Researchers Find Surprising New Reason for Long COVID—Here’s What to Know
Long COVID symptoms, like brain fog, may be caused by a reduction in serotonin levels, per new research.
Researchers also found that the reason behind long COVID neurological symptoms could start in the gut.
Experts explain the findings.
Nearly one in five adult COVID-19 survivors have long COVID, but experts are still not conclusive on what actually causes the long-term string of complications. Now, new research shows a possible explanation as to why some long COVID patients experience brain fog and other cognitive deficits: a reduction in serotonin levels.
A study published in Cell sheds new light on the mechanisms of how persistent inflammation after contracting the SARS-CoV-2 virus (the virus that causes COVID-19) can cause long-term neurological symptoms. Researchers evaluated the effects of long COVID in blood and stool samples from various clinical studies and in small animal models.
According to the news release from Penn Medicine, the researchers determined that a subset of patients with long COVID had traces of the SARS-CoV-2 virus in their stool samples even months after a COVID-19 infection. This suggests that components of the virus remain in the gut of some patients long after infection. Researchers found that this remaining virus triggers the immune system to release proteins that fight the virus, called interferons. These interferons cause inflammation that reduces the absorption of the amino acid tryptophan in the gastrointestinal (GI) tract.
Tryptophan is a building block for several neurotransmitters, including serotonin, which is primarily produced in the GI tract and carries messages between nerve cells in the brain and throughout the body. It plays a key role in regulating memory, sleep, digestion, wound healing, and other functions that maintain homeostasis within the body. Serotonin is also an important regulator of the vagus nerve, a system of neurons that mediate the communication between the body and the brain.
The researchers discovered that when the virus remains in the gut long after the original COVID infection, tryptophan absorption is reduced and therefore, serotonin is depleted. This leads to a disruption in vagus nerve signaling, which in turn can cause several of the symptoms associated with long COVID, such as memory loss, brain fog, etc.
What are the symptoms and signs of long COVID?
Long COVID is a group of symptoms that persist well after a COVID infection has resolved, says Amit Sachdev, M.D., director of the Division of Neuromuscular Medicine at Michigan State University. “This can include many neurologic symptoms such as fatigue, lightheadedness, headache, and sensory loss,” he explains. Unfortunately, Dr. Sachdev notes, many patients with long COVID are unable to return to their previous level of function.
Long COVID is an umbrella term for a constellation of persistent symptoms that individuals experience at least three months after infection, adds infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.
Why do patients with long COVID exhibit a reduction in serotonin?
There is a hypothesis that some individuals may harbor SARS-CoV-2 in their gastrointestinal tract and its presence disrupts the absorption of a key precursor to serotonin, says Dr. Adalja. “This deficit in serotonin could explain some of the symptoms of long COVID in a subset of patients,” he adds.
Many of the symptoms of long COVID are central or brain symptoms, says Dr. Sachdev. “This includes mental fogginess, mental fatigue, and slowing of the body movements (psychomotor retardation),” he notes. There is some overlap between these symptoms of long COVID and the sorts of symptoms we can see in states such as depression, says Dr. Sachdev.
It is not clear why serotonin reduction occurs, but improving serotonin levels may be beneficial in the right patient, adds Dr. Sachdev.
Could anti-depressants treat long COVID?
Right now, it’s unclear whether SSRI (anti-depressant) treatment may be helpful, says Dr. Adalja, “Clinical trials could answer this question.”
As of now, given that it is not clear why long COVID occurs, it would not be wise to attempt to prevent it with SSRI medications, says Dr. Sachdev.
The bottom line
This is an early study that offers a hypothesis about a subset of long COVID, says Dr. Adalja. “Further studies will be needed to further [explain this connection],” he says. For those with long COVID, more is being done to understand it, says Dr. Sachdev.
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