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For many organisms (including humans), tryptophan is an essential amino acid. This means that it cannot be synthesized by the organism and therefore must be part of its diet. Amino acids, including tryptophan, act as building blocks in protein biosynthesis. In addition, tryptophan functions as a biochemical precursor for the following compounds (see also figure to the right):

Serotonin (a neurotransmitter), synthesized via tryptophan hydroxylase.[8][9] Serotonin, in turn, can be converted to melatonin (a neurohormone), via N-acetyltransferase and 5-hydroxyindole-O-methyltransferase activities.[10]
Niacin is synthesized from tryptophan via kynurenine and quinolinic acids as key biosynthetic intermediates.[11]
Auxin (a phytohormone) when sieve tube elements undergo apoptosis tryptophan is converted to auxins.[12]

The disorders fructose malabsorption and lactose intolerance cause improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood[13] and depression.[14]