Tryptophan can be converted into kynurenine, which can form the neurotoxin quinolinic acid in the brain, associated with depression and neurological disorders.

“however tryptophan is also a precursor for another compound called kynurenine which can cross the blood-brain barrier and in the brain form a neurotoxin called Quin olynyk acid Quin allylic acid can cause cells to die in the brain it's also associated with depression and other mental health and neurological disorders”