L-Tryptophan
Evidence Fact Sheet
L-Tryptophan is an essential amino acid and precursor to serotonin, melatonin, and (via the kynurenine pathway) NAD+. Research is mixed: meta-analytic signals for sleep quality and mood in healthy adults, but limited and inconclusive evidence in clinical depression. Permitted as a food ingredient/amino acid; no authorized health claim in the EU.
Also known as: Tryptophan · L-Trp · TRP
Overview
L-Tryptophan (L-Trp) is an essential amino acid obtained from dietary protein. It is the metabolic precursor for serotonin and melatonin synthesis, and it also feeds NAD+ production through the kynurenine pathway, which underlies its mechanistic interest for mood, sleep, and metabolic research. In supplement form it is typically studied across a dose range from the RDA (about 5 mg/kg/day) up to roughly 500–2000 mg/day for mood and sleep contexts, with caution noted around concurrent SSRI/MAOI use (serotonin-syndrome risk). Regulatory status: FDA GRAS as an amino-acid food ingredient (post-1995 reinstatement); EFSA rejected mood claims under Reg 432/2012 with no authorized health claim; ANVISA permits it within the dietary-supplement framework; China permits it as a GB 14880/GB 2760 nutritional fortifier. This page reports research findings in studied populations and is not clinical guidance.
Mechanism of Action
De novo NAD+ via kynurenine pathway (60 mg Trp -> 1 mg niacin equivalent) · Serotonin precursor · Melatonin precursor
Body systems: CNS · METABOLISM
Evidence-Based Benefits
Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — L-Tryptophan is not characterized as a treatment for any disease.
Sleep Quality
Meta-analysis supported- -81.03 min/gwake after sleep onset · P=0.017
- SMD -1.0895%CI -1.89 to -0.28
A meta-analysis and meta-regression found that L-tryptophan supplementation shortened wake after sleep onset, with a larger effect at doses of 1 g or more. However, the analysis found no effect on other sleep components, so the benefit appears specific to time spent awake after falling asleep rather than overall sleep architecture.
Reported effect: Trp supplementation can shorten wake after sleep onset (-81.03 min/g, P = 0.017; SMD, -1.08 min [95%CI, -1.89 to -0.28]); group receiving >=1 g had shorter WASO than <1 g (56.55 vs 28.91 min; P = 0.001)
“Trp supplementation can shorten wake after sleep onset (-81.03 min/g, P = 0.017; SMD, -1.08 min [95%CI, -1.89 to -0.28]) ... The group receiving >=1 g Trp supplementation had a shorter wake after sleep onset than the group with Trp < 1g supplementation (Trp <1 g vs Trp >=1 g: 56.55 vs 28.91 min; P = 0.001) ... However, Trp supplementation did not affect other sleep components.”
Source: PMID 33942088 · Sutanto 2022 · Nutr Rev
Mood in Healthy Adults
Meta-analysis supported- 11 RCTssystematic review pool
- 4 RCTssignificant mood effect
- 0.14-3 g/daystudied dose range
A systematic review of randomized controlled trials in healthy adults found that, among 11 eligible RCTs, four showed significant effects of tryptophan intake on negative and happy feelings versus control. Effects on aggressive feelings were not seen, and the authors framed the benefit as decreasing anxiety and increasing positive mood in healthy individuals rather than treating any disorder.
Reported effect: 11 RCTs met the criteria; four RCTs showed significant effects on negative feelings and happy feelings vs control; effectiveness for aggressive feelings was not recognized
“As a result, 11 RCTs met the criteria and were accepted. Four RCTs showed the effects of tryptophan intake on negative feelings and happy feelings in healthy individuals, with significant differences between the treatment and the control groups ... the effectiveness of TRP for aggressive feelings was not recognized ... 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.”
Source: PMID 32272859
Depression (Clinical)
Meta-analysis supported- OR 4.10vs placebo · 95%CI 1.28-13.15
- 2 trialsmet quality criteria
- 64 patientstotal pooled
This Cochrane review of 5-HTP and tryptophan for depression pooled only two trials of sufficient quality and found the substances were better than placebo at alleviating depression. The authors stressed that the evidence was of insufficient quality to be conclusive, so this is a positive but low-certainty signal.
Reported effect: Peto Odds Ratio 4.10 (95% CI 1.28-13.15; RD 0.36; NNT 2.78) from 2 trials totalling 64 patients; evidence of insufficient quality to be conclusive
“Only two trials, involving a total of 64 patients, were of sufficient quality to meet inclusion criteria ... The available evidence suggests these substances were better than placebo at alleviating depression (Peto Odds Ratio 4.10; 95% confidence interval 1.28-13.15; RD 0.36; NNT 2.78) ... However, the evidence was of insufficient quality to be conclusive.”
Source: PMID 11869656 · Shaw 2002 · Cochrane Database Syst Rev
Kynurenine Pathway in Depression (Biomarker)
Null / no benefit Meta-analysis supportedThis meta-analysis of clinical studies examined circulating kynurenine-pathway metabolites in mood disorders. Peripheral tryptophan was non-significantly reduced in major depression versus healthy controls (the confidence interval crossed zero), while kynurenine itself was significantly lower. This characterizes pathway alterations in depression and does not test tryptophan supplementation.
Effect size: not quantified on this page — see the linked study below for the reported figures.
Source: PMID 29940237 · Arnone 2018 · Neurosci Biobehav Rev
Adjunctive Use with Antidepressants
Null / no benefit Meta-analysis supportedA systematic review and meta-analysis of nutraceuticals as add-ons to antidepressants concluded that current evidence supports adjunctive SAMe, methylfolate, omega-3, and vitamin D to reduce depressive symptoms. Amino-acid precursors such as tryptophan were notably not among the supported adjuncts, so this is an honest negative for that specific use. No standalone tryptophan effect size was reported in the abstract.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“Current evidence supports adjunctive use of SAMe, methylfolate, omega-3, and vitamin D with antidepressants to reduce depressive symptoms.”
Source: PMID 27113121 · Sarris 2016 · Am J Psychiatry
Dosage (research context · not a recommendation)
5 mg/kg/day RDA · 500-2000 mg/day mood/sleep (caution SSRI interaction · Tier C · Arnone 2018 PMID 29940237 kynurenine pathway mood SR+MA)
Regulatory Status · 4 Markets
- US · FDA
- GRAS (amino acid food ingredient · post-1995 reinstatement)
- EU · EFSA
- Reg 432/2012 rejected mood claims 2010 · no authorized health claim
- CN · China
- China: essential amino acid permitted as GB 14880 / GB 2760 nutritional fortifier/food-additive in food and supplement formulas; no tryptophan-specific health-food functional claim approved.
- BR · ANVISA
- RDC 243/2018 dietary supplement framework · IN 28/2018 no Anexo V claim
Safety
Drug interaction with SSRI/MAOI (serotonin syndrome risk) · pregnancy generally safe at RDA · 1989 EMS outbreak associated with contaminated supplement (not pure tryptophan)
Related
Goals: cognitive-support
Lifestyles: high-stress
References
PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.
- PMID 33942088 · Sutanto 2022 · Nutr Rev — Sleep Quality
- PMID 32272859 — Mood in Healthy Adults
- PMID 11869656 · Shaw 2002 · Cochrane Database Syst Rev — Depression (Clinical)
- PMID 29940237 · Arnone 2018 · Neurosci Biobehav Rev — Kynurenine Pathway in Depression (Biomarker)
- PMID 27113121 · Sarris 2016 · Am J Psychiatry — Adjunctive Use with Antidepressants
Frequently Asked Questions
1. Does tryptophan actually improve sleep?
A meta-analysis (Sutanto 2022, PMID 33942088) found L-tryptophan shortened wake after sleep onset (SMD -1.08), with a larger effect at doses of 1 g or more, but it did not affect other sleep components. So the evidence points to less time awake after falling asleep rather than a broad change in sleep architecture.
2. Is tryptophan proven for clinical depression?
Not conclusively. A Cochrane review (Shaw 2002, PMID 11869656) pooled only two quality trials (64 patients) and found 5-HTP and tryptophan better than placebo (OR 4.10), but the authors stated the evidence was of insufficient quality to be conclusive. A separate nutraceutical meta-analysis (Sarris 2016, PMID 27113121) did not list tryptophan among the supported add-ons to antidepressants.
Last evidence review: 2026-06-08