A group of scientists is putting forth a novel hypothesis to account for certain instances of long Covid . Their research, featured in a recent publication in the journal Cell, indicates that individuals experiencing the complex condition exhibit lower levels of serotonin. The study, conducted at the University of Pennsylvania, posits that this serotonin depletion may be linked to lingering traces of the virus in the gut. This reduction in serotonin could particularly elucidate memory issues and certain neurological and cognitive symptoms associated with long Covid, according to their findings.
Numerous recent studies have unveiled distinct biological changes occurring in individuals with long Covid, providing crucial insights into a condition that manifests in diverse ways and often eludes detection through standard diagnostic tools like X-rays.
These findings hold promise for potential treatments, potentially involving medications designed to enhance serotonin levels. The authors of the study suggest that their research could establish a common biological pathway connecting various theories about the origins of long Covid, including the persistence of viral remnants, inflammation, increased blood clotting, and autonomic nervous system dysfunction.
Christoph Thaiss, one of the study’s lead authors and an assistant professor of microbiology at the Perelman School of Medicine at the University of Pennsylvania, remarked, “All these different hypotheses might be connected through the serotonin pathway,” and he added, “Even if not everybody experiences difficulties in the serotonin pathway, at least a subset might respond to therapies that activate this pathway.”
Akiko Iwasaki, an immunologist at Yale University, described the study as “an excellent study that identifies lower levels of circulating serotonin as a mechanism for long Covid.” She mentioned her own research, which, along with colleagues at the Icahn School of Medicine at Mount Sinai, identified other biological changes associated with some long Covid cases, including fluctuations in the hormone cortisol. These studies may hint at specific subtypes of long Covid or different biological indicators at various stages of the condition.
The study examined 58 long Covid patients, ranging from three to 22 months post-infection, and compared their blood samples with those of 30 individuals with no post-Covid symptoms and 60 patients in the acute stage of coronavirus infection.
Maayan Levy, a lead author and assistant professor of microbiology at the Perelman School of Medicine, noted that serotonin levels, along with other metabolites, were altered immediately following a coronavirus infection, a pattern observed after other viral infections. However, in long Covid patients, serotonin was the only significant molecule that failed to return to pre-infection levels.
The research team also analyzed stool samples from some long Covid patients and detected residual viral particles. By combining these findings with research involving mice and miniature human gut models, where serotonin is primarily produced, the team identified a potential pathway underlying certain long Covid cases.
The proposed mechanism suggests that viral remnants trigger the production of interferons by the immune system, leading to inflammation that impairs the body’s ability to absorb tryptophan, an amino acid essential for serotonin production in the gut. Blood clot formation following a coronavirus infection may further hinder serotonin circulation. Depleted serotonin disrupts the vagus nerve system, responsible for transmitting signals between the body and the brain, potentially leading to memory problems and cognitive issues common in long Covid patients.
While the study offers valuable insights, it is important to note that it was not extensive, and its findings should be confirmed through additional research. Some long Covid studies with patients experiencing milder symptoms did not consistently show depleted serotonin, suggesting that this may occur primarily in individuals with multiple severe symptoms.
The next step in understanding long Covid involves identifying biomarkers, measurable biological changes to aid in diagnosis. The study suggests three potential biomarkers: the presence of viral remnants in stool, low serotonin levels, and elevated interferon levels. Most experts believe that a single biomarker for long Covid may not exist, as multiple indicators could emerge, varying based on symptoms and other factors. instances of long Covid
Addressing the pressing need for effective long Covid treatments, several clinical trials are currently underway. Dr. Levy and Dr. Thaiss plan to initiate a clinical trial testing fluoxetine, a selective serotonin reuptake inhibitor often marketed as Prozac, and potentially tryptophan. Dr. Levy expressed hope that supplementing serotonin or preventing its degradation might help restore vagal signals and improve memory, cognition, and other related aspects of the condition.