Is human body temperature changing? 

With the dawn of the COVID-19 pandemic, body temperature has been catapulted to the forefront of public attention. Checking body temperature used to be conducted when you had reason to believe somebody was ill. Now, it has become a routine requirement for everyday activities such as entering workplaces, schools, or even travelling abroad. Regular temperature checking has become the new normal. However, the common belief that 'normal' body temperature is 98.6°F has recently been questioned and found to be outdated. Surprisingly, the human body appears to have changed over the last 160 years.

How did 98.6°F become normal? 

So where did this specific temperature come from? Well, in 1851 a German doctor, Carl Reinhold August Wunderlich, took the temperature of 25,000 patient’s armpits, finding it to lie between 97.2- 99.5 °F, with 98.6°F concluded as the average. This figure has been considered normal ever since. That is until now… New research suggests that the average body temperature is in fact lower.

Is human temperature decreasing?

A recent research study published in ELife looked at human temperatures across three periods of time. 1860–1940 (24,000 Civil War veterans), 1971–1975 (15,000 people in the National Health and Nutrition Examination Survey), and 2007–2017 (150,000 people in the Stanford Translational Research Integrated Database Environment). The findings of this extensive study show that the temperature of the average American now seems to run more than one degree lower - closer to 97.5°F! These results are supported by a study, which found that the average temperature of 35,000 British people was 97.9°F, again much lower than previously considered normal.

Why is the average body temperature less than it used to be?

There are several reasons why researchers speculate body temperature has decreased. For a start, technology has improved and thermometers are more accurate than they used to be. People have access to warm clothes and controlled home heating/cooling systems. This means that our bodies are less likely to experience temperature extremes. When the body faces extreme heat or cold, it increases the metabolic system to cope with the situation, and it is thought that increasing the metabolic system increases the body temperature. A lower metabolic rate is also thought to be linked to a higher body mass and better overall health, both of which are more common in today's society.

The effect of infection on body temperature.

It must also be taken into consideration that Wunderlich’s study measured temperature when the average life expectancy was just 38 years old. Infections such as tuberculosis and chronic inflammatory conditions affected large numbers of the population. In today's generation, people are much less likely to develop an infectious disease. There is a general decline in infection and inflammation, and inflammation is what increases our metabolism, which also raises body temperature. This theory is supported by research conducted over recent years on indigenous people who live in Bolivia. The 2016 study identified that high levels of infection led to high metabolism rates and that lower metabolism was associated with lower body temperature. Interestingly, a small study was conducted in Pakistan, where there is a high incidence of tuberculosis and other chronic infections. The results of this study showed that the participants had an average temperature much closer to the original 98.6°F identified by Wunderlich. When looking at the various research studies conducted over recent years, the change in inflammation and infection levels seems to be the most viable explanation for the decline seen in body temperature.

Basically, our bodies have evolved and are generally healthier, as has our ability to cope with illness and extremes of temperature. Overall, life expectancy has increased, we are less likely to experience infection or inflammation, and are in more control of the surrounding environment.

Don't forget - a fever is still a fever.

Despite this new evidence, experts are all in agreement about one fact - a fever is still a fever and this definition should not be changed. The Centers for Disease Control and Prevention defines a fever as a temperature of 100.4 °F or above. It is a symptom that helps to recognize the presence of disease, inflammation, and other problems within the human body. Temperature can help to determine how unwell you are or how you are responding to treatment. Therefore, it is important to remember that any temperature above 100.4°F is a fever and needs treating as advised by a medical professional.

What does this all mean?

Well, there is no need for a decline in average body temperature to cause alarm. At present, it is merely an interesting development, and further research is needed to gain a better understanding of the implications. The parameters of what is considered a 'normal' temperature may change over time, but the definition of fever remains extant and should always be treated as a warning sign.

References:

Adhi M, Hasan R, Noman F, Mahmood SF, Naqvi A, Rizvi AU. Range for normal body temperature in the general population of Pakistan. J Pak Med Assoc. 2008;58(10):580-584. Cdc.gov. 2020. Definitions Of Symptoms For Reportable Illnesses | Quarantine | CDC. [online] Available at: [Accessed 10 July 2020]. Gurven, M., Trumble, B., Stieglitz, J., Yetish, G., Cummings, D., Blackwell, A., Beheim, B., Kaplan, H. and Pontzer, H., 2016. High resting metabolic rate among Amazonian forager-horticulturalists experiencing high pathogen burden. American Journal of Physical Anthropology, 161(3), pp.414-425. Obermeyer, Z., Samra, J. and Mullainathan, S., 2017. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ, p.j5468. Protsiv, M., Ley, C., Lankester, J., Hastie, T. and Parsonnet, J., 2020. Decreasing human body temperature in the United States since the Industrial Revolution. eLife, 9. Wunderlich, C., Seguin, E., Wunderlich, C. and Seguin, E., 1871. Medical Thermometry, And Human Temperature. New York: Wood.

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