Just because it's fed by veterinarians doesn't make it a good food. They are bribed from day one in vet school. Getting free stuff (Dog food, scrubs, stethoscopes, paper, pens, toiletries (yes even in the bathroom!)), and making sure they weasel their way into the vet's office. We don't ask our normal practitioner's about nutrition advice, so we shouldn't expect vet's to know it either. Vet's only get about one training seminar in nutrition unless they specialize in nutrition. Sure, vets should take more time out of their day to learn about nutrition before being led blindly... but they do a lot of studying on how to fix injuries and such, that I don't blame them. So just because your vet has it doesn't make it a good food. Once I finish my DVM, Science Diet will not be anywhere near my clinic. More like Orijen, Evo, California Natural, and premade raw. (If your intructor's pitbull has a lot of allergies, she should try California Natural... very limited ingredients as to not upset an allergic dog. VERY good food for allergies) Just take a look at the ingredients for yourself... Corn shouldn't be the number one ingredient... (This is the #1 thing dogs are allergic to!!!) even bad foods like Nutrish say meat should be #1. (Just because meat is number one, doesn't mean there aren't 5 more corn products down the line to add up to more than the meat)

EDIT: I did just look up Z/D, I have to say, at least it doesn't contain corn!!! But starch really shouldn't be the first ingredient. Nor should it have cellulose... they are both fillers. Doggies like meat!!!! Basically, Science Diet works for some dogs, and that's great! But there are definitely cheaper, better alternatives out there. (Like Cal Nat is cheaper than SD)