I am wondering what the best food is for my 2 Westies

I have 2 Westies who are both eating the Hills's Prescription Diet I/D Canine Gastrointestinal Health canned and dry food. The 13 year old dog was prescribed I/D because she has IBS but the food does not seem to help and she has an episode of IBS at least once a week. The 3 year old has gained weight and looks bloated. Both dogs are constantly hungry although they get the prescribed daily amount of food.

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