Remnants of the virus that causes COVID-19 may linger in the gut, ultimately causing a reduction in circulating levels of serotonin. This may explain a number of long COVID-19 symptoms, such as brain fog and memory problems, according to new research from scientists at the University of Pennsylvania in Philadelphia.
To conduct the study, researchers looked at blood and stool samples from various clinical studies and animal studies. A subset of patients who were experiencing long COVID-19 had traces of the SARS-CoV-2 virus in their stool samples months after acute infections, indicating remnants of the virus lingered in their gut.
Researchers found the viral persistence triggers the immune system to fight the virus off, which causes inflammation. The inflammation in turn reduces the absorption of tryptophan, an amino acid that is a building block of serotonin, in the GI tract.
"When tryptophan absorption is reduced by persistent viral inflammation, serotonin is depleted, leading to disrupted vagus nerve signaling, which in turn can cause several of the symptoms associated with long COVID, such as memory loss," Penn Medicine said in a news release on the findings.
The study, published Oct. 16 in Cell, may connect many of the major hypotheses surrounding the causes of long COVID-19, such as lingering virus components, inflammation and increased blood clotting, the lead study author told The New York Times.
"All these different hypotheses might be connected through the serotonin pathway," Christoph Thaiss, PhD, assistant professor of microbiology at the Perelman School of Medicine at the University of Pennsylvania, told the news outlet. "Second of all, even if not everybody experiences difficulties in the serotonin pathway, at least a subset might respond to therapies that activate this pathway."