Medical Research

Long covid, the gut and vagus nerve

Kristen Sparrow • October 31, 2023

This NPR transcript, which you can read below,  says that scientists think that there may be some corona virus that remains in the gut causing the “Brain Fog” symptoms of Long Covid.  The cause of the brain fog, is a reduction in serotonin.  The virus, or pieces of the virus, may be leading to inflammation that interferes with serotonin production. Serotonin is the target of many anti-depressants, called Selective Serotonin Reuptake Inhibitors, or SSRI’s.” We discussed Long covid    and autonomic activity.  Also  here.  Acupuncture and Covid here.  Long covid and TAVNS here.

they focussed on a cytokine called type 1 interferon, revealing that this signaling protein was driving inflammation and interfering with serotonin levels in the bloodstream in several ways.

The gut produces 90% of serotonin in the body. The amino acid tryptophan is critical to this task — it’s a precursor to serotonin and gets absorbed in the gastrointestinal tract from the food we eat. Except, this inflammatory response in the gut actually impaired the absorption of tryptophan.

Something I did NOT know was that the gut is responsible for 90% of serotonin in the body!!  This only reinforces the wisdom of ancient Chinese medicine, that all systems affect each other.  We know that inflammation plays a key role in depression, and we also know that altered autonomic activity increases inflammation.  But here we have the mind-blowing reality that the gut and its vagal activity can actually affect serotonin production.  Amazing.  I wish I could add that to my book!

Scientists have made significant progress in understanding the neurological symptoms of long COVID, such as brain fog, memory loss, and fatigue. They discovered that low serotonin levels in the body could predict whether someone would experience persistent symptoms after a COVID-19 infection.

Researchers at the University of Pennsylvania found that the depletion of serotonin might be linked to long COVID symptoms. They suggested a pathway starting in the gut, traveling through the vagus nerve, and affecting the brain, leading to serotonin reduction. Serotonin is crucial for cognitive function and mood regulation.

Their experiments, mostly conducted on mice, indicated that chronic viral infections, similar to what might occur in long COVID patients, reduced serotonin levels. The immune response played a role by causing inflammation and interfering with serotonin absorption in the gut.

The researchers also found that serotonin reduction impaired communication between the vagus nerve and the brain, affecting the hippocampus, a brain region involved in memory and cognition. Fortunately, cognitive symptoms observed in mice could be reversed by reactivating the vagus nerve or restoring serotonin signaling, although the applicability to humans remains uncertain.

The study suggests that immune problems outside the nervous system can have far-reaching consequences on the brain and other functions post-COVID-19. However, the research doesn’t provide a complete explanation for all long COVID symptoms, as there may be various mechanisms at play.

The findings have prompted scientists to explore the serotonin pathway as a potential target for clinical trials and interventions for long COVID patients. However, further research is needed to confirm these findings in humans and to develop effective treatments.

While the study sheds light on one aspect of long COVID, the condition remains complex and heterogeneous, with multiple manifestations and mechanisms. Researchers continue to work on unraveling the mysteries of long COVID to develop better diagnostics and treatments.