Fiber is far more than simply an inert indigestible substance - it is vital for the establishment of a healthy population of gut bacteria flora. It is estimated that 97% of Americans are eating a fiber deficient diet. If a person is having problems eating a fiber rich diet then it is likely that they have gut flora that is unhealthy. Eating fiber will eventually correct this.

"...Fiber has long been linked to better health, but new research shows how the gut microbiota might play a role in this pattern. One investigation discovered that adding more fiber to the diet can trigger a shift from a microbial profile linked to obesity to one correlated with a leaner physique. Another recent study shows that when microbes are starved of fiber, they can start to feed on the protective mucus lining of the gut, possibly triggering inflammation and disease.
"Diet is one of the most powerful tools we have for changing the microbiota," Justin Sonnenburg, a biologist at Stanford University, said earlier this month at a Keystone Symposia conference on the gut microbiome. "Dietary fiber and diversity of the microbiota complement each other for better health outcomes."..."

Fiber-Famished Gut Microbes Linked to Poor Health - Scientific American