My dog won't eat and has tummy issues. What food helps digestion?

Thanks for using Petco Pet Education Center, formerly Petcoach! Just like you pointed out, there are many different diets in the market to the point that it is hard to decide which one is best for patients. For any concerns on digestion, I prefer a veterinary prescription diet called Purina EN which is easily digestible and at the same time very good in terms of nutrition. It is also palatable and most dogs seem to like it. Alternatively, you could consider another diet which is the Hill's id which is similar to the Purina EN though in my experience it is not as palatable and some dogs will not eat it. Your veterinarian would have to write up a prescription for this diet. Good luck and have a good night!