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Tr5.5
Tryptophan Research
Likely helps
154 peer-reviewed studies
What the evidence says
Likely helps
Tryptophan appears to help in 5 of 7 studies with measurable effects — the evidence leans clearly favourable.
Most evidence is from high-quality meta-analyses and randomised trials published 1970–2026 with a typical study size of 66 participants.
Based on 154 studies · 12 meta-analyses · 108 RCTs · 25,367 total participants
Confidence
High confidence
What the studies found
5helped2unclear· 147 more without graded effect data
By outcome
Depression & moodImproved mood through serotonin support · 1-2 weeks
Patients with major depressive disorder with melancholic and psychotic features1
Osteoarthritis patients1
Active research area
56 studies in the last 5 years · Latest meta-analysis: 2026
197019982026
1Depressive disorder symptomsMeta-AnalysisCited 1×n=17,437 · very large study2025
This extensive systematic review and NMA of nutraceuticals for treating depressive disorders indicated a number of nutraceuticals that could offer benefits, either as adjuncts or monotherapies.
Cheng YC et al. · Psychological medicine (2025)
Adjunctive nutraceuticals consistently showed better efficacy than antidepressants (ADT) alone in outcomes including SMD, remission, and response.
This extensive systematic review and NMA of nutraceuticals for treating depressive disorders indicated a number of nutraceuticals that could offer benefits, either as adjuncts or monotherapies.
2Tryptophan and metabolite levels in severe depressionMeta-AnalysisCited 23×n=4,647 · very large study2022
Patients with affective disorders with melancholic and psychotic features and suicidal behaviors showed normal IDO enzyme activity but a lowered availability of plasma/serum TRP to the brain, which is probably due to other processes such as low albumin levels.
Almulla AF et al. · Cells (2022)
Large benefit
← WorseNo effectBetter →
Likely real
Severe patients showed significant lower (p < 0.0001) TRP (standardized mean difference, SMD = -0.517, 95% confidence interval, CI: -0.735; -0.299) and TRP/CAAs (SMD = -0.617, CI: -0.957; -0.277) levels with moderate effect sizes, while no significant difference in CAAs were found.
Kynurenine (KYN) levels were unaltered in severe MDD/BD phenotypes, while the KYN/TRP ratio showed a significant increase only in patients with psychotic features (SMD = 0.224, CI: 0.012; 0.436).
Quinolinic acid (QA) was significantly increased (SMD = 0.358, CI: 0.015; 0.701) and kynurenic acid (KA) significantly decreased (SMD = -0.260, CI: -0.487; -0.034) in severe MDD/BD.
3Differential metabolites identificationMeta-AnalysisCited 13×n=495 · medium study2023
The main differential metabolites between OA and healthy subjects were amino acids and their derivatives, including tryptophan, lysine, leucine, proline, phenylalanine, glutamine, dimethylglycine, citrulline, asparagine, acetylcarnitine and creatinine, which could be potential biomarkers for predicting OA.
Liao Z et al. · Nutrients (2023)
In the 13 studies, 132 kinds of small molecule differential metabolites were extracted, 58 increased, 57 decreased and 17 had direction conflicts.
Among them, 37 metabolites appeared more than twice.
At least 13 studied genes with polymorphisms were involved in MDD development according to MF and BP, but not significantly.
Suktas A et al. · BMC psychiatry (2024)
A similar result was observed for BDNF rs6265 GG (OR = 1.26; 95% CI = 0.78-2.06; P = 0.35) and BDNF rs6265 AA genotypes (OR = 1.12; 95% CI = 0.77-1.64; P = 0.56).
At least 13 studied genes with polymorphisms were involved in MDD development according to MF and BP, but not significantly.
These results suggest that MDD development risk factors might require genetic and other factors for interaction and induction.
5Mood and emotional functioning improvementSystematic ReviewCited 44×2021
In order to estimate the optimum amount of TRP intake more accurately, further studies need to be conducted with more appropriate settings of intake period, intake frequency, and intake method.
Kikuchi AM et al. · Journal of dietary supplements (2021)
This suggests that TRP intake may be an effective approach to decrease anxiety and increase positive mood in healthy individuals.
On the other hand, the effectiveness of TRP for aggressive feelings was not recognized.
Reviewing these 11 RCTs, we concluded that taking 0.14-3 g of TRP per day in addition to the usual meal can be expected to improve the mood of healthy individuals.
The TRYCAT pathway appears to be downregulated in BD patients.
Hebbrecht K et al. · Frontiers in immunology (2021)
Noticeable benefit
← WorseNo effectBetter →
Likely real
Peripheral levels of tryptophan (SMD = -0.44; p < 0.001), kynurenine (SMD = - 0.3; p = 0.001) and kynurenic acid (SMD = -.45; p = < 0.001) were lower in BD patients versus healthy controls.
The TRYCAT pathway appears to be downregulated in BD patients.
There is a need for more and high-quality studies of peripheral and central TRYCAT levels, preferably using longitudinal designs.
7Association between serotonin and depressionSystematic ReviewCited 496×n=17 · very small study2023
This meta-analysis examined the effects of Tryptophan.
Moncrieff J et al. · Molecular psychiatry (2023)
The main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations.
Some evidence was consistent with the possibility that long-term antidepressant use reduces serotonin concentration.
Conclusions: Gut microbiota dysbiosis may contribute to anxiety vulnerability in perimenopausal women through interconnected immune, metabolic, and neuroendocrine mechanisms.
Dietary modulation of the intestinal microbiota represents a biologically plausible and low-risk complementary approach to support emotional well-being during this transitional period.
However, these findings should be interpreted cautiously due to substantial heterogeneity and limited control for key confounders such as antipsychotic medication, diet, and life-style factors.
Militaru AM, Pietreanu AC, Trifu S, Popa GL. · International journal of molecular sciences (2026)
Meta-analyses revealed no statistically significant differences in alpha diversity indices (Shannon, Simpson, Chao1, ACE, Observed) between patients and controls, despite high heterogeneity.
Taxonomic synthesis identified recurrent but heterogeneous dysbiotic patterns characterized by the depletion of short-chain fatty acid-producing taxa (e.g., Faecalibacterium , Roseburia , Lachnospiraceae) and enrichment of pro-inflammatory taxa (e.g., Proteobacteria, Fusobacterium ).
Schizophrenia is associated with evidence of compositional alterations and functional shifts rather than a global loss of microbial richness.
The goal for clinical research is to explore options for TRP-targeted therapies and their integration into new therapeutic strategies.
Holeček M · Nutrients (2026)
The final sections are devoted to the benefits and adverse effects of TRP supplementation, the therapeutic use of various TRP metabolites, and the pharmacological targeting of enzymes, transporters, and receptors involved in TRP catabolism.
It is concluded that all pathways of TRP catabolism are altered across a broad spectrum of human illnesses, and further investigation is needed to understand their role in disease pathogenesis better.
The goal for clinical research is to explore options for TRP-targeted therapies and their integration into new therapeutic strategies.
Exploring the mechanisms linking tryptophan metabolism to embryo implantation and decidualization may pave the way for novel treatments for reproductive disorders.
Liu Z et al. · Tissue & cell (2026)
New evidence supports a close relationship between tryptophan metabolism and embryo implantation; the levels or ratios of tryptophan metabolites, such as kynurenine, serotonin, and melatonin, significantly correlate with various decidualization markers.
In this review, we summarize the latest research progress on tryptophan and its metabolites in embryo implantation and their effects on decidualization.
Exploring the mechanisms linking tryptophan metabolism to embryo implantation and decidualization may pave the way for novel treatments for reproductive disorders.
In addition to the roles of the KP in the pathogenesis of the abovementioned diseases, the related advancements in treatment are discussed.
Zhang Q et al. · Reviews in the neurosciences (2026)
The kynurenine pathway (KP), a crucial route of tryptophan (TRP) catabolism, has emerged as a focal point of investigation because of its complex involvement in regulating central nervous system (CNS) function.
This metabolic pathway, which operates in various cell types within the CNS, closely links immune responses, neurotransmission, and neuroinflammation.
In addition to the roles of the KP in the pathogenesis of the abovementioned diseases, the related advancements in treatment are discussed.
16Sleep and circadian disruption effects on microbiomeReview2026
We highlight directions for future mechanistic research, particularly in translational models, to clarify causal pathways and support microbiome-informed strategies for mitigating the health consequences of sleep and circadian disruption.Published by Elsevier Ltd.
Maki KA et al. · Neuroscience and biobehavioral reviews (2026)
Across studies, consistent links emerged between sleep and circadian disruption and changes in host homeostasis through microbiome-mediated mechanisms.
These include activation of the hypothalamic-pituitary-adrenal axis, altered bile acid metabolism, inflammation-related microbial shifts, and disrupted tryptophan pathways.
Together, these findings suggest that altered sleep influences metabolic and immune function via gut microbial signaling pathways.
The "paradox" is therefore not that Trp changes chemistry across settings, but that the same nutrient is routed through different cellular contexts, enzymes, ligands, and cell states.
Cho A et al. · International journal of molecular sciences (2026)
We also argue that the potential metabolite biomarker interpretation should be context-dependent.
Finally, we propose a clinical-context-specific framework for intervention.
Dietary and microbiome-based strategies may be most effective in prevention, premalignant states, or supportive care, whereas established cancers are more likely to require biomarker-guided targeting of tumor-associated catabolic pathways and convergent signaling mechanisms.
This systematic-review examined the effects of Tryptophan.
Zhang Y et al. · Cell communication and signaling : CCS (2026)
Finally, we propose a conceptual "diet-microbiota-drug" triad to guide precision interventions, and discuss current challenges such as interindividual variability, the lack of standardized assessment tools, and the need for integrative multi‑omics and clinical validation.
A deeper mechanistic understanding of gut-organ crosstalk may pave the way for innovative therapies to restore systemic metabolic homeostasis.
Broader application of this research could ultimately refine personalized psychiatry, expand therapeutic horizons, and contribute to global mental health resilience.
Chehadi AC et al. · International journal of molecular sciences (2026)
Special emphasis is placed on translational potential, methodological limitations, and the need for harmonized research frameworks.
Here we highlight that phytochemical interventions represent a mechanistically informed and biocompatible strategy for advancing depression management.
By bridging neurobiology and clinical psychiatry, these insights may pave the way for next-generation therapeutics that integrate dietary, microbiota-targeted, and anti-inflammatory approaches.
The KP may play a key role in the pathophysiology of CS-associated behaviors, and could serve to highlight potential targets for novel therapeutic strategies, specifically for treatment resistance.
de Bartolomeis A, Fornaro M, Scopetta E, Ricci C, Irano A, De Simone G, Comai S, Iasevoli F, Caiazza C. · European neuropsychopharmacology : the journal of the European College of Neuropsychopharmacology (2025)
We performed a systematic review and meta-analysis following a pre-determined protocol (PROSPERO:CRD42023459414), considering KP metabolites and enzymes as outcomes (Standardized Mean Difference=SMD).
CS proved to increase hippocampal and cortical overall concentrations of kynurenine (SMD=1.71,95 %C.I.[0.99,2.42], p < 0.01,I 2 =91.3 %,k = 23,n = 508; SMD=1.54,95 %C.I.[1.08,2.01],p < 0.01,I 2 =57.77 %,k = 16,n = 247), as well as kynurenine/tryptophan ratio and quinolinic acid.
KP enzyme expression/activity, including Indoleamine-2,3-Dioxygenase, Tryptophan-2,3-Dioxygenase, Kynurenine-3-Monoxygenase, increased.