My family doctor first told me about psyllium when I first had allergies in answer to my question of how to get enough fiber. Psyllium husks are the outer seed covering of the psyllium plant seeds. They are very high in fiber and contain a compound that when mixed with water, makes a mucilage. This mucilage is very healing to the bowel, provides a soft fiber to ease constipation (or avoid it), absorb toxins, help lower cholesterol, regulate blood sugar, and stop diarrhea.
The natural product, Metamucil which is used for relieving constipation and promoting regularity is simply Psyllium. Interestingly, Psyllium is not habit forming: meaning, your colon does not come to depend on you ingesting Psylium in order to eliminate.
You can buy Psyllium at health food stores. I get it a store called "Nutter's" in a bulk bin for a fairly reasonable price. Actually the price on the bin does not look reasonable, but Psyllium husks are so light that they do not add up fast. You can also buy it in capsules which are "easier to swallow" if taking it for your bowel health - as opposed to drinking the slimy mucilage. If you take the husks dry in capsules, you just have to drink a full glass of water and the the slimy stuff forms inside :)
Of course for baking purposes, you will want to get it bulk. If you just can not find it, you can buy the Metamucil at any drug store and just use a couple Tbsp of that in the bread - just make sure that you do not get a flavored kind... "pink lemonade rice bread" just does not sound appealing! However, if you can find it bulk, it will be less expensive.
Just google "Metamucil" or "Psyllium Husks" to get more information.