Tryptophan

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

Function
Tryptophan is an essential amino acid that humans cannot live without consuming. Amino acids function as building blocks in protein biosynthesis.

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

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 that is toxic, causing hallucinations and delusions. Tryptophan has also been indicated as an aid for schizophrenic patients.

Dietary sources
Tryptophan, found as a component of dietary protein, is particularly plentiful in chocolate, oats, bananas, dried dates, milk, cottage cheese, meat, fish, turkey, chicken, sesame and peanuts.

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

In 1989, a large outbreak of a new, disabling, and in some cases deadly autoimmune illness called eosinophilia-myalgia syndrome (EMS) was traced to L-tryptophan. The bacterial culture used to synthesize tryptophan by a major Japanese manufacturer, Showa Denko KK, had recently been genetically engineered to increase tryptophan production; 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, resulting in the presence of an end-product contaminant responsible for the toxic effects. The FDA was unable to establish with certainty that this was the sole cause of the outbreak. Tryptophan was banned from sale in the US, and other countries followed suit.

Though it is indisputable that Showa Denko KK did produce and sell a contaminated batch of L-tryptophan, there are some concerns that the FDA's handling of this accident unfairly favoured the pharmaceutical industry and the new antidepressant Prozac if only because of its curiously fortuitous timing. The March 22, 1990 ban on public sale of L-tryptophan came only four days before the media announcement of Prozac on March 26, 1990 in Newsweek magazine. Both L-tryptophan and Prozac affect serotonin in the brain, though in different ways, and were promising in the treatment of depression. At the time of the ban the FDA did not know, or did not indicate, that EMS was caused by a contaminated batch, and yet even when the contamination was discovered and the process fixed, the FDA maintained that L-tryptophan was unsafe. In February 2001 the FDA loosened the restrictions on marketing (though not on importation), but still expressed the following concern:
 * "Based on the scientific evidence that is available at the present time, we cannot determine with certainty that the occurrence of EMS in susceptible persons consuming L-tryptophan supplements derives from the content of L-tryptophan, an impurity contained in the L-tryptophan, or a combination of the two in association with other, as yet unknown, external factors.".

In recent years, compounding pharmacies and some mail-order supplement retailers have begun selling tryptophan to the general public. 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. Also, most health-food stores sell a cheap metabolite of tryptophan called 5-HTP to get around the ban and the resulting artificially high cost of the amino acid itself. 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).

Tryptophan and turkey
Contrary to popular belief, tryptophan in turkey meat does not cause drowsiness. Turkey does contain tryptophan, which does have a documented sleep-inducing effect. However, tryptophan is effective only when taken on its own as a free amino acid. Tryptophan in turkey is found as part of a protein, and, in small enough amounts, this mechanism seems unlikely.

A more-likely hypothesis is that the ingestion of large quantities of food, such as at a Thanksgiving feast, means that large quantities of both carbohydrates and branched-chain amino acids are consumed. Like carbohydrates, branched-chain amino acids require insulin to be transduced through the myocyte membranes, which, after a large meal, creates a competition among the amino acids and glucose for insulin, while simultaneously creating tryptophan's reduced competition with other amino acids for the Large Neutral Amino Acid Transporter protein for transduction across the blood-brain barrier. The result is a greater availability of tryptophan, via the Large Neutral Amino Acid Transporter, for conversion into serotonin by the raphe nuclei, which is then available for conversion into melatonin by the pineal gland. Drowsiness is the outcome.