Tryptophan

Tryptophan is an essential amino acid and precursor of serotonin, melatonin and niacin (B3). With chronic inflammation, tryptophan is diverted by the enzyme IDO (indoleamine 2,3-dioxygenase) into the kynurenine route – less serotonin, more potentially neurotoxic kynurenine metabolites. IDO activity can be estimated in Prevent 360 via the ratio of kynurenine to tryptophan.

What tryptophan does in the body

Tryptophan is the rarest essential amino acid in dietary proteins. In the blood it is largely bound to albumin; only the free fraction competes at the blood-brain barrier with the branched-chain amino acids, tyrosine and phenylalanine for transport into the brain. It is processed further along three routes: a small part becomes serotonin with BH4, iron and vitamin B6, and at night melatonin in the pineal gland. The largest part goes via the kynurenine pathway: in the liver it is started by the enzyme TDO, which is stimulated by cortisol and by a lot of tryptophan; in immune cells by the enzyme IDO, which inflammatory messengers such as interferon-gamma switch on. Kynurenine gives rise to kynurenic acid, which dampens the NMDA receptor, the excitatory quinolinic acid and finally NAD+, for which vitamins B6 and B2 are needed. Gut bacteria also make indoles from tryptophan, which influence the gut barrier and immune cells via the aryl hydrocarbon receptor.

Low tryptophan – what happens in the body?

When little tryptophan is available, less starting material is available for serotonin in the gut and brain and for melatonin in the evening. During inflammation, the lowering is part of the defence: IDO withdraws tryptophan from pathogens and at the same time slows overactive T cells – in the process more tryptophan flows into the kynurenine pathway and less into serotonin production. Cortisol also shifts breakdown in this direction via TDO. Low values also occur with low protein intake and impaired absorption in the gut; in studies they have been observed, among other things, with depressed mood and fructose malabsorption.

High tryptophan – what does it mean?

More tryptophan in the blood does not automatically mean more serotonin, because production is limited mainly by the activity of tryptophan hydroxylase. A high value can show that breakdown via the kynurenine pathway is currently not very active, for example when there is no inflammation. The free fraction rises when albumin is low or when free fatty acids displace tryptophan from albumin – this happens during prolonged physical exertion; more tryptophan then reaches the brain, which studies link with central fatigue.

The interpretation is given in your report.