Mention has already been made of bran and fiber. It hardly gains credulity when you consider precisely what it is. Take wheat, for example. If this is stored for any length of time it becomes bad. The reason for this is quite simple.
Stored wheat becomes bad because the fat in the germ turns rancid.
To stop this happening wheat has to be ground, or milled, to remove the wheat germ. Further examination of it would seem unlikely to persuade you otherwise. Bran, which forms the wall not just of wheat but of all plant cells is made of indigestible carbohydrates. This means that it enters and leaves the digestive tract completely intact. Unless a substance is absorbed from the alimentary tract it cannot really be considered to have entered the body.