I am not particularly actually going to even recommend a high fiber food.
I would actually talk to your about doing a food that is actually designed for diabetes.
That would include Science Diet M/D or Purina DM diets.
If your vet carries neither, ask for a written prescription and you can simply get it from someone else. These foods are literally formulated to help deal with diabetes, including the restricted carbohydrate content. That is what I would do.
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