the reason for the body’s efforts to keep the temperature at about 98.6 F is part of the homeostatic mechanism. In other words, the body is constantly and actively seeking to keep the life processes that are running in the background to maintain the hugely complex chemistry and biology of life, as well as to deny access to fungi. This keeps the organs of the body and the chemical processes in stable operation.
The new study shows that this ideal temperature sought by the human physiology may have shifted away from 98.6 F. In fact, since the 19th century, the average American has had a steady drop in the body temperature by about 0.05 F (0.02 C) per decade. Thus, for those who were born in the early years of the 21st century, the average body temperature is about 1.06 F (0.58 C) lower than those in the corresponding years of the 19th century. For women, the gap is about 0.58 F (0.32 C).
To nail down the change, the current study retrieved data from multiple sources, including the American Civil War, the 1970s and the early years of this century. Armed with all this information, the researchers separated the temperature readings for analysis. They had over 670,000 measurements at the end!
As they classified the readings, they found that the average temperature was going down steadily over the years. They wondered if the improved sensitivity of the thermometers in use in later years might possibly account for this. To exclude this, they looked at the set of data from the Civil War period, where the same group was being monitored for decades, probably with the same kind of thermometers. They found the same tendency to cooling, though the thermometers being used by the three groups were widely different.
The conclusion they drew was that human beings have changed their physiological functioning over time, and part of that is a colder body temperature.
An English study in 2017 based on 35,000 subjects, in whom the temperature was measured about 250,000 times, also showed a decrease in the average from 98.6F, at about 97.88 F or 36.6 degrees Celsius.
The researchers admit that they don’t yet know much about how this very slow cooling down will affect human health or lifespan. However, says Parsonnet, this trend plays a significant part in a larger pattern: modern humans are not so vulnerable to infections as our forefathers were in the days of Wunderlich.
Researcher Dr. Julie Parsonnet thinks she knows the reason why. She has specialized for years in the study of the organism Helicobacter, which colonizes the human stomach. While normally harmless, it can also cause esophageal, gastric and intestinal ulcers. However, the current trend in Helicobacter infections shows a steady decline. Says Parsonnet, “I became aware, because I worked on it for 30 years, that that organism is disappearing from populations in the United States.”
In the 1800s, she points out, people were constantly being attacked by malaria, wounds and injuries, tuberculosis, dental infections and dysentery, in an unending succession for the most part. This would lead to a lot of inflammatory processes going on over the average human’s lifespan. This in turn results in a higher level of cytokines, or inflammatory cell signaling molecules. These cause an enhanced rate of metabolism, that makes the body hotter.
On the other hand, infectious disease plays a much more minor part in today’s disease spectrum. Advances in potable water availability and quality, coupled with immunization and antibiotics, have caused many such infectious conditions to come under control. This would inevitably lead to a significant change in the functioning of the human body over the decades.
Another factor they suggest is the modern access to effective temperature regulatory devices such as fans, air conditioners and the like. As a result, they say, humans no longer need to make much effort to keep themselves balanced with respect to body temperature. In the US, notes Parsonnet, “It’s always 70 F (21.1 C) in our houses.”
However, this doesn’t hold good in the vast majority of the earth’s peoples. The study says that there may be a large range of body temperatures that are normal for the healthy body, depending on the place where people stay. In Pakistan, for instance, the average temperature is still about 98.6 F, as studied in 2008. On the other hand, there is no obvious correlation between a temperature rise and body functioning.
Parsonnet acknowledges that cooling might alter the way micro-organisms live and operate, but there is no data on this question right now. On the macroscopic human scale, health status changes are shown only by radical shifts in body temperature such as a fever or hypothermia.
Admittedly descending into the realm of speculation, Parsonnet goes on to hint that as medicine continues to find new and better ways to manage inflammatory and infectious conditions, the average body temperature may keep falling as the lifespan becomes longer. Calling the body temperature "a marker of inflammatory state, “ she says, “If you can take the temperature of a population, you might be able to predict their life expectancy, she says, sharing her expectation that someday, both life expectancy and body temperature will likely level off and remain consistent into the future. On the other hand, there might not be a real “healthy number” for individual temperatures. If you feel hotter than usual, and your temperature is above 100.4 F, you might think you were feverish, but otherwise, fluctuations in temperature aren’t really abnormal.
Decreasing human body temperature in the United States since the industrial revolution, Myroslava Protsiv, Catherine Ley, Joanna Lankester, Trevor Hastie, Julie Parsonnet, eLife 2020;9:e49555 DOI: 10.7554/eLife.49555, https://elifesciences.org/articles/49555