Pathophysiology: Tryptophan and Serotonin Metabolism

1 Kavyani et al. 2022 — Kynurenine Pathway Review

Full Citation:: Kavyani B, Lidbury BA, Schloeffel R, et al. Could the kynurenine pathway be the key missing piece of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) complex puzzle? Cellular and Molecular Life Sciences. 2022;79(8):412. DOI:: 10.1007/s00018-022-04380-5 PMID:: 35821534 PMCID:: PMC9276562 Type:: Comprehensive review

Key Findings: This review proposes the kynurenine pathway (KP) as a unifying mechanism in ME/CFS pathophysiology. Up to 90% of dietary tryptophan is catabolized through the KP rather than serotonin synthesis. Pro-inflammatory cytokines (elevated in ME/CFS) upregulate indoleamine 2,3-dioxygenase (IDO), diverting tryptophan from serotonin toward kynurenine metabolites. Key consequences include: (1) serotonin depletion contributing to mood and sleep disturbances; (2) quinolinic acid accumulation causing neurotoxicity and excitotoxicity; (3) reduced kynurenic acid decreasing neuroprotection; (4) NAD+ depletion via quinolinic acid-induced PARP activation, contributing to mitochondrial energy deficits.

Relevance: Provides mechanistic link between immune activation, neurotransmitter abnormalities, and energy metabolism dysfunction—three core domains of ME/CFS pathophysiology. The KP model explains symptom clusters including fatigue (NAD+ depletion), cognitive dysfunction (neurotoxicity), mood disturbances (serotonin depletion), and immune dysregulation (cytokine-IDO axis). Suggests therapeutic targets: IDO inhibitors, NAD+ precursors (nicotinamide riboside), kynurenine aminotransferase activators.

Certainty Assessment:

  • Quality: High (comprehensive review from Macquarie University ME/CFS group; lead author Guillemin is president of International Society for Tryptophan Research)
  • Limitations: Review article synthesizing evidence; direct KP metabolite measurements in ME/CFS limited; animal model validation needed
  • Replication: Multiple independent groups have shown tryptophan/kynurenine alterations in ME/CFS

2 Abujrais et al. 2024 — Metabolomic Analysis

Full Citation:: Abujrais S, Vallianatou T, Bergquist J. Untargeted Metabolomics and Quantitative Analysis of Tryptophan Metabolites in Myalgic Encephalomyelitis Patients and Healthy Volunteers: A Comparative Study Using High-Resolution Mass Spectrometry. ACS Chemical Neuroscience. 2024;15(19):3525–3534. DOI:: 10.1021/acschemneuro.4c00444 PMID:: 39269261 PMCID:: PMC11450765 Study Design:: Case-control metabolomics (n=38 ME/CFS, n=24 controls)

Key Findings: Using high-resolution mass spectrometry, researchers from Uppsala University’s ME/CFS Collaborative Centre found significantly lower 3-hydroxykynurenine (\(p\)=0.003) and 3-hydroxyanthranilic acid (\(p\)=0.021) in ME/CFS patients. Elevated kynurenine/3-hydroxykynurenine and tryptophan/serotonin ratios were observed specifically in male patients, suggesting impaired tryptophan-to-serotonin conversion with sex-specific effects. Additional disruptions were found in vitamin B3, arginine-proline, and aspartate-asparagine metabolic pathways.

Relevance: Most recent (2024) quantitative evidence of tryptophan pathway dysregulation in ME/CFS. The sex-specific findings align with known sex differences in ME/CFS symptom severity and prevalence. Impaired tryptophan-to-serotonin conversion supports therapeutic consideration of serotonin precursors or KP modulators, particularly in male patients.

Certainty Assessment:

  • Quality: High (rigorous metabolomics methodology; established ME/CFS research center)
  • Sample: Moderate size (n=62 total); larger studies needed for subgroup analyses
  • Limitations: Cross-sectional design; dietary tryptophan intake not controlled; replication needed

3 Simonato et al. 2021 — Tryptophan and Cytokines

Full Citation:: Simonato M, Dall’Acqua S, Zilli C, et al. Tryptophan Metabolites, Cytokines, and Fatty Acid Binding Protein 2 in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Biomedicines. 2021;9(11):1724. DOI:: 10.3390/biomedicines9111724 PMID:: 34829954 PMCID:: PMC8615774 Study Design:: Case-control (serum analysis)

Key Findings: Lower serum kynurenine and serotonin with higher 3-hydroxykynurenine in ME/CFS patients compared to controls. Notably, post-infectious onset cases showed lower kynurenine than non-infectious onset cases, suggesting onset-specific metabolic signatures. Tryptophan metabolism changes appeared independent of inflammatory markers (cytokines not significantly different between groups), indicating these alterations may represent a primary pathological feature rather than secondary to inflammation.

Relevance: Distinguishes metabolic profiles between infection-triggered and gradual-onset ME/CFS, supporting disease subtyping. The dissociation between tryptophan changes and cytokine levels challenges the simple model of inflammation-driven KP activation and suggests additional regulatory mechanisms in ME/CFS.

Certainty Assessment:

  • Quality: Moderate to High (peer-reviewed case-control study with metabolite quantification)
  • Limitations: Sample size not specified in abstract; onset-type comparison may be underpowered; dietary controls unclear
  • Replication: Findings consistent with Abujrais 2024 and broader tryptophan dysregulation literature

4 Lee et al. 2024 — Central Serotonin Hyperactivity

Full Citation:: Lee JS, Kang JY, Park SY, et al. Central 5-HTergic hyperactivity induces myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS)-like pathophysiology. Journal of Translational Medicine. 2024;22:14. DOI:: 10.1186/s12967-023-04808-x PMID:: 38178138 PMCID:: PMC10773012 Study Design:: Translational (mouse model + human validation)

Key Findings: First experimental evidence demonstrating that central serotonin hyperactivity can induce ME/CFS-like symptoms. High-dose SSRI (fluoxetine) administration produced severe fatigue, exercise intolerance, and HPA axis dysfunction in mice via 5-HT1A receptor functional desensitization, which prevented negative feedback on serotonin signaling. Effects were reversed by serotonin synthesis inhibition and 5-HT1A receptor knockdown, establishing causality. Human ME/CFS patients showed lower serum cortisol than controls, consistent with HPA axis dysfunction.

Relevance: Provides critical experimental validation of the “hyper-serotonergic hypothesis” in ME/CFS. The apparent paradox of central serotonin hyperactivity alongside peripheral serotonin depletion (Simonato et al. 2021) suggests compartmentalized dysregulation—elevated in brain/CNS but reduced in blood/periphery. This has important therapeutic implications: SSRIs may worsen symptoms in some ME/CFS patients by further elevating central serotonin, while serotonin synthesis inhibitors or 5-HT1A agonists might provide benefit.

Certainty Assessment:

  • Quality: High (rigorous experimental design with reversal experiments establishing causality)
  • Limitations: Animal model may not fully recapitulate human ME/CFS; human validation limited to cortisol measurement
  • Clinical Implication: Suggests caution with SSRIs in ME/CFS; potential for 5-HT1A-targeted therapies

5 Dehhaghi et al. 2022 — Kynurenine and NAD+ Metabolism

Full Citation:: Dehhaghi M, Kazemi Shariat Panahi H, Kavyani B, et al. The Role of Kynurenine Pathway and NAD+ Metabolism in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Aging and Disease. 2022;13(3):698–711. DOI:: 10.14336/AD.2021.0824 PMID:: 35656108 PMCID:: PMC9116917 Type:: Review

Key Findings: KP hyperactivation diverts tryptophan from serotonin synthesis, contributing to mood disturbances. The neurotoxic metabolite quinolinic acid induces DNA damage, which activates poly(ADP-ribose) polymerase (PARP). PARP activation consumes NAD+, leading to NAD+ and consequently ATP depletion—providing a direct mechanistic link between tryptophan metabolism and the profound fatigue of ME/CFS. Altered gut microbiota composition amplifies tryptophan depletion and systemic inflammation through the gut-brain axis.

Relevance: Links three major pathophysiological domains: tryptophan metabolism, energy production, and gut microbiome. The KP → quinolinic acid → PARP → NAD+ depletion cascade provides a testable mechanism for ME/CFS fatigue. Authors recommend clinical trials of NAD+ precursor supplementation (nicotinamide riboside, nicotinamide mononucleotide) based on this mechanistic model.

Certainty Assessment:

  • Quality: High (comprehensive review from established ME/CFS tryptophan metabolism research group)
  • Limitations: Review article synthesizing evidence; direct experimental validation of PARP-NAD+ depletion pathway in ME/CFS needed
  • Clinical Translation: NAD+ precursor trials are feasible and low-risk; preliminary evidence exists from aging research