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Here's Why You Get Diarrhea When You're Sick
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Nobody likes diarrhea. But is the icky and uncomfortable experience actually the body's way of flushing bad stuff out of your system?

In a new study in mice, researchers set out to answer the question of whether diarrhea is simply a symptom of an illness or, instead, a way for the body to quickly get rid of germs.

In fact, diarrhea's purpose — or lack thereof — has been the subject of much scientific debate.

"The hypothesis that diarrhea clears intestinal pathogens has been debated for centuries," senior study author Dr. Jerrold Turner, a professor of pathology at Brigham and Women's Hospital in Boston, said in a statement. But the role that diarrhea plays in the progression of intestinal infections "remains poorly understood." [The Poop on Pooping: 5 Misconceptions Explained]

It's not uncommon for the symptoms that accompany an illness to have a positive effect. For example, a fever happens because the body is trying to speed up the immune responses that will kill germs. And when someone's eyes water, the body is trying to flush away anything that shouldn't be there.

In the new study, the researchers "sought to define the role of diarrhea and to see if preventing it might actually delay pathogen clearance and prolong disease," Turner said. In other words, could using certain medicines to prevent diarrhea make an illness worse?

To study the role of diarrhea, the researchers infected mice with a bacterium called Citrobacter rodentium, which is the mouse equivalent of Escherichia coli and then studied what went on in the animals' intestines. The researchers found that within two days of infection, the permeability of the walls of the mice's intestines increased, meaning that more water and other molecules could flow into the intestines. (When a person or animal has diarrhea, the poop is very watery.)

Importantly, the researchers found that this increase in permeability happened before the walls of the intestines became inflamed and damaged by the infection, which suggests that this increased permeability helps to defend the gut, as opposed to being the result of gut damage. Indeed, the researchers also found that the influx of water into the intestine, and then out of the body in the form of feces, helped clear the germs out of the gut and ultimately limited the severity of the diarrhea.

Two molecules were involved in the changes that the researchers observed in the mice. One was interleukin-22, which is an immune molecule that signals cells to increase their levels of the other molecule, called claudin-2 has been shown in earlier studies of diarrhea to increase the permeability of the intestinal wall.

In fact, some researchers have proposed making drugs that could inhibit claudin-2 in order to help prevent diarrhea. But the new findings suggest that blocking this molecule could prolong an infection, the researchers wrote. Increased levels of this molecule and increased gut permeability "are essential to host defense," they wrote.

Because the study was done in mice, more research is needed to confirm the results in humans.

The study was published on June 14 in the journal Cell Host & Microbe

Originally published on Live Science.