The overall human body temperature continues to drop. What is going on?

As a warm-blooded animal, we all know that our normal body temperature is 37 degrees. Some people are curious about how 37 degrees came about. In fact, it was measured by Carl Reinhold August Wunderlich in 1851.

However, in 2020, researchers at Stanford University in the United States found that over the past century, normal human body temperature has been declining. By analyzing more than 677,000 temperature data collected since 1860, the researchers found that compared with the end of the 19th century, men and women had dropped by 0.59°C and 0.32°C, respectively.

Why does body temperature drop? Today we will explain that the overall temperature drop may be caused by: reduced infection, decreased basal metabolism, and ambient temperature.

01. Observed decrease in body temperature

In 1851, a German doctor named Carl Reinhold August Wunderlich measured the body temperature of subjects in Leipzig, Germany for millions of times, and calculated for the first time that the average normal human body temperature is 37°C. Since then, body temperature has been used as a key vital sign in clinical judgment of health status.

In fact, not only humans, but many mammals have a body temperature around 37°C, so much so that some people are wondering why it is this temperature, is it chosen by God?

However, American researchers have found that human body temperature appears to be decreasing [1].

This time they included a total of 3 groups:

1. Union Army veterans of the American Civil War (N = 23,710; measured between 1860 and 1940) – blue

2. National Health and Nutrition Examination Survey I (N = 15,301; 1971-1975) – Green

3. Stanford Translational Research Integrated Database Environment (N = 150,280; 2007-2017) – Orange

From the above picture, we can see several pieces of information.

1. The icon is divided into male and female, with the upper half of the chart representing males and the lower half representing females.

2. Body temperature decreases over time (from top to bottom)

This can be seen in the male graph, where the curve for the Civil War soldiers (blue) is higher than that for the 1970s, and both are higher than the temperatures in the early 21st century.

Of course, you can see it in the female image as well, but without the Civil War soldier curves.

3. Body temperature tends to decrease with age (from left to right).

It can be seen that with age, body temperature gradually decreases, which is generally believed to be caused by a decrease in metabolic rate.

02. Why is it declining?

Temperature regulation is one of the basic survival lines of the human body. Do not adjust body temperature unless necessary.

But if the overall human body temperature drops, then that must mean there are major changes.

1. Immune issues: Reduced infections.

Recently, the new coronavirus has infected many people. It is estimated that many people have felt the impact of this wave of epidemic, and one of the most important impacts is: fever . The purpose of hoarding ibuprofen or acetaminophen is mainly to reduce fever.

Infection and fever are the body's basic regulatory mechanisms.

A review in Nature in 2015 mentioned the immune heat signaling pathway, especially the important role of cytokine IL-6 and the heat-related adrenaline signaling pathway. Fever is a basic immune response of the body to infection and a strong protective mechanism of the body. A large amount of evidence shows that a limited increase in the body's core temperature is beneficial for the treatment of infection. For example, an increase in body temperature is a basic response to infection and immune diseases, and it is a protective mechanism of the body formed after millions of years of natural selection. A large amount of evidence shows that during fever caused by many types of infections, the core temperature increases by 1 to 4 ° C, and the patient's survival rate after infection can be improved, while the use of antipyretics to reduce fever will increase the mortality rate of influenza patients by 5%.

And lower body temperatures in humans are generally associated with fewer infections.

You know, in the early days, humans basically lacked the means to deal with infectious diseases. However, with the emergence of antibiotics such as penicillin represented by Fleming, humans have important means to fight infectious diseases, thereby reducing the possibility of large-scale overall infection. This is a huge contribution.

For example, the picture below shows the situation of tuberculosis. Over time, global infections have decreased.

Similar situations exist in many infectious diseases, which is also the result of the improvement of human overall health.

2. Decreased basal metabolism

Resting metabolic rate is the largest component of energy expenditure for a typical modern person, and heat is a byproduct of metabolism. It is currently believed that the overall metabolic rate of humans may have decreased [2].

3. Ambient temperature

The ambient temperature will also affect the body temperature. The most intuitive reason is that the human body needs to exchange heat with the outside world. If the ambient temperature fluctuates, the body's metabolism will also fluctuate. In fact, in order to maintain a constant temperature, the human body uses 50-70% of its daily calories for this purpose.

In the 19th century, heating in the United States was irregular, and people often had to shiver to keep warm. With the change of heating, especially the invention of air conditioning, many people have better ambient temperature systems, thus reducing metabolic needs.

Of course, the author does not mention whether larger global climate change also has an impact.

1 Myroslava Protsiv, Catherine Ley, Joanna Lankester, Trevor Hastie, Julie Parsonnet (2020) Decreasing human body temperature in the United States since the Industrial Revolution eLife 9:e49555

2 Frankenfield, David, et al. "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review." Journal of the American Dietetic association105.5 (2005): 775-789.