Short answer. Tryptophan is an essential amino acid and the only building block the body uses to make serotonin, and then melatonin. Only about 3% of it becomes serotonin; roughly 90% runs down a different route, the kynurenine pathway (Richard 2009). Adults need about 3.5 to 6.0 mg per kg of body weight a day.
What is tryptophan, and what does it do?
Tryptophan is one of the nine essential amino acids, which means the body can't make it and has to get it from the protein in food. Beyond building protein, it has one job that gets most of the attention: it is the only raw material the body uses to make serotonin, a messenger involved in mood, appetite, gut movement and the timing of sleep (Richard 2009). The same amino acid is also the starting point for melatonin and, through a separate route, for a small amount of niacin (vitamin B3) (Badawy 2017).
Because it sits at the head of the serotonin line, a lot of research has tried to shift serotonin by changing how much tryptophan reaches the brain. The catch, which the rest of this page comes back to, is that very little tryptophan ends up as serotonin.
How does tryptophan become serotonin and melatonin?
In two enzyme steps, with two more at night. First an enzyme that needs iron turns tryptophan into 5-HTP (5-hydroxytryptophan). A second enzyme, which needs vitamin B6 in its active form (P-5-P), turns 5-HTP into serotonin. After dark, the pineal gland takes that serotonin through two further steps to melatonin, the signal that helps set the sleep-wake clock.
| Step | What it makes | What the step needs |
|---|---|---|
| Tryptophan to 5-HTP | 5-HTP | An enzyme that needs iron |
| 5-HTP to serotonin | Serotonin | An enzyme that needs vitamin B6 (P-5-P) |
| Serotonin to melatonin, at night | Melatonin | Two pineal-gland steps, triggered by darkness |
Two things follow. B6 is a helper, not raw material, so it never becomes serotonin itself, yet the body can't finish the job without it. And 5-HTP is one step closer to serotonin than tryptophan is, which is why some products use 5-HTP instead.
How much tryptophan do you need?
Adults need roughly 3.5 to 6.0 mg per kg of body weight a day, which works out to about 250 to 425 mg daily (Richard 2009). A separate review put the basic requirement at about 5 mg per kg (Fernstrom 2012), so the two line up. Most people already pass that from food alone: a typical daily intake is around 900 to 1,000 mg (Richard 2009).
The NIH Office of Dietary Supplements doesn't publish a separate allowance or upper limit for tryptophan. A varied diet with enough protein covers the requirement without any effort on your part.
Which foods are high in tryptophan, and does turkey make you sleepy?
Tryptophan is in most protein foods. Richard 2009 names oats, bananas, dried prunes, milk, tuna, cheese, bread, chicken, turkey, peanuts and chocolate among common sources. We are not giving milligrams per serving, because the figures we can source from PubMed and the NIH do not cover them.
Turkey gets the blame for the post-dinner slump, but its tryptophan is an unlikely cause. Here's the twist: a protein-heavy meal makes it harder, not easier, for tryptophan to reach the brain. Tryptophan shares one carrier across the blood-brain barrier with several other large amino acids, and that carrier is close to full at normal levels (Richard 2009). A big serving of protein floods the blood with those competitors, so tryptophan's share of the carrier drops. A carbohydrate-rich meal does the opposite: in rats, it raised brain tryptophan and brain serotonin, because insulin clears the competing amino acids into muscle (Fernstrom 1971). So if a holiday dinner leaves you drowsy, the large plate and the side dishes explain it better than the bird.
Where does most of your tryptophan go?
Down the kynurenine pathway, not toward serotonin. Only about 3% of dietary tryptophan is used to make serotonin anywhere in the body, and only about 1% in the brain; the kynurenine route handles roughly 90% of tryptophan breakdown (Richard 2009). A separate review puts the kynurenine share at about 95% (Badawy 2017). Either way the message is the same: serotonin is a small side branch, not the main road.
Stress and inflammation can widen that branch. Stress hormones switch on the liver enzyme at the start of the kynurenine pathway, and inflammation switches on its partner enzyme elsewhere in the body, so long-running stress and infections can pull more tryptophan toward kynurenine and leave less for serotonin (Höglund 2019; Badawy 2017). Short bursts of stress seem to work the other way and can briefly raise brain tryptophan (Höglund 2019). This is one reason the useful goal is not simply more tryptophan, but steady supply, with the cofactors to finish the job and without stress constantly diverting the raw material.
What do studies show about tryptophan and mood?
The honest summary: the system clearly matters, but taking tryptophan as a supplement has thin, mixed support, and no study shows it reliably lifts mood. Researchers probe the system with depletion, where volunteers drink a large amino acid mix with tryptophan left out, blood tryptophan falls for a few hours, and mood is measured. It is an experiment, never a remedy. The best-established finding is that lowering serotonin this way lowers mood in some people, though not in everyone (Young 2013).
| Study | Who took part | What happened |
|---|---|---|
| Smith 1997 | 15 women who had recovered from a diagnosed mood disorder, off medication | A tryptophan-free drink cut plasma tryptophan by 75%; 10 of the 15 had a temporary return of low-mood symptoms |
| Ruhé 2007, meta-analysis | 45 tryptophan-depletion studies pooled | No mood drop in healthy volunteers; a small drop in people with a family history of a mood disorder; a moderate drop in those with a past mood disorder in remission |
| Kikuchi 2021, systematic review | 11 randomized trials in healthy adults | Adding 0.14 to 3 g of tryptophan a day to normal meals may improve mood; the authors called for better studies |
| Lindseth 2015 | 25 healthy young adults, diet crossover | Higher positive-mood scores after 4 days of a high-tryptophan diet than a low one |
| Shaw 2002, Cochrane | Adults with a diagnosed mood disorder | 108 trials found, but only 2 (64 people) were good enough to count; the evidence was too weak to be conclusive |
Read together, these point to a pattern rather than a promise. Depletion bites hardest in people who are already vulnerable, which the reviewers read as a trait rather than proof that low serotonin causes low mood (Ruhé 2007). Adding tryptophan has shown small, short-term mood effects in healthy volunteers (Kikuchi 2021; Lindseth 2015), while the supplement trials in people with a diagnosed mood disorder were too few and too small to settle anything (Shaw 2002). Taking a building block away and putting one back are not mirror images.
Is a tryptophan supplement safe?
For most healthy adults, modest amounts are generally well tolerated, with two things worth knowing. The first is historical. In 1989 a serious illness called eosinophilia-myalgia syndrome appeared among tryptophan users, and it was traced to a contaminant in product from a single manufacturer rather than to tryptophan itself: 29 of 30 patients in one investigation had used that company's tryptophan (Belongia 1990). Later reviews blame the contaminant, not the amino acid (Fernstrom 2012), but it is why tryptophan quality and sourcing still matter.
The second is interactions. Tryptophan raises serotonin, so combining it with serotonin-raising medicines such as antidepressants, or with 5-HTP, can in rare cases cause too much serotonin activity, a dangerous state known as serotonin syndrome (Fernstrom 2012). Other side effects, such as drowsiness, nausea or tremor, turn up mainly at high doses, in the range of 70 to 200 mg per kg a day, far above the daily requirement (Fernstrom 2012). So the rules are short: use one serotonin-precursor product at a time, and if you take medicine for mood, ask your prescriber before adding tryptophan and do not combine it with a 5-HTP product on your own. If you are in crisis or thinking about harming yourself, call or text 988 in the US, or your local emergency number, right away.
Which ASDHelp products contain tryptophan?
Two ASDHelp products list tryptophan on their labels, and one of them pairs it with 5-HTP. Vitamin B6 shows up across several of these, so count it. We keep total supplemental vitamin B6 at or under 25 mg a day for adults and 10 mg for children unless a clinician sets a different amount, because high-dose B6 can damage sensory nerves over time (Schaumburg 1983).
- Spectrum Neurotransmitter Support: 80 mg of tryptophan and 290 mg of tyrosine per scoop, from 6 g of pea and rice protein, alongside NAC, DHA, CoQ10 and phosphatidylserine. The label serving is one scoop for ages 4 and up; adults take it with a practitioner's guidance. It has no added B6.
- SeroTonic+ Spray: 16 mg of L-tryptophan and 16 mg of 5-HTP per two-spray serving, with 2 mg of B6. The label allows up to three servings a day, so up to 48 mg of each precursor. Because it carries both tryptophan and 5-HTP, count it as one serotonin-precursor product, not two, and do not add another on top. Because it contains 5-HTP, it's never taken with antidepressants; its own label also tells you to check with your healthcare professional first if you take any medicines.
- Vitamin B6 P-5-P: 30 mg of the active cofactor per capsule, the form the body uses at the 5-HTP-to-serotonin step. One capsule is already above the 25 mg adult ceiling we keep to, so it is for when B6 is the only gap, not an add-on to a product that already includes B6. Under 18 it is practitioner-directed. Our vitamin B6 guide covers the limits by age.
- Amino contains no tryptophan, so it is not a tryptophan source. It does supply several large neutral amino acids, including leucine, valine, isoleucine and phenylalanine, that share the brain-entry carrier with tryptophan, so taken close together they compete for the same door. It also adds 14 mg of B6, which counts toward your daily total, and it contains phenylalanine, so it is not for anyone told to limit phenylalanine.
Why we use one precursor at a time, and never double up on the same active, is in one of each; for the wider picture, see how amino acids become the brain's messengers. Children's amounts always go to the child's clinician, and most of these labels give no children's dose.
Tryptophan or 5-HTP?
They feed the same pathway from different points, and neither is clearly better for everyone. Tryptophan is the natural starting material. 5-HTP is the next step, already past the slowest enzyme, and other amino acids don't get in the way of its absorption (Birdsall 1998). Each has trade-offs, and the research on both for mood is limited (Shaw 2002).
| Feature | Tryptophan | 5-HTP |
|---|---|---|
| Where it acts | Start of the pathway | One step before serotonin |
| Brain entry | Competes with other amino acids for the carrier | Absorbed without that competition; crosses into the brain easily |
| Also makes | Serotonin, melatonin and some niacin | Serotonin, and onward to melatonin |
| Shared rule | Both are serotonin precursors: use one at a time. 5-HTP is never taken with antidepressants; for tryptophan, ask your prescriber first if you take medicine for mood | |
Our full write-up of the second option, with doses and cautions, is on the 5-HTP page. Tyrosine, the amino acid behind dopamine rather than serotonin, is a different lever; see our L-tyrosine guide. Whichever you pick, add one precursor at a time and give it several weeks before judging it.
Questions to take to your doctor
- Given my diet and protein intake, is there any reason to think I am short on tryptophan, and would testing tell us anything?
- I take medicine that affects serotonin or mood. Is it safe for me to try a tryptophan or 5-HTP product at all?
- If I use more than one of these products, how much vitamin B6 am I getting in total, and is it under the limit for my age?
- For a child, what amount, if any, is appropriate, and which product fits?
Sources
- Richard DM et al. L-Tryptophan: basic metabolic functions, behavioral research and therapeutic indications. Int J Tryptophan Res 2009. PMID 20651948
- Badawy AA. Kynurenine pathway of tryptophan metabolism: regulatory and functional aspects. Int J Tryptophan Res 2017. PMID 28469468
- Höglund E et al. Tryptophan metabolic pathways and brain serotonergic activity: a comparative review. Front Endocrinol 2019. PMID 31024440
- Fernstrom JD, Wurtman RJ. Brain serotonin content: increase following ingestion of carbohydrate diet. Science 1971 (rat study). PMID 5120086
- Young SN. Acute tryptophan depletion in humans: a review of theoretical, practical and ethical aspects. J Psychiatry Neurosci 2013. PMID 23428157
- Smith KA et al. Relapse of depression after rapid depletion of tryptophan. Lancet 1997. PMID 9093253
- Ruhé HG et al. Mood is indirectly related to serotonin, norepinephrine and dopamine levels in humans: a meta-analysis of monoamine depletion studies. Mol Psychiatry 2007. PMID 17389902
- Kikuchi AM et al. A systematic review of the effect of L-tryptophan supplementation on mood and emotional functioning. J Diet Suppl 2021. PMID 32272859
- Lindseth G et al. The effects of dietary tryptophan on affective disorders. Arch Psychiatr Nurs 2015. PMID 25858202
- Shaw K et al. Tryptophan and 5-hydroxytryptophan for depression. Cochrane Database Syst Rev 2002. PMID 11869656
- Fernstrom JD. Effects and side effects associated with the non-nutritional use of tryptophan by humans. J Nutr 2012. PMID 23077193
- Belongia EA et al. An investigation of the cause of the eosinophilia-myalgia syndrome associated with tryptophan use. N Engl J Med 1990. PMID 2370887
- Birdsall TC. 5-Hydroxytryptophan: a clinically-effective serotonin precursor. Altern Med Rev 1998. PMID 9727088
- Schaumburg H et al. Sensory neuropathy from pyridoxine abuse. N Engl J Med 1983. PMID 6308447
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure or prevent any disease. This article is educational and is not medical advice.