E2 ModerateModerate confidencePEM not requiredCross-SectionalPeer-reviewedMachine draft
Tryptophan catabolites, inflammation, and insulin resistance as determinants of chronic fatigue syndrome and affective symptoms in long COVID.
Al-Hakeim, Hussein Kadhem, Khairi Abed, Anwar, Rouf Moustafa, Shatha et al. · Frontiers in molecular neuroscience · 2023 · DOI
Quick Summary
This study looked at 90 Long COVID patients and measured specific molecules in their blood related to how their body processes an amino acid called tryptophan. The researchers found that patients with the most severe Long COVID symptoms had low tryptophan levels and high levels of a related compound called kynurenine, along with markers of inflammation. These three measurements together explained about 40% of why patients had fatigue, pain, depression, and anxiety symptoms.
Why It Matters
This research is relevant to ME/CFS because it identifies a potential biological mechanism—tryptophan depletion and inflammatory activation—that could explain why post-viral patients develop persistent fatigue, pain, and mood symptoms. Understanding these shared molecular pathways may help researchers develop targeted treatments and biomarkers for severe Long COVID and potentially inform ME/CFS research directions.
Observed Findings
- Severe Long COVID patients (22% of cohort) had significantly lower tryptophan and higher kynurenine levels compared to milder cases.
- The three biomarkers (CRP, KYN/TRY ratio, and insulin resistance) together explained approximately 40% of variance in physiosomatic and affective symptoms.
- Peak body temperature and reduced oxygen saturation during acute COVID-19 predicted both symptom severity and kynurenine/tryptophan ratio at follow-up.
- All symptom domains (chronic fatigue, fibromyalgia-like pain, depression, anxiety) loaded onto a single latent factor, suggesting they share common biological origins.
- Patients with severe Long COVID showed very high C-reactive protein and insulin resistance alongside depleted tryptophan.
Inferred Conclusions
- The physio-affective phenome of Long COVID (fatigue, pain, depression, anxiety as one phenomenon) is driven substantially by inflammatory responses initiated during acute infection and sustained thereafter.
- Tryptophan pathway dysfunction, reflected in low tryptophan and elevated kynurenine metabolites, contributes meaningfully to the symptom burden in severe Long COVID.
- Severity of acute infection (measured by fever and hypoxia) predicts both biomarker abnormalities and Long-term symptom burden, suggesting acute phase severity is a prognostic marker.
Remaining Questions
- Does tryptophan depletion play a causal role in symptoms, or is it an epiphenomenon of inflammation? Does this mechanism apply to ME/CFS cases without documented preceding COVID infection?
What This Study Does Not Prove
This study does not prove that tryptophan depletion causes Long COVID symptoms; it shows correlation only. The cross-sectional design cannot establish temporal sequence or causality, and the findings describe Long COVID specifically, not necessarily ME/CFS or other chronic fatigue conditions. The biomarkers explained only 40% of symptom variance, leaving 60% unexplained by mechanisms not measured here.
Tags
Symptom:Cognitive DysfunctionPainFatigue
Biomarker:CytokinesMetabolomicsBlood Biomarker
Phenotype:Infection-TriggeredSevereLong COVID Overlap
Method Flag:PEM Not DefinedWeak Case DefinitionNo Controls
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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