Tryptophan is an essential amino acid that is the precursor of both serotonin and melatonin. Melatonin is a hormone that is produced by the pineal gland in animals, which regulates sleep and wakefulness. Serotonin is a brain neurotransmitter, platelet clotting factor, and neurohormone found in organs throughout the body. Metabolism of tryptophan into serotonin requires nutrients such as vitamin B6, niacin, and glutathione. Niacin (also known as vitamin B3) is an important metabolite of tryptophan. It is synthesized via kynurenine and quinolinic acids, which are products of tryptophan degradation. There are a number of conditions or diseases that are characterized by tryptophan deficiencies. For instance, fructose malabsorption causes improper absorption of tryptophan in the intestine, which reduces levels of tryptophan in the blood and leads to depression. High corn or other tryptophan-deficient diets can cause pellagra, which is a niacin-tryptophan deficiency disease with symptoms of dermatitis, diarrhea, and dementia. Hartnup's disease is a disorder in which tryptophan and other amino acids are not absorbed properly. Symptoms of Hartnup's disease include skin rashes, difficulty coordinating movements (cerebellar ataxia), and psychiatric symptoms such as depression or psychosis. Tryptophan supplements may be useful for treating Hartnup's. Assessment of tryptophan deficiency is done through studying excretion of tryptophan metabolites in the urine or blood. Blood may be the most sensitive test because the amino acid tryptophan is transported in a unique way. Increased urination of tryptophan breakdown products (such as kynurenine) correlates with increased tryptophan degradation, which occurs with oral contraception, depression, mental retardation, hypertension, and anxiety states. The requirement for tryptophan and protein decreases with age. The minimum daily requirement for adults is 3 mg/kg/day or about 200 mg a day. There is 400 mg of tryptophan in a cup of wheat germ. A cup of low fat cottage cheese contains 300 mg of tryptophan and chicken and turkey contain up to 600 mg of tryptophan per pound (http://www.dcnutrition.com). Tryptophan plays a role in "feast-induced" drowsiness. Ingestion of a meal rich in carbohydrates triggers the release of insulin. Insulin, in turn, stimulates the uptake of large neutral branched-chain amino acids (BCAAs) into muscle, increasing the ratio of tryptophan to BCAA in the bloodstream. The increased tryptophan ratio reduces competition at the large neutral amino acid transporter (which transports both BCAAs and tryptophan), resulting in greater uptake of tryptophan across the blood-brain barrier into the cerebrospinal fluid (CSF). Once in the CSF, tryptophan is converted into serotonin and the resulting serotonin is further metabolized into melatonin by the pineal gland, which promotes sleep. Under certain situations, tryptophan can be a neurotoxin and a metabotoxin. A neurotoxin is a compound that causes damage to the brain and nerve tissues. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of tryptophan can be found in glutaric aciduria type I (glutaric acidemia type I or GA1). GA1 is an inherited disorder in which the body is unable to completely break down the amino acids lysine, hydroxylysine, and tryptophan due to a deficiency of mitochondrial glutaryl-CoA dehydrogenase (EC 1.3.99.7, GCDH). Excessive levels of their intermediate breakdown products (e.g. glutaric acid, glutaryl-CoA, 3-hydroxyglutaric acid, glutaconic acid) can accumulate and cause damage to the brain (and also other organs), but particularly the basal ganglia. Babies with glutaric acidemia type I are often born with unusually large heads (macrocephaly). Other symptoms include spasticity (increased muscle tone/stiffness) and dystonia (involuntary muscle contractions resulting in abnormal movement or posture), but many affected individuals are asymptomatic. High levels of tryptophan have also been implicated in eosinophilia-myalgia syndrome (EMS), an incurable and sometimes fatal flu-like neurological condition linked to the ingestion of large amounts of L-tryptophan. The risk of developing EMS increases with larger doses of tryptophan and increasing age. Some research suggests that certain genetic polymorphisms may be related to the development of EMS. The presence of eosinophilia is a core feature of EMS, along with unusually severe myalgia (muscle pain). It is thought that both tryptophan and certain unidentified tryptophan contaminants may contribute to EMS (PMID: 1763543 ). It has also been suggested that excessive tryptophan or elevation of its metabolites could play a role in amplifying some of the pathological features of EMS (PMID: 10721094 ). This pathological damage is further augmented by metabolites of the kynurenine pathway (a tryptophan degradation pathway).