This is a confusing area of research. To understand we must first take a step back.
Fiber is an indigestible carbohydrate found in plant foods. It comes in two main forms, soluble (dissolves in liquid) and insoluble.
Prebiotics is the name given to fiber that “feeds” (is fermented by) our gut bacteria. Fermentation can cause beneficial changes in the activity or balance of gut bacteria, one reason dietary fiber is so important (
10).
Now while all prebiotics are fiber, not all fiber acts as prebiotic.
To confuse the issue further, many fiber-containing foods are high in FODMAPs, as are many prebiotics. In fact, the majority of known prebiotics are oligosaccharides (the “O” in FODMAP) (
10).
Given these overlapping categories and definitions, it’s not surprising the evidence for prebiotic and fiber supplement use is mixed for IBS patients.
Research suggests that soluble (but not insoluble) fiber supplements may help reduce both IBS-related diarrhea and constipation (
12,
13,
14).
It’s thought that soluble fiber helps to make stools a kind of soft yet thick texture. This helps stools move along during constipation, yet firms them up considerably during diarrhea.
Most of the research has looked at the soluble fiber supplement psyllium (or psyllium husk), which is a weak prebiotic.
In one clinical trial of IBS patients, taking 10 grams (about 2 teaspoons) of psyllium per day reduced perceived abdominal pain and discomfort by 90 points compared to just 58 points in the bran group and 49 points for placebo (
15).