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Tryptophan is an amino acid and essential in human nutrition. It is one of the 20 amino acids in the genetic code (codon UGG), and its symbol is Trp or W. Only the L-stereoisomer appears in mammalian protein.

Tryptophan is also a precursor for serotonin (a neurotransmitter) and melatonin (a neurohormone). The functional group of tryptophan is indole, see that article for more on its chemical properties.

Dietary sources

Tryptophan is particularly plentiful in chocolate, oats, bananas, dried dates, milk, cottage cheese, meat, fish, turkey, and peanuts. Tryptophan is associated with serotonin and melatonin production.

Tryptophan has been implicated as a possible cause of schizophrenia in people who cannot metabolize it properly. When improperly metabolized it creates a waste product in the brain which is toxic and causes hallucinations and delusions.

Tryptophan has also been indicated as an aid for schizophrenic patients.

Use as a dietary supplement

For some time, tryptophan was available in health food stores as a dietary supplement. Many people found tryptophan to be a safe and reasonably effective sleep aid, probably due to its ability to increase brain levels of serotonin (a calming neurotransmitter when present in moderate levels) and/or melatonin (a drowsiness-inducing hormone secreted by the pineal gland in response to darkness or low light levels). Clinical research tended to confirm tryptophan's effectiveness as a natural sleeping pill and for a growing variety of other conditions typically associated with low serotonin levels or activity in the brain. In particular, tryptophan showed considerable promise as an antidepressant, alone and as an "augmentor" of antidepressant drugs. Other promising indications included relief of chronic pain and reduction of impulsive, violent, manic, addictive, obsessive, or compulsive behaviours and disorders.

In 1989, a large outbreak of a mysterious, disabling, and in some cases deadly autoimmune illness called eosinophilia-myalgia syndrome was traced to an improperly prepared batch of tryptophan. The bacterial culture used to synthesise tryptophan by a major Japanese manufacturer had recently been genetically engineered to increase tryptophan production: unfortunately, with the higher tryptophan concentration in the culture medium, the purification process had also been streamlined to reduce costs, and a purification step that used charcoal absorption to remove impurities had been omitted. This allowed another bacterial metabolite through the purification, and this contaminant of the end-product had been responsible for the toxic effects. Regardless of the origin of the toxicity, tryptophan was banned from sale in the US, and other countries followed suit.

Though tryptophan supplements are still banned in the US from over-the-counter sale, properly produced pharmaceutical-grade tryptophan continues to be used legally as an essential nutrient in infant formulas and intravenous feeding and, in recent years, compounding pharmacies and some mail-order supplement retailers have begun selling tryptophan to the general public-sometimes as a supplement for "pets". Tryptophan has also remained on the market as a prescription drug (Tryptan) which some psychiatrists continue to prescribe, particularly as an augmenting agent for people who are unresponsive to antidepressant drugs. Indeed, tryptophan has continued to be used in clinical and experimental studies employing human patients and subjects. Several of these studies suggest tryptophan can effectively treat the fall/winter depression variant of seasonal affective disorder (SAD).


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