People are — quite literally — getting colder, and new research may help explain why

The average body temperature of us humans has dropped since 1860. New research now suggests that this has something to do with what’s going on in our gut.

This is the researchers’ conclusion after examining data from sepsis patients, among others. Their results can be read in the journal American Journal of Respiratory and Critical Care Medicine.

It’s one of the first things doctors measure when reporting an inflammatory complaint to a hospital, for example: your temperature. “Temperature matters,” researcher Kale Bongers stresses. “You can easily measure it and the temperature provides more information about both the metabolic state and the inflammatory state in the body.”

Normal body temperature is officially around 37 degrees. But in practice, core temperature still varies greatly from person to person. We know that people today are generally “cooler” than they used to be. Since 1860, the average human body temperature has decreased.

Search for explanations
The reason for the differences we see when we compare the core temperatures of healthy people living today is shrouded in mystery. And researchers can’t yet properly explain the decline in average body temperature since 1860. Reason enough for scientists to get their teeth into it. And they did this by focusing on patients with sepsis. Sepsis is a very violent reaction of the human body to bacteria, viruses, fungi or parasites. This reaction is often accompanied by a drastic change in body temperature. For example, some patients develop a fever. In other patients, the temperature actually drops. “We know that in sepsis the change in temperature is important because it gives a good indication of who survives and who dies,” said researcher Robert Dickinson. But why one patient has a high temperature and another has a low temperature is also not clear.

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The relationship between intestinal microbiota and temperature
To get more information about this, the scientists analyzed rectal swabs taken from 116 patients with sepsis. The swabs revealed which bacteria could be found in the intestines of these patients. The first thing that struck me was that the intestinal flora of these patients was incredibly different. “Our patients have more diversity in their microbiomes than in their genes,” Bongers says. For each of the two patients, their genomes overlapped by over 99%, while they showed literally 0% overlap in their gut bacteria. But — and now it gets interesting — it turns out that differences in gut flora are related to a patient’s body temperature.

Very concretely, for example, bacteria belonging to the strain were found packages associated with fever in sepsis patients. We’re talking about bacteria that feel at home in the human gut, but the extent to which they appear in people’s gut varies from person to person.

experiments on mice
The data tentatively suggested that the bacteria in the gut — at least in sepsis patients — influence body temperature. The researchers decided to take a closer look by conducting a number of experiments on mice. They worked with “normal” mice and their genetically identical counterparts that — unlike “normal” mice — did not harbor any bacteria in their gut. The mice in both groups were then subjected to sepsis. Remarkably, the body temperature of “normal” mice underwent drastic changes, while the body temperature of mice without a microbiota changed much less.

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But even when the mice were still healthy, the mice without gut flora were shown to have lower basal temperatures than their conventional counterparts. Meanwhile, antibiotic treatment — which also alters the number of bacteria in the gut — has been found to lower the body temperature of “normal” mice. The researchers concluded that there are several different indicators that the bacteria in the gut influence body temperature.

Change in diet and hygiene
With this, the scientists also shed new light on the mystery of hypothermia. “Although we have not yet demonstrated that changes in the microbiome can explain the decrease in our body temperature, we think this is a reasonable hypothesis,” Bongers said. “Our genetic material hasn’t changed much in the past 150 years, but changes in diet, hygiene, and antibiotics have a huge impact on our gut flora.”

However, much more research is needed. Not least with regard to sepsis patients. For example, the researchers want to see if it is possible to modify the body temperature of these patients — through alterations in the microbiome — and thus increase their chances of survival.

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