Tryptophan is an essential amino acid that is metabolised in the brain to synthesise serotonin. Because tryptophan cannot be synthesised in humans, the diet is the only source of the amino acid. Dietary manipulation can alter mood because it changes the availability of brain tryptophan and this alters the synthesis rate of serotonin, an important determinant of mood. However, the gut microbiota are also able to metabolise tryptophan. Three major metabolites of tryptophan are produced in the gut including serotonin, kynurenine and indole derivatives. There is evidence that these metabolites are then able to interact with human tissues and cause physiological responses. In particular, serotonin produced in this way is known to be able to enter the central nervous system where it can elicit the normal functions of endogenously produced serotonin. Another role for the metabolites of tryptophan produced in the gut is in the modulation of the immune system which may involve significant anti-inflammatory effects.
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