Constipation SIBO versus FBD (Functional Bowel Disorder)

Hello everyone,

For the past several months my doctor has been treating my slowed motility, which he attributes to stress. The official diagnosis is Functional Bowel Disorder, with the symptoms being very similar to constipation caused SIBO. From the treatment, most of the symptoms have improved, but very slowly (even in his opinion). He doesn't think that this is SIBO due to the fact that there is no diarrhea, and says that a slow transit time will cause an increase in gas (hence no need for breath test).

Is there really no SIBO that causes constipation? Have any of you had success in improving the motility of your gut, or in treating constipation SIBO?