Other Nutrients

Tryptophan

By All The Nutrients Editorial Team Published: 6 min read

Tryptophan is an essential amino acid — one of the nine amino acids the body cannot synthesise and must obtain from dietary protein. It holds a unique position in amino acid biochemistry as the precursor to three biologically critical molecules: serotonin (the key neurotransmitter for mood, appetite, and sleep regulation), melatonin (the hormone governing circadian rhythms), and niacin (vitamin B3, via the kynurenine pathway — approximately 60 mg of tryptophan converts to 1 mg of niacin equivalent). The RDA for tryptophan is 5 mg/kg of body weight per day (350 mg/day for a 70 kg adult). Despite its importance, tryptophan is consistently the least abundant essential amino acid in most dietary proteins — it is the rate-limiting amino acid in most plant-based diets. See the top food sources of tryptophan ranked from USDA data.

Top Source

Sea lion, Steller, meat with fat (Alaska Native)

1.60 g

What Does Tryptophan Do?

  • Serotonin synthesis and mood regulation — tryptophan crosses the blood-brain barrier (via the large neutral amino acid transporter) where it is hydroxylated to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase (TPH2) and then decarboxylated to serotonin (5-HT). Serotonin is the primary neurotransmitter regulating mood, emotional processing, appetite, pain sensitivity, and impulsive behaviour. Low central serotonin activity is implicated in depression, anxiety, and seasonal affective disorder — conditions that respond to SSRIs (selective serotonin reuptake inhibitors). Dietary tryptophan influences serotonin synthesis, though the relationship is modulated by competing amino acids for brain uptake.
  • Melatonin synthesis and sleep-wake cycle regulation — serotonin produced in the pineal gland is converted to N-acetylserotonin and then to melatonin by N-acetyltransferase and hydroxyindole O-methyltransferase. Melatonin secretion rises 2–3 hours before habitual sleep onset in darkness and coordinates the circadian timing system. The dietary tryptophan → serotonin → melatonin pathway means that tryptophan adequacy directly supports normal sleep architecture, though timing and carbohydrate co-ingestion modulate its brain availability.
  • Niacin (vitamin B3) synthesis — in the absence of adequate dietary niacin, tryptophan is the body's backup niacin source via the hepatic kynurenine pathway. Approximately 60 mg of tryptophan yields 1 mg of niacin equivalent — which is why classical pellagra (niacin deficiency) did not develop in populations consuming corn-based diets as long as protein quality and tryptophan intake remained adequate. This dual role means tryptophan deficiency can present as both reduced serotonin function and pellagra.
  • Immune regulation via the kynurenine pathway — most dietary tryptophan (~90–95%) is metabolised through the kynurenine pathway by indoleamine 2,3-dioxygenase (IDO) — an enzyme upregulated during immune activation, inflammation, and infection. Kynurenine pathway metabolites (kynurenic acid, quinolinic acid) regulate immune tolerance, modulate T-cell differentiation, and have direct effects on the central nervous system. IDO activation during chronic inflammation "steals" tryptophan from the serotonin pathway, potentially contributing to the depression and fatigue of inflammatory conditions.

Signs of Tryptophan Deficiency

Tryptophan deficiency severe enough to cause clinical symptoms is rare in adults consuming adequate total protein, but relative tryptophan insufficiency can affect serotonin availability. In experimental depletion studies, acute tryptophan depletion (consuming a tryptophan-free amino acid drink) reliably lowers mood and increases vulnerability to depression in people with a personal or family history of depression — confirming the serotonergic significance of dietary tryptophan. Populations most at risk of absolute tryptophan deficiency are those with very low protein intake or diets based on low-tryptophan staples (maize without nixtamalisation).

  • Low mood, irritability, and increased anxiety from reduced serotonin synthesis
  • Sleep disturbances and circadian rhythm disruption from reduced melatonin production
  • Pellagra-like syndrome (in severe combined tryptophan + niacin deficiency) — the "4 Ds": dermatitis, diarrhoea, dementia, death
  • Hartnup disorder — genetic impairment of tryptophan intestinal absorption causes pellagra-like skin lesions, cerebellar ataxia, and psychiatric symptoms despite apparently adequate diet
  • Impaired wound healing and immune function in protein-deficient states where all essential amino acids are reduced

How to Get the Most from Tryptophan

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Eat carbohydrates with tryptophan-rich foods for better brain uptake

Tryptophan competes with other large neutral amino acids (LNAA: leucine, isoleucine, valine, phenylalanine, tyrosine) for the LAT1 transporter across the blood-brain barrier. When you eat carbohydrates, insulin secretion reduces blood levels of most competing LNAAs (by driving them into muscle cells), increasing tryptophan's relative plasma ratio and improving its brain uptake. This is the likely mechanism behind carbohydrate cravings in low mood and the sedative effect of high-carbohydrate meals — often misattributed to simple "food coma."

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Evening tryptophan-rich foods may support sleep — timing matters

Consuming tryptophan-rich foods in the evening (turkey, eggs, dairy, pumpkin seeds) alongside a moderate carbohydrate source may support melatonin synthesis and improve sleep onset latency. Clinical studies with 1–3 g of tryptophan supplementation show modest improvements in sleep onset (approximately 10 minutes faster) and subjective sleep quality. Food-based amounts are lower but contribute to overall tryptophan status.

Common Tryptophan Questions

Does eating turkey make you sleepy from tryptophan?

The "turkey makes you sleepy" claim is mostly a myth — or at least an exaggeration. Turkey contains tryptophan (approximately 300 mg/100g), but no more than chicken, beef, or many other proteins. The sleepiness after a large holiday meal is far more attributable to the high calorie and carbohydrate content of the whole meal, alcohol, and the social/relaxed context. That said, tryptophan does support melatonin synthesis and carbohydrates do improve tryptophan's brain uptake — so the combination is not entirely without basis, just massively overstated.

What does tryptophan do?

Tryptophan is an essential amino acid used for protein synthesis and as the precursor to three key compounds: serotonin (neurotransmitter governing mood, appetite, and sleep), melatonin (hormone regulating circadian rhythms and sleep onset), and niacin (vitamin B3, via the kynurenine pathway). It also plays a role in immune regulation through the kynurenine pathway activated during inflammation.

Which foods are highest in tryptophan?

Tryptophan per 100g: spirulina dried (929 mg), pumpkin seeds (576 mg), sesame seeds (588 mg), soybeans roasted (575 mg), Parmesan cheese (559 mg), beef (219–279 mg), turkey breast (293 mg), chicken breast (290 mg), eggs (167 mg), and tofu (141 mg). Pumpkin seeds are the most concentrated food source per gram of protein.

Does tryptophan help with depression and mood?

Tryptophan is the dietary precursor to serotonin, and low serotonin activity is implicated in depression. Acute tryptophan depletion in clinical studies reliably lowers mood in vulnerable individuals. Tryptophan supplements (1–3 g/day) and its metabolite 5-HTP have shown modest antidepressant effects in small trials, but evidence is insufficient for clinical recommendation. Tryptophan supplementation is not a replacement for evidence-based depression treatment — consult a healthcare provider for mood disorders.

Is tryptophan the same as 5-HTP?

No — they are sequential molecules in the serotonin pathway. Tryptophan is hydroxylated (by tryptophan hydroxylase) to 5-HTP (5-hydroxytryptophan), which is then decarboxylated (by aromatic amino acid decarboxylase) to serotonin. 5-HTP supplements bypass the first rate-limiting step, making them more directly serotonergic than tryptophan supplements — but also raising more concerns about serotonin syndrome at high doses. Both are sold as supplements; neither is a standard medical treatment.