The study design is a double-blind, randomized, controlled, parallel-group, 6-week study comparing the effects of gluten rich versus gluten free diets in diarrhea or diarrhea predominant IBS patients. All participants will keep a daily bowel pattern diary throughout the study. All participants will have negative serum tissue transglutaminase (TTg) assay, and anti-endomysial antibody test, if TTg is positive or equivocal. All participants will have the following studies performed both before and after the 4-week dietary intervention: