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Well, there are worse ingredients to worry about than artificial Vit. K.  I would keep him on the EN for at least 4-8 weeks to stabilize his G-I. Adding a probiotic would be helpful--not enzymes, but a probiotic.  I use a human product, but since your vet has the Purina products, you may want to ask about Forti-Flora---I know several people who have had great success with that particular product.  Sounds like he needs to get his intestinal flora back in balance.  Some dogs do not do well with a lot of switching and/or rotating of the diet.  He may just be such a dog.

As for sunflower oil, I have not heard it linked to cancer, but it is not as easily digested as say, fish oil or salmon oil.

I say stay the course with the EN for now, at least for a good while.  I know that you have had success with Eagle Pack Holistic Select, maybe after you stabilize his G-I, that may be something to go back to, if you have reservations about the Cal.Nat.

Honestly I'd not worry about the menadione.  I asked my Animal Science nutrition prof about it when I was in college and he scoffed at the "oh it's horrible, toxic, cancer-causing, etc. etc." comments I had heard.  This was NOT a class sponsored by a petfood company (or any company) and the prof was a researcher in animal nutrition.

Thank you for the input! I'm going to stick with the Natural Balance a little longer, but if I end up having to switch to EN I feel so much better about it.