There are dozens of different soluble fibers. As the IBS research findings and Heather have pointed out some rapidly fermenting soluble fibers such as inulin and FOS are not tolerated by many IBSers and normal people as well.
The effects of soluble fiber are real effects that take place in the gut irrespective of what one's brain thinks or one believes! Some soluble fibers supplements have negative effects on some individuals. These negative effects are definitely not imaginary! IMHO
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS