A diet change is the recommend first course of action for diabetic cats. The blander foods are easier on their stomachs and likely because of the possibility of the pancreatitis he prescribed the bland food.
Here is a site that has answers to lots of questions about cats and diabetes. You can also discuss a substitute food with your vet if the prescription is too expensive for you.
http://www.felinediabetes.com/fdmb-faq.htm
A/D is what's called a "recovery food." It is used primarily when pets have heart disease, diabetes, or even anorexia. Vets use it because it's gentle on the stomach and pets really like it. They will eat it when they won't eat anything else. It also has very few fillers and is good for the diabetic pet.
Bland foods (like A/D) are great for cats with pancreatitis as well.
While it is a little expensive, it is definitely worth the price. Eukanuba makes an A/D as well so you might want to compare prices and see if you can get it cheaper. You should also keep in mind that prices can vary greatly between vets so you will want to shop around and get a good price.
Let me know if you don't understand or if you have more questions.
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