Psyllium and bentonite may have their value but shouldn't be used to substitute bulk and fibre from real food. Digestible fibre from cooked veggies are better suited for a smoot, efficient digestive process than rough fibre that, at first, works like a broom moving out waste, but then it irritates the intestinal lining and leaches out minerals from the intestinal wall.
Added bran has a similar effect. It contains a lot of antibodies that the grain uses to to ward off invading organism, worms and fungus. When ingested in this form, the body's immune system inthe intestinal tract tries to get rid of these antobodies as soon as it can, hence the "improved" bowel function. But this also can lead to loss of fluids and minerals, bowel exhaustion, and bloating in the large intestine (due to an increased number of bacteria that know how to break down the undigestible fibre).
The best way to add natural fibre is through salads, cooked veggies (especially green leafy), grain foods, pulses, etc, according to body type (see The Key to Health and Rejuvenaiton). For example, Brown basmati rice is good for the VATA type but very upsetting and gas-forming for the PITTA type, who does very well with white Basmati rice. Pink and brown lentils upset PITTA types but are excellent for Vata types.