Even if your dog is healthy it doesn't hurt to put him/her on a good quality diet without preservatives and minus those ingredients that cause allergies (the beef/wheat connection, for example). I note that places like PetSmart have a brand or two that promote themselves as extra healthy--read the label and see. My dogs get a brand available at feed stores and garden centers in Texas and Oklahoma (I'm not sure how much wider it is distributed) called Muenster's Natural. It has some of the trace ingredients that seem to contribute distinctly to good health (diatomaceous earth, for example).

SRS