There are several brands of dog food available which are designed to provide dogs with such a high fiber diet. This may be a good alternative since any sudden addition of a lot of fruit or vegetables to a dog’s diet can lead to stomach upset. However, if you introduce the "roughage" very gradually you should not experience any problems.

You could replace some of your dog’s regular diet with a high quality bran or any of the following: Carrots, pear, banana or pepper (non spicy variety!). If your dog prefers food which is not crunchy, try grating or pureeing the additions. Also be aware that some apparently everyday foods can be highly toxic to dogs, such as onions, grapes and raisins; and the pips of many fruits.