5-HTP administration significantly increased mucosal levels of 5-HIAA , the main metabolite of 5-HT , in both healthy controls ( 7.1 1.7 vs. 2.5 0.7 pmol/mg , 5-HTP vs. placebo , P = 0.02 ) and IBS patients ( 20.0 4.8 vs. 8.1 1.3 pmol/mg , 5-HTP vs. placebo , P = 0.02 ) , with the latter group showing a significantly larger increase .