The only true way to diagnose IBD is with a biopsy, your vet is just guessing.
Fiber will not necessarily help your cat move things along, they are not humans, they are cats and they need food that the cat's digestive tract is made for and that is MEAT, BONES, ORGANS. Not fiber, not potatoes, not corn.
Please read the info in the link below:
http://www.catinfo.org/#Prescription/Therapeutic_Diets_
This bulletin half way down:
Inflammatory Bowel Disease (IBD): IBD can cause vomiting, diarrhea, and/or constipation in the cat. IBD can also present with weight loss as the only clinical sign. There are many unanswered questions with respect to this disease process, but it is logical to start to “treat” a gastrointestinal problem in the cat with a species-appropriate diet considering that it is food that 'bathes' the problem area.
Too often these cats are treated with a high level of steroids and a prescription grain-laden dry food diet. I feel very strongly that this common therapeutic regimen dismisses the very significant role that a proper diet plays in our IBD patients. There are an impressive number of anecdotal reports of cats that were terribly ill with IBD that exhibit dramatic improvement when all dry food was removed from their diet and a grain-free/low-carb canned food was fed instead.
Taking it even one step further, there are many reports of cats with IBD that improved tremendously on a balanced, grain-free, homemade diet. (See Making Cat Food for a balanced recipe.)
Some cats do need steroids (temporary or long-term) but we need to focus more heavily on feeding these patients an appropriate diet rather than simply relying on immunosuppressive medications.
If you truly want to help your cat then change it's diet, get rid of the vet food, which in my opinion is doing your cat
no good, along with the pred, which may also be damaging your cat more than is necessary. You may even have to change to a 100% raw diet on a protein source that doesn't irritate your cat's colon.
Pred will only mask the problem and it is a harsh medication that suppresses the immune system.
My friend had to go all the way to a raw diet with no chicken to get her cat who was severely ill with IBD to cure his colon. It took just a couple of months and is now healthy. Her cat can't eat any processed food.
Most cat food is not made with a cat's digestive tract in mind!
Some Felidae has grain in it and others has potatoes in it. It could be the carbs that is affecting your cat, or chicken, or beef.
Has your cat been checked for giardia? Coccidia?