Many of us are familiar with this scenario: we get a nasty stomach bug, and suddenly rush to the toilet with a bad case of vomiting and diarrhea. The runs are often caused by a stomach virus or a meal that didn't sit well with us, but we're not sure why this happens. Now, a team of researchers from Brigham and Women’s Hospital in Massachusetts have found diarrhea serves a purpose when we're feeling ill — it helps clear the bacteria that's causing the infection.

"The hypothesis that diarrhea clears intestinal pathogens has been debated for centuries," said co-author Jerrold Turner, MD, PhD, of the BWH Departments of Pathology and Medicine, in a statement.

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In the study, published in Cell Host and Microbe, Turner and his colleagues found infecting mice with an intestinal bacterium, known as Citrobacter rodentium, which is similar to E.coli, led them to produce proteins that caused microscopic leaks in the intestinal wall. These leaks allowed an excess of water to enter the intestines, which made the stool watery, and as a result, limited disease severity. This suggests proteins in the body can help promote infection clearance via diarrhea.

The researchers concluded the two proteins involved in causing diarrhea include claudin-2 and interleukin-22 — both produced by the human body.

In the experiment, the researchers tested three different types of mice — regular mice; genetically modified mice that produced large amounts of the protein claudin-2; and mice that didn't make any claudin-2. When the mice became infected, immune cells traveled to the intestinal wall, and first released interleukin-22. This protein bound itself to the cells in the intestines, which led to the release of claudin-2. This reaction started the leak that let more water in, resulting in diarrhea.

The regular mice had diarrhea when they got sick, and the mice that made more claudin-2 always had diarrhea, even when they were healthy. Those that didn't make claudin-2 experienced more severe injuries to their intestinal lining, but they still had diarrhea since the immune system attacked the cells to produce a watery stool.

This means claudin-2 seems to be an important component of the cause of diarrhea. The researchers believe the interaction between claudin-2 and interleukin-22 may be vital in fighting infections, particularly during the early stages of disease. However, it's unknown whether this immune mechanism is important in combating all infections, and if it could be applied to humans.

Evidently this study warrants further research on humans to see what implications, if any, these findings could have on our health. But, it does suggest diarrhea is good for the body. This is the body's way of telling us it's getting rid of infection, and lost nutrients need to be replaced.

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Minerals like sodium and potassium that are lost with diarrhea should be replaced with mineral-rich foods. Moreover, it's best to drink fluids during the day, and less at meals. This will prevent malnutrition, and slowly help restore firmer stool.

So, next time you get the runs, just remember you'll feel relieved - eventually. 

Source: Tsai PY, Zhang B, He WQ et al. L-22 upregulates epithelial claudin-2 to drive diarrhea and enteric pathogen clearance. Cell Host & Microbe. 2017.

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