Glad you asked this question. My girl Roxy isn't underweight, however I had been doing research on different types of food to ensure she was getting the best. I found and like (even talked to the vet about it Saturday) Taste of the Wild. There is only 1 place where I live that sells it, so I think I'm going to grab a small bag and gradually change her over and see if she likes it. The vet even said if she likes it to let them know and they will do further research with what they have available to give me some more insight.
What I liked about it was that it was grain free and used very natural products (even the fish) in their dog food. Very high protein content and uses sweet potatoes and white potatoes as its filler.
If anyone else has used this food or has any insight on it I would love to know.
*Sorry for hijacking the post, just thought this would help you too!

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