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Here you can find a .pdf relating a study made on a man who suffered from Crohn's disease.

He was cured using amino acids.

What are your thoughts, and could it be useful to IBS? I tried so many things like probiotics, pre,...

Interesting, but unfortunately the evidence is very limited.  We know that Crohn's and its close relative ulcerative colitis are triggered by bacteria such as mycobacterium avium complex and there is immune suppression through the adrenals.  We also know that serotonin and dopamine both have regulatory effects on the immune system.  So there may be something to it, but really more evidence is required.  Still, these amino acids are relatively safe at recommended doses so it would not hurt to try.  Just make sure to not go overboard on the 5-HTP in particular.