Weird Drop in Human Body Temperatures May Have an Explanation

Scientists think they may have finally discovered the reason why human body temperature has been decreasing over the past few centuries—gut microbes.

A paper published in the American Journal of Respiratory and Critical Care Medicine found that in patients with sepsis—a major and potentially fatal infection—the body temperature was directly linked to the variety of species in their gut microbiome. Some species make the patients more effective at increasing their body temperature.

This may help scientists explain the strange dip in human body temperatures seen over the past 150 years.

A 2020 study in the journal eLife showed that on average, the human body has dropped by 0.05 F every ten years since the 1800s, with men born in the early to mid-1990s having an average body temperature 1.06 F lower than that of men born in the early 1800s. A similar pattern was found in women, with the body temperature of women born in the early to mid-1990s being on average 0.58 F lower than that of women born in the 1890s.

From the 116 people with sepsis involved in the most recent study, the researchers found that these microbiome-driven variations in the temperature of their body also indicated their clinical outcome. Essentially, their temperature, and therefore their gut microbiome, gave clues to whether they would survive.

"We know that temperature response is important in sepsis because it strongly predicts who lives and who dies," microbiologist and immunologist Robert Dickson from the University of Michigan, said in a statement.

"But we don't know what drives this variation and whether it can be modified to help patients."

Fever is an immune response to infection. The body is attempting to kill the virus or bacteria that is causing the infection by raising its internal temperature to a point where the microbe cannot properly function. Additionally, the body's immune system works better at higher temperatures.

Therefore, this research indicates that the presence of certain species in the gut microbiome makes the immune system more efficient at fighting infection.

The researchers also did the same tests on mice, and found that a similar pattern of gut microbiota affecting temperature response to infection was present.

"Among conventional mice, heterogeneity of temperature response in sepsis was strongly explained by variation in gut microbiota," the authors wrote in the paper.

The presence of bacteria of the Firmicutes phylum, also known as Bacillota, in a patient's gut microbiome was found to be the most strongly associated with increased fever response and therefore higher body temperatures. These bacteria produce metabolite chemicals that can influence a person's immune response and metabolism.

The patients involved in the study had Firmicutes bacteria present in varying numbers, with those possessing the bacteria having higher temperatures and faring better against the infection.

"Arguably, our patients have more variation in their microbiota than they do in their own genetics," Kale Bongers, a clinical instructor at the University of Michigan Department of Internal Medicine and lead author of the study, said in the statement. "Any two patients are more than 99 percent identical in their own genomes, while they may have literally 0 percent overlap in their gut bacteria."

These findings were also consistent with the mice experiments: a variation in the mice's temperature response was strongly correlated with the same bacterial type, Firmicutes.

"We found that the same kind of gut bacteria explained temperature variation both in our human subjects and in our laboratory mice," said Dickson. "This gave us confidence in the validity of our findings and gives us a target for understanding the biology behind this finding."

The authors note in the statement that more research is needed to understand if changing the microbiome to change the body's ability to increase its temperature and fight infection could be key to improving the survival for patients with sepsis.

"The gut microbiome is a key modulator of body temperature variation both in health and critical illness, and is thus a major, understudied target for modulating physiologic heterogeneity in sepsis," the paper's authors wrote.

The study also gives clues as to the role of the gut microbiome in affecting body temperature, and therefore may be part of the explanation for the strange and as yet unexplained decrease in global body temperature over the past 150 years.

"While we certainly haven't proven that changes in the microbiome explain the drop in human body temperature, we think it is a reasonable hypothesis," said Bongers. "Human genetics haven't meaningfully changed in the last 150 years, but changes in diet, hygiene, and antibiotics have had profound effects on our gut bacteria."

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