Since synthetic indigo is produced on a massive scale by chemical routes, i.e. 50,000 tons/y (2011), the prospect of a biological pathway is of practical interest.

[1] A reaction, similar to that used to produce indigo dye, is seen in the normal population,[2] who excrete small amounts of the chemical in their urine.

[5][failed verification] Individuals affected by blue diaper syndrome exhibit a defect in tryptophan metabolism.

Indican is then excreted into the urine and from there into the diaper where, upon exposure to air, it is oxidised by atmospheric oxygen to indigo blue dye.

It can cause gastrointestinal symptoms in patients whose protein absorption is reduced, as in Hartnup's disease, allowing for greater bacterial decomposition of the tryptophan to indole and its conversion to indican.