Tryptophan is an α-amino acid that is used in the biosynthesis of proteins. 

Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a non-polar aromatic amino acid. 

It is an essential amino acid,  and cannot synthesized.

Tryptophan must be obtained from the diet. 


Tryptophan is also a precursor to the neurotransmitter serotonin, the hormone melatonin, and vitamin B3.

Tryptophan is encoded by the codon UGG.

Tryptophan is a zwitterion at physiological pH.

Amino acids, including tryptophan, are used as building blocks in protein biosynthesis, and proteins are required to sustain life. 

Tryptophan is among the less common amino acids found in proteins.


Tryptophan it plays important structural or functional roles whenever it occurs. 

Tryptophan and tyrosine residues anchor membrane proteins within the cell membrane. 

Tryptophan, is important in glycan-protein interactions. 

Tryptophan is a biochemical precursor of:

Serotonin synthesized by tryptophan hydroxylase.

Melatonin is in turn synthesized from serotonin.

Niacin, also known as vitamin B3, is synthesized from tryptophan via kynurenine and quinolinic acids.

Fructose malabsorption causes improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood,and depression.

Indole is produced from tryptophan by bacteria that express tryptophanase.


Lactobacillus species metabolize tryptophan into indole-3-aldehyde (I3A) increasing interleukin-22 (IL-22) production.

Indole itself triggers the secretion of glucagon-like peptide-1 (GLP-1) in intestinal L cells.

Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction.

Activated charcoal, an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma.

Clostridium sporogenes metabolizes tryptophan into indole and subsequently 3-indolepropionic acid.

3-indolepropionic acid is a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals.

IPA binds to the pregnane X receptor (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function.

Following its absorption from the intestine and distribution to the brain, it confers a neuroprotective effect against cerebral ischemia and Alzheimer’s disease.



People with major depressive disorder (MDD), bipolar disorder, or schizophrenia appear to metabolize tryptophan in the brain differently than people without mental health disorders.



Tryptophan can be metabolized to either a route where serotonin is produced, or to the kynurenine pathway.



Patients with mood disorders and schizophrenia do not have decreased levels of tryptophan.



Tryptophan is being broken down more often in the kynurenine pathway, shifting away from the production of serotonin.



Serotonin is made from tryptophan that is thought to regulate anxiety and improve mood.



A meta-analysis suggests a shift from serotonin production to the kynurenine pathway in patients with mental health disorders.


Recommended Dietary Allowance (RDA) of 5 mg/kg body weight/day of Tryptophan for adults 19 years and over.

Tryptophan is present in most protein-based foods or dietary proteins. 

Tryptophan is plentiful in: chocolate, oats, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, almonds, sunflower seeds, pumpkin seeds, buckwheat, spirulina, and peanuts. 

Tryptophan content in turkey is typical of poultry.

Tryptophan is converted into 5-hydroxytryptophan (5-HTP) which is then converted into the neurotransmitter serotonin: proposed that consumption of tryptophan or 5-HTP may improve depression symptoms by increasing the level of serotonin in the brain. 

It is sold over the counter in the United States as a dietary supplement for use as an antidepressant, anxiolytic, and sleep aid. 

Tryptophan levels are unlikely to be altered by diet.

Consuming purified tryptophan increases the serotonin level in the brain, whereas eating foods containing tryptophan does not.

The transport system that brings tryptophan across the blood–brain barrier also transports other amino acids which are contained in protein food sources, and prevent the plasma concentration of tryptophan from increasing brain concentration levels.

The use of tryptophan as an adjunctive therapy in addition to standard treatment for mood and anxiety disorders is not supported by the scientific evidence.

Potential side effects of tryptophan supplementation include:  nausea, diarrhea, drowsiness, lightheadedness, headache, dry mouth, blurred vision, sedation, euphoria, and nystagmus. 

Tryptophan as a dietary supplement has the potential to cause serotonin syndrome when combined with antidepressants of the MAOI or SSRI class or other strongly serotonergic drugs.

As an essential amino acid, tryptophan is not synthesized from simpler substances.

 It needs to be present in the diet in the form of tryptophan-containing proteins. 

Eosinophilia–myalgia syndrome has linked to intake of tryptophan/contaminants.

The amount of tryptophan in turkey is comparable to that contained in other meats, and drowsiness after eating it may be caused by other foods eaten with the turkey, particularly carbohydrates.

Carbohydrate ingestion triggers the release of insulin, which in turn stimulates the uptake of large neutral branched-chain amino acids, but not tryptophan, into muscle, increasing the ratio of tryptophan to such amino acids  in the blood stream. 

The increased tryptophan ratio reduces competition at the large neutral amino acid transporter resulting in more uptake of tryptophan across the blood–brain barrier into the cerebrospinal fluid (CSF).

Once in the CSF, tryptophan is converted into serotonin by the normal enzymatic pathway,  and further metabolized into melatonin by the pineal gland.

All of this suggests that postprandial somnolence, may be the result of a heavy meal rich in carbohydrates, which indirectly increases the production of melatonin in the brain, and thereby promotes sleep.





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